okabass 4/12/2018 6:14 AM
Measuring pickups Capacitance?
It seems that PU's capacitance is not so easy to measure. I think normal method is to define the resonance frequency and inductance and then calculate the capacitance.
It would be nice if you could get the capacitance easier to calculate the resonance frequency and have a feeling how the PU will behave.

I thought that not so sophisticated or brutal method would be to make a tap at half point the coil. Then wind the rest. Then disconnect or cut the coil at half tap, measure the capacitance at the start and end.

So if you are winding 10000 turns, tap it at 5000 turns. Wind the rest, cut the wire at the tap, measure the capacitance, solder the wire back.
Never tried, but if someone has, please tell if it doesn't work.
 
Helmholtz 4/12/2018 7:41 AM
Sorry, your method will not give useful information on the capacitances that determine the PU's resonance(s). The equivalent circuit of your arrangement consists of the series wiring of two coupled windings of a transformer with distributed and reflected capacitances and a rather small interwinding capacitance (<50pF or so) in between. (Please see thread titled "Switchable additional winds"). The result will be more or less the interwinding capacitance. But this does not relate to any PU resonance.

You can measure PU capacitance with an LCR meter that allows measuring at 100KHz. With typical high impedance PUs the accuracy is very good. The absolute measuring error (always negative) caused by the presence of inductance is below 3pF for a PU inductance of 1H and decreases proportionally with higher inductances.

Generally the importance of PU capacitance on the frequency response seems overrated. Its impact on the loaded resonant frequency is much smaller than inductance. A difference of 100pF corresponds to a 3 foot change in length of a typical guitar cable. Thus cables of different lengths can be used to evaluate audible effects of different PU capacitances.
 
okabass 4/12/2018 8:12 AM
HI

Thank you. Really informative answer. I thought that if the measuring the cap. is so easy, it have been used.

Yes the pots, cable etc. impact is very big: several kHz. It is surprisingly big, when you first time notice the difference with lunloaded and loaded PU. Like:
http://i.imgur.com/ryNSHbQ.png
 
bbsailor 4/12/2018 12:38 PM
Quote Originally Posted by okabass View Post
It seems that PU's capacitance is not so easy to measure. I think normal method is to define the resonance frequency and inductance and then calculate the capacitance.
It would be nice if you could get the capacitance easier to calculate the resonance frequency and have a feeling how the PU will behave.

I thought that not so sophisticated or brutal method would be to make a tap at half point the coil. The wind the rest. Then disconnect or cut the coil at half tap, measure the capacitance at the start and end.

So if you are winding 10000 turns, tap it at 5000 turns. Wind the rest, cut the wire at the tap, measure the capacitance, solder the wire back.
Never tried, but if someone has, please tell if it doesn't work.
Pickups sound a certain way due to the unintended consequences of winding enough wire on the pickup to generate enough voltage to drive the input stage of early guitar amplifiers which were derived from tube- based PA amplifiers. These amplifiers typically had a high impedance of about 1M ohm and the consequence of this high impedance design has audible effects. This early design has not evolved much. Les Paul recorded his "Recording" model guitars directly into mic inputs using low impedance pickups without the typical electric guitar sound. Below is the reason for the electric guitar sound.

1. The amp input acts as a bridging input impedance that is about 10 times higher than the source impedance to minimize loading to get the maximum input voltage. At pickup resonance, this bridging impedance affects pickup loading the most as the pickup resonance point is where the pickup impedance is the highest.

2. Using thin magnet wire AWG 42 or 43 allows many thousand turns to be placed on the bobbin to get a high enough output voltage to drive the amplifier required input level. Since this thin wire also has a very thin non conductive coating, there is a very slight capacitance between adjacent strands laying in very close proximity to each other. Multiply this very slight capacitance by the thousands of pickup turns and you begin to get enough capacitance to form a resonant peak in the region of human hearing where the ear is most sensitive. See this link https://sonicscoop.com/wp-content/up...son-curves.png

3. Hand winding pickups puts more air between the winds than machine winding where the wire density is more compact. Thus, hand winding has lass capacitance than machine winding. Typically, pickups have between 80 pf to about 160 pf of capacitance but this is heavily overshadowed by the typical guitar cable capacitance of about 35 pf per foot or about 350 pf for a 10 foot cable. To hear what a guitar sounds like look up "Tillman buffer amplifier" where you can see how to build a JFET in the guitar end of a guitar cable to put a 1Meg to 3 Meg ohm input load right at the guitar end of the cable and also isolate the pickup from the cable capacitance to hear a higher shift in the natural pickup resonance due to the elimination of cable capacitance effects.

4. Guitars have a fundamental frequency range from 82 Hz on the open low E string to 1312 HZ on the 24th fret of the high E string. However, guitar pickups respond to the way a string is strummed as the horizontal movement affects the 2nd harmonic (twice the fundamental frequency) more because when a string passes a pickup pole piece it generates a peak voltage each time is passes the pole piece. Strings typically oscillate in an oblong pattern with both horizontal and vertical movement and depending on which pickup you use, where you strike the strings and how you strike the strings can have an audible impact on the initial string movement and sound. The first 30 up to 50 milliseconds has the most hearing impact on the perceived quality of the sound. Try this simple experiment either by just listening or by listening and observing the pickup output on an oscilloscope. Turn on the neck pickup. Pinch the low E string over the pole piece and pull sideways about .125 inch and then let go. Now, observe and listen the very first few oscillations. Pinch the string again in the same place above the pickup pole piece and pull it up about .125 inch and release (or a little less for low action guitars) and now listen and observe the sound. This vertical pick should sound a little louder as the vertical string motion contributes more to string output level then horizontal string motion. Try it on the other open strings also.

As you can see, pickup capacitance, cable capacitance and other loading (amp input impedance, volume controls and tone controls) can contribute to the perceived sound but there are more variables involved. Playing at a low level where the ear is most sensitive between 3 KHz to 5 KHz and the bass frequency sensitivity is very low is a lot different than when playing in a band where the typical listening level is higher and the threshold of hearing sensitivities becomes more flat across the guitar fundamental and harmonic range up to about 5 KHz where the typical high impedance pickup response tends to quickly fall off after the pickup resonant peak.

I hope this helps?

Joseph J. Rogowski
 
okabass 4/12/2018 1:08 PM
HI

Thank you for your in-depth answer.

I've wind only some dozens PU as hobby, but it is nice to have theory also. My interest is now in 51 7ender Precision bass PU type. It is quite simple but sounds surprisingly good.Try to find a good balance with clarity and lo end. Now It feels that around 10000 turns 42 AWG (7,7 kΩ), 3,3 Henrys is quite close.
 
Antigua 4/12/2018 3:31 PM
Quote Originally Posted by bbsailor View Post
4. Guitars have a fundamental frequency range from 82 Hz on the open low E string to 1312 HZ on the 24th fret of the high E string. However, guitar pickups respond to the way a string is strummed as the horizontal movement affects the 2nd harmonic (twice the fundamental frequency) more because when a string passes a pickup pole piece it generates a peak voltage each time is passes the pole piece. Strings typically oscillate in an oblong pattern with both horizontal and vertical movement and depending on which pickup you use, where you strike the strings and how you strike the strings can have an audible impact on the initial string movement and sound. The first 30 up to 50 milliseconds has the most hearing impact on the perceived quality of the sound. Try this simple experiment either by just listening or by listening and observing the pickup output on an oscilloscope. Turn on the neck pickup. Pinch the low E string over the pole piece and pull sideways about .125 inch and then let go. Now, observe and listen the very first few oscillations. Pinch the string again in the same place above the pickup pole piece and pull it up about .125 inch and release (or a little less for low action guitars) and now listen and observe the sound. This vertical pick should sound a little louder as the vertical string motion contributes more to string output level then horizontal string motion. Try it on the other open strings also.
I came up with a way to demonstrate the horizontal vs. vertical string movement difference. You abut the guitar string with a guitar pick (or something similar), directly over the selected pickup, and then pull and release the string with your fingers, while the string is still against the pick. The presence of the pick forces the string to only move along one axis. Of course, the friction between the pick and the string causes the string to stop moving very quickly, but for the brief transient, you can hear the differences. If the guitar pick is parallel to the face of the guitar, or "side to side", and you release the string, you hear almost nothing. If you re-orient the guitar pick so that it's perpendicular to the face, or "up and down", year hear the transient very clearly. This shows how one axis of string movement produces a clearly audible signal, while movement along the other axis does not.
 
Antigua 4/12/2018 3:38 PM
Quote Originally Posted by Helmholtz View Post

You can measure PU capacitance with an LCR meter that allows measuring at 100KHz. With typical high impedance PUs the accuracy is very good. The absolute measuring error (always negative) caused by the presence of inductance is below 3pF for a PU inductance of 1H and decreases proportionally with higher inductances.
How interesting. The DE-5000 has a 100kHz setting. I've calculated the capacitance of dozens of pickups based on the peak frequency. I'll give C measurements a try with a DE-5000 and see how close it comes to those peak f based measurements.
 
Helmholtz 4/12/2018 4:56 PM
Quote Originally Posted by Antigua View Post
How interesting. The DE-5000 has a 100kHz setting. I've calculated the capacitance of dozens of pickups based on the peak frequency. I'll give C measurements a try with a DE-5000 and see how close it comes to those peak f based measurements.
Just make sure to select parallel mode to measure Cp at 100kHz and serial mode to measure inductance (Ls) at low frequency.

If you find a noticeable deviation from the capacitance calculated via frequency response, the reasons are:

1) Inductance at resonance frequency is lower than the value measured at a lower frequency. This is typical for PUs with high steel cores, where inductance decreases with frequency caused by the magnetic skin effect. An indication is the 1kHz inductance being lower than the 100Hz value.

2)The peak frequency of a low Q parallel resonant circuit is not identical to the theoretical resonant frequency. (I think this is discussed in Terman's book)

In any case the 100kHz measuring gives the most reliable result.
 
Antigua 4/15/2018 10:21 PM
Quote Originally Posted by Helmholtz View Post
Just make sure to select parallel mode to measure Cp at 100kHz and serial mode to measure inductance (Ls) at low frequency.

If you find a noticeable deviation from the capacitance calculated via frequency response, the reasons are:

1) Inductance at resonance frequency is lower than the value measured at a lower frequency. This is typical for PUs with high steel cores, where inductance decreases with frequency caused by the magnetic skin effect. An indication is the 1kHz inductance being lower than the 100Hz value.

2)The peak frequency of a low Q parallel resonant circuit is not identical to the theoretical resonant frequency. (I think this is discussed in Terman's book)

In any case the 100kHz measuring gives the most reliable result.
It works! I don't know how you knew this would work, but you're exactly correct. I measured a Seymour Duncan SSL-1 neck pickup, and got the exact same capacitance value 103pF , reported here a few months ago Seymour Duncan SSL-1, Analysis and Review | Fender Stratocaster Guitar Forum

Here's a pic of the measurement:

[ATTACH=CONFIG]48494[/ATTACH]

Same story with a Seymour Duncan SH-1N or "'59 neck", measured 98pF here Seymour Duncan SH-1N "59" Neck, Analysis & Review | GuitarNutz 2 , showing 99pF with the DE-5000

[ATTACH=CONFIG]48495[/ATTACH]

100kHz was the only test frequency that worked. For the SSL-1, at 10kHz, it shows 4.86pF in PAR, 39pF in SET, both of which are way off, and lower frequncies are increasingly off, showing capacitance values in the nano-farad range. At 100kHz, switching between SER and PAL shows the same value to within 1pF, for bot the SSL-1 and the SH-1N, so while I assume there is a good reason to use PAL mode, SER appears to be very close as well.

Note in the picture that I have the lead wires far apart from each other, and not touching, as when they're brought close together, you instantly see the capacitance rise by a few picofarads.

Since it appears the DE-5000 reliably measures capacitance as well as inductance (only SER mode is suitable for inductance, and the lowest frequency setting of 100Hz is most accurate), and that these value agree with those derived from impedance plots, it's therefore possible to calculate the resonant peak of the pickup using nothing more than the DE-5000.

The resonant peak itself is rarely useful, since in an electric guitar, a high degree of additional capacitance will bring the true resonant frequency down to a much lower value. For the surveying of pickups I've done, I also include resonant peaks with a 470pF capacitor across the pickup, which is intended to represent a guitar cable, and give a more realistic representation of what the pickup will do in situ. I will try putting a 470pF cap across the pickup, then I'll measure the capacitance again, and see how closely the capacitances sum, as well as determine how closely the calculated resonant peak "with load" comes to the peak measured with an oscilloscope. If it turns out the DE-5000 can effectively acquire all these data points, then the only advantage that would remain for bode plot measurements is in determining how much resonant damping occurs due to eddy currents.

Thanks again for tipping me off to the fact that an LCR meter can calculate the capacitance of a pickup at 100kHz, this is very valuable information to have.
 
okabass 4/16/2018 3:23 AM
Thank you for the tip Hemholtz and Antigua. Just ordered DE-5000 LCR meter from eBay.
 
Helmholtz 4/16/2018 6:30 AM
You are welcome.

I knew that it would work before I bought an LCR meter with the 100kHz option some years ago. My considerations were based on understanding of:

- the PU's equivalent circuit
- the measuring principle of LCR meters in series and parallel modes.

The measuring error can be estimated from the formula for the apparent capacitance in parallel resonant circuits, which can be derived from the imaginary part of the admittance (1/Z). It is essential that the measuring frequency is far above the circuits resonance, where the frequency response of the impedance shows a clear -6dB/octave (= purely capacitive) behavior.
A measuring frequency closer to resonance (i.e. 10khz) will give a lower meter reading, as the meter shows apparent capacitance. The latter decreases towards resonance and = 0 exactly at resonance.
 
Antigua 4/16/2018 1:20 PM
Quote Originally Posted by Helmholtz View Post
You are welcome.

I knew that it would work before I bought an LCR meter with the 100kHz option some years ago. My considerations were based on understanding of:

- the PU's equivalent circuit
- the measuring principle of LCR meters in series and parallel modes.

The measuring error can be estimated from the formula for the apparent capacitance in parallel resonant circuits, which can be derived from the imaginary part of the admittance (1/Z). It is essential that the measuring frequency is far above the circuits resonance, where the frequency response of the impedance shows a clear -6dB/octave (= purely capacitive) behavior.
A measuring frequency closer to resonance (i.e. 10khz) will give a lower meter reading, as the meter shows apparent capacitance. The latter decreases towards resonance and = 0 exactly at resonance.
Since most inductors have a much lower inductance, in the mH range, and would have a very high resonant peak, is it somewhat unusual to be able to find the parasitic capacitance of an inductor with an LCR meter, even if it does have a 100kHz test range?
 
Helmholtz 4/16/2018 1:42 PM
I am not sure, if I understand your question.

As noted earlier, the measurement error increases with 1/L and the measuring frequency has to lie in a frequency region, where the impedance drops proportionally with increasing frequency (i.e. -6dB/octave). In practice this means that the measuring frequency (100kHz) should be at least about a factor 10 higher than the resonant frequency, as a rule of thumb.
 
Antigua 4/16/2018 2:07 PM
Quote Originally Posted by Helmholtz View Post
I am not sure, if I understand your question.

As noted earlier, the measurement error increases with 1/L and the measuring frequency has to lie in a frequency region, where the impedance drops proportionally with increasing frequency (i.e. -6dB/octave). In practice this means that the measuring frequency (100kHz) should be at least about a factor 10 higher than the resonant frequency, as a rule of thumb.
So if 100kHz needs to be 10x above the resonant frequency, so this means 10kHz would be the highest resonant frequency for which this would work with accuracy. AFAIK, very few inductors have a resonant frequency that is as low as 10kHz, and that guitar pickups are special in that that have a very high inductance, generally above 2 henries. In a way, that means this is sort of a "trick", in that that this method would not work for most inductors, just those with especially low resonant peaks, and so we've "lucked out", in a sense.

Thanks again for participating here, your insights have really moved the ball forward in my pickup research. Are you involved with Gitec?
 
Helmholtz 4/16/2018 2:40 PM
Quote Originally Posted by Antigua View Post
So if 100kHz needs to be 10x above the resonant frequency, so this means 10kHz would be the highest resonant frequency for which this would work with accuracy. AFAIK, very few inductors have a resonant frequency that is as low as 10kHz, and that guitar pickups are special in that that have a very high inductance, generally above 2 henries. In a way, that means this is sort of a "trick", in that that this method would not work for most inductors, just those with especially low resonant peaks, and so we've "lucked out", in a sense.

Thanks again for participating here, your insights have really moved the ball forward in my pickup research. Are you involved with Gitec?
I would not call it a trick, just applied physics. Fortunately it works fine with typical high impedance PUs. You could use the method for resonant frequencies above 10kHz but would need to calculate the error caused by inductance and compensate. Inductors with even higher resonant frequencies require meters/equipment with higher measuring frequencies.
I know GITEC and had some contact. They don't seem to be interested in exchange/communication with people who refuse to become a member of the club. But I highly appreciate Manfred Zollner's book "Physik der Elektrogitarre".
 
Mike Sulzer 4/21/2018 4:58 AM
 
Mike Sulzer 4/21/2018 5:18 AM
Quote Originally Posted by Helmholtz View Post
Just make sure to select parallel mode to measure Cp at 100kHz and serial mode to measure inductance (Ls) at low frequency.

If you find a noticeable deviation from the capacitance calculated via frequency response, the reasons are:

1) Inductance at resonance frequency is lower than the value measured at a lower frequency. This is typical for PUs with high steel cores, where inductance decreases with frequency caused by the magnetic skin effect. An indication is the 1kHz inductance being lower than the 100Hz value.

2)The peak frequency of a low Q parallel resonant circuit is not identical to the theoretical resonant frequency. (I think this is discussed in Terman's book)

In any case the 100kHz measuring gives the most reliable result.
The lower apparent inductance measured at frequencies above about 100 Hz is mostly due to eddy currents in metal pickup parts. For example, the cores act like the secondary of a poorly coupled transformer. I write "apparent" because the meter is capable of measuring two numbers, the real and imaginary parts (or amplitude and phase) of the impedance, and therefore can be used to model one reactive component (L or C) and one resistance (series or parallel). A pickup is a more complicated circuit, with the eddy current losses involving loss (resistance) as well as inductance. (The Q of some pickups is determined mostly by the eddy current losses rather than wire resistance.) Therefore the measured inductance above 100 Hz is somewhat in error because a correct measurement requires more than two numbers. At a high enough frequency, the the inductive reactance of the eddy current effect should dominate, and therefore the capacitance as measured at 100 KHz should be very close. But i think it is important to evaluate the error with all this in mind.
 
Antigua 4/21/2018 10:52 AM
Regarding capacitance measurements with the DE-5000 at 100kHz, I mentioned some problems in this thread http://music-electronics-forum.com/t46007-3/#post493453 where an SSL-1 measures the same as what could be derived by measuring the inductance and the peak resonant frequency (about 100pF), but a Fender Fat 50 pickup had given a reading that was much too low (about 60pF instead of 120pF). I'm moving this over to this thread since this one is about capacitance and that one is about tapped single coils, which is have issues that go beyond capacitance.

I created extended impedence plots for both the SSL-1 and Fat 50, directly driving the pickup with a function generator, as opposed to using an external inducer coil.

Fat 50
[ATTACH=CONFIG]48571[/ATTACH]

SSL-1
[ATTACH=CONFIG]48572[/ATTACH]

This shows that very near the test frequency of 100kHz, the Fat 50 has some sort of secondary resonance at 98kHz. The SSL-1 has a similar secondary resonance, but it's 153kHz. It appears that the overlap of the DE-5000's test frequency and the secondary resonance prevents the Fat 50 from measuring correctly. Any idea what the source of that secondary inductance and resonance is? Could it be related to the lead wires?
 
Joe Gwinn 4/21/2018 11:38 AM
Quote Originally Posted by Antigua View Post
This shows that very near the test frequency of 100kHz, the Fat 50 has some sort of secondary resonance at 98kHz. The SSL-1 has a similar secondary resonance, but it's 153kHz. It appears that the overlap of the DE-5000's test frequency and the secondary resonance prevents the Fat 50 from measuring correctly. Any idea what the source of that secondary inductance and resonance is? Could it be related to the lead wires?
Without disturbing the lead dress, short the coil and use the DE-5000 to measure the inductance of its own leads. Given that number, how big must the capacitance be to explain the observed resonant frequency? This may be a clue.

Change the lead dress: First time, with leads twisted together. Second time, with leads as far apart as possible.

Does bringing a piece of soft ferrite, steel, copper, stainless steel close to the coil have any effect? And so on.
 
Helmholtz 4/21/2018 12:05 PM
What are all these needle artefacts? I don't see them in my impedance measurements. Also the high and low frequency slopes appear not to be correct. Did you measure as I proposed without the field coil?
The zigzag anomality is the result of an additional series and parallel resonance with higher resonant frequencies. I could show in simulations that such behaviour can be the result of partially shorted windings. Another explanation could be a very sloppy wind, where the winding is not carefully layered but outer turns are used to fill lower spaces thereby causing an uneven distribution of the distributed capacitance. I have not found a way yet to prove this idea wrong or right, as I am not winding.
 
Antigua 4/21/2018 12:19 PM
Quote Originally Posted by Helmholtz View Post
What are all these needle artefacts? I don't see them in my impedance measurements. Also the high and low frequency slopes appear not to be correct. Did you measure as I proposed without the field coil?
The zigzag anomality is the result of an additional series and parallel resonance with higher resonance frequencies. I could show in simulations that such behaviour can be the result of partially shorted windings. Another explanation could be a very sloppy wind, where the winding is not carefully layered but outer turns are used to fill lower spaces thereby causing an uneven distribution of the distributed capacitance. I have not found a way yet to prove this idea wrong or right, as I am not winding.
I don't know what causes the spikes, I assume some sort of extraneous noise, but patters such as resonances are evident regardless of the noise, so it's not preventing me from conducting tests.

Those are both interesting possibilities: 1) a particularly uneven layer distribution causing a non uniform distributed capacitance, or 2) an internal short that would essentially create a small shorted coil within the larger coil. I'm not so sure about #1 because the SSL-1 has this second peak too, and they're known to be machine wound.

#2 seems like an attractive explanation, a short creating a smaller shorted coil within the coil, but that raises more questions, such as how does the short occur, and why would there only happen to be one of them per each tested coil?

I'll test some more single coils and get more data points on the second high freq. resonances.
 
Helmholtz 4/21/2018 1:45 PM
Please do not use the field coil. It seems to distort the impedance frequency response. The field coil coupling only makes sense for plotting the transfer response, but tends to introduce EMI effects and distorts especially high frequency response.
 
Antigua 4/21/2018 1:46 PM
Quote Originally Posted by Helmholtz View Post
Please do not use the field coil. It seems to distort the impedance frequency response. The field coil coupling only makes sense for plotting the transfer response, but tends to introduce EMI effects and distorts especially high frequency response.
These recent plots are direct, with a 1meg resistor, no inducer coil.

I'm gathering more data points now, I just want to keep it all to one post.
 
Helmholtz 4/21/2018 1:58 PM
Here are some of my measurements of strat PUs, some showing anomalies:[ATTACH=CONFIG]48574[/ATTACH]
Please note the straight +/- 6dB/octave slopes below and above resonance. If the anomaly lies at 100kHz, the LCR meter will give a wrong result, as it can only read impedance/admittance at the single 100kHz frequency.
I used a 100k series resistor, as I did not care for open loop Q and 100K is close to the loading with two 250K pots and 1M amplifier input impedance.
 
Antigua 4/21/2018 2:21 PM
Quote Originally Posted by Helmholtz View Post
Here are some of my measurements of strat PUs, some showing anomalities:[ATTACH=CONFIG]48574[/ATTACH]
Please note the straight +/- 6dB/octave slopes below and above resonance. If the anomality lies at 100kHz, the LCR meter will give a wrong result, as it can only read impedance/admittance at the single 100kHz frequency.
Thanks for providing the plots. It's too bad arbitrary test frequencies can't be specified with the affordable meters. I'd think you could test a handful of higher frequencies and achieve good accuracy that way.

I'm trying to determine if it could be the lead wires could be involved, but so far it doesn't seem likely.

I notice your plots show about one prominent anomaly per pickup, all somewhat close together, with similar Q factors, and all above 100kHz.
 
Helmholtz 4/21/2018 2:40 PM
BTW, the one with almost no anomaly (Fralin) sounds by far best to me. It is the only one that has the great brilliance of a good vintage strat PU. (I own a set of original '59 strat PUs for reference.)
 
Antigua 4/21/2018 2:44 PM
Quote Originally Posted by Helmholtz View Post
BTW, the one with almost no anomaly (Fralin) sounds by far best to me. It is the only one that has the great brilliance of a good vintage strat PU. (I own a set of original '59 strat PUs for reference.)
I'm testing a Lollar Blackface neck, so far I'm up to 200kHz with no anomaly. The Fralin and the Lollar and hand guided pickups, where as the Fat 50 and SSL-1 are high volume production pickups, there might be something to that.
 
Antigua 4/21/2018 3:20 PM
Quote Originally Posted by Joe Gwinn View Post
Without disturbing the lead dress, short the coil and use the DE-5000 to measure the inductance of its own leads. Given that number, how big must the capacitance be to explain the observed resonant frequency? This may be a clue.
The anomalies are observed strictly with the Velleman bode plotter, and seem to vary from pickup to pickup, so I think the rig itself is mostly ruled out.

Quote Originally Posted by Joe Gwinn View Post
Change the lead dress: First time, with leads twisted together. Second time, with leads as far apart as possible.

Does bringing a piece of soft ferrite, steel, copper, stainless steel close to the coil have any effect? And so on.
That's a good idea about twisting the lead wires. I gave that a try and it didn't change the frequency at which the anomalous peak occurred.

I tried putting 470pF across the pickup, the resonant peak dropped form ~7kHz down to 3.9kHz, but the anomolous peak only appeared to drop very slightly, from 98kHz down to around 94kHz, with a Q that was lower by about half. Those higher frequency figures are sort of rough estimates, as you can see from the rather low resolution of the plot images. Cap values higher than 470pF seem to drown out the anomalous resonance.

As for placing permeable materials around the pickup, with steel Tele baseplates I've only ever been able to increase the inductance by about 150mH, so I don't think that wouldn't alter the circuit much. If theres value in seeing how inductance changes the anomalous peak, the better trick would probably by to find an steel pole Strat pickup with the anomaly, and then remove the pole pieces. That's easier said than done though, so I'd only do that if there were a hypothesis in place first.
 
Mike Sulzer 4/21/2018 4:32 PM
Quote Originally Posted by Helmholtz View Post
BTW, the one with almost no anomaly (Fralin) sounds by far best to me. It is the only one that has the great brilliance of a good vintage strat PU. (I own a set of original '59 strat PUs for reference.)
The pickup is loaded with about 500 pf and played through a system with about 5KHz bandwidth. It is hard to believe that anomalies at about 100 KHz have any effect on the sound.

It is also hard to believe that old Fender pickups have any particular special qualities. The brilliance of Fender pickups as compared to for example, humbuckers, is the result using Alnico cores that have lower conductance than steel. I suppose you could argue that the Alnico produced then has different eddy current losses than that produced now, but I have my doubts that this is a significant effect.
 
Helmholtz 4/22/2018 6:14 AM
The pickup is loaded with about 500 pf and played through a system with about 5KHz bandwidth. It is hard to believe that anomalies at about 100 KHz have any effect on the sound.
Right, the sound difference described can hardly be explained by the shown frequency responses. But I thought I should mention it nevertheless.

If it wasn't for such hard to understand sound effects, I would not waste my time (and some money) with countless parameter and response measurements, material analyses, simulations, literature researches, listening tests and so on. The standard PU filter response and parameter measurements can get quite boring and frustrating over time, as they often don't vary much and can explain only part of the PU's sound. But I am a physicist and want to find out.
 
Antigua 4/22/2018 12:40 PM
I measured four other machine wound Fender pickups, three from a Mexican Strat with a DCR or 6.9k, and one from Japan with a DCR of 5.4k, the three Mexican pickups showed single secondary peaks of 114kHz, 125kHZ and 150kHz, and the Japan made single coil have a second peak at 150kHz.

I made a LTSpice model that seems to result in a similar plot, it has another resonant coil in series with the lumped capacitance. Maybe this models what is happening, maybe not:

[ATTACH=CONFIG]48582[/ATTACH]

Another possible clue is that this doesn't appear to be something that effects inductors in general, as far as I can tell from Google searches, so aspects that make a pickup like most ordinary inductors can probably be ruled out, leaving qualities that are more specific to Stratocaster pickups.

The second peak frequency is definitely specific to the pickup, if I test the same pickups a second time, they show the same second peak. I've tried fiddling with the lead wires, but moving them around, twisting them, etc. doesn't make any difference.

All four tested pickups have steel pole pieces, and the three Mexican pickups are more or less identical in shape and size and probably turn count. If the pole pieces were a factor, I'd think the frequencies would all be a lot closer.

It's interesting that the second resonance varies from 100kHz to 150kHz from pickup to pickup. These machine wound pickups have coils that are rather flat, so it's likely the traversal is fairly uniform, and in fact the "hand guided" pickups don't seem as likely to have the anomalous peak. So oddly enough, the pickups with a uniform manufacturing method show a randomness in this second peak, where as the hand guided pickups, with randomly laid wire, are possibly more uniform, insofar as they don't have this peak, though it could be the case that's it's just at a much higher frequency and/or the resonance is suppressed to the point of being unobservable.




Quote Originally Posted by Helmholtz View Post
Right, the sound difference described can hardly be explained by the shown frequency responses. But I thought I should mention it nevertheless.
You have to account for cognitive bias in order to draw a conclusion with respect to hearing.
 
Mike Sulzer 4/22/2018 1:32 PM
The capacitance of a coil is a surprising subject. The capacitance of a single layer coil is a transmission line effect, not what you might think. A multilayer coil is complicated, and there is certainly some effect from electrostatic coupling between layers. But it might be with certain winding techniques you effectively get a multiple component circuit perhaps involving more than one transmission line (with capacitive reactance).

Quote Originally Posted by Antigua View Post
I measured four other machine wound Fender pickups, three from a Mexican Strat with a DCR or 6.9k, and one from Japan with a DCR of 5.4k, the three Mexican pickups showed single secondary peaks of 114kHz, 125kHZ and 150kHz, and the Japan made single coil have a second peak at 150kHz.

I made a LTSpice model that seems to result in a similar plot, it has another resonant coil in series with the lumped capacitance. Maybe this models what is happening, maybe not:

[ATTACH=CONFIG]48582[/ATTACH]

Another possible clue is that this doesn't appear to be something that effects inductors in general, as far as I can tell from Google searches, so aspects that make a pickup like most ordinary inductors can probably be ruled out, leaving qualities that are more specific to Stratocaster pickups.

The second peak frequency is definitely specific to the pickup, if I test the same pickups a second time, they show the same second peak. I've tried fiddling with the lead wires, but moving them around, twisting them, etc. doesn't make any difference.

All four tested pickups have steel pole pieces, and the three Mexican pickups are more or less identical in shape and size and probably turn count. If the pole pieces were a factor, I'd think the frequencies would all be a lot closer.

It's interesting that the second resonance varies from 100kHz to 150kHz from pickup to pickup. These machine wound pickups have coils that are rather flat, so it's likely the traversal is fairly uniform, and in fact the "hand guided" pickups don't seem as likely to have the anomalous peak. So oddly enough, the pickups with a uniform manufacturing method show a randomness in this second peak, where as the hand guided pickups, with randomly laid wire, are possibly more uniform, insofar as they don't have this peak, though it could be the case that's it's just at a much higher frequency and/or the resonance is suppressed to the point of being unobservable.






You have to account for cognitive bias in order to draw a conclusion with respect to hearing.
 
Mike Sulzer 4/22/2018 1:50 PM
Quote Originally Posted by Helmholtz View Post
Right, the sound difference described can hardly be explained by the shown frequency responses. But I thought I should mention it nevertheless.

If it wasn't for such hard to understand sound effects, I would not waste my time (and some money) with countless parameter and response measurements, material analyses, simulations, literature researches, listening tests and so on. The standard PU filter response and parameter measurements can get quite boring and frustrating over time, as they often don't vary much and can explain only part of the PU's sound. But I am a physicist and want to find out.
An excellent goal, IMO. My own bias is to look near the resonance for effects that can alter the harmonic content from what one might expect. For example, a pickup with steel cores must have a somewhat non-standard impedance shape around the resonance because of frequency varying eddy current losses (and reactance). I have not yet tried to compare the measured shape to what one expects with no eddy current losses, but it is a project for the future. My favorite way to measure pickup impedance is to put a resistor (say 2K) in series and drive this series combination with a signal with a broad frequency spectrum, sampling across the series combination, and across the resistor. Then with suitable processing, you can get the voltage across the pickup and the current through it. The ratio is the impedance with essentially no loading effects. You can measure hundreds of frequency points at once, giving a very good measurement of the frequency variation.
 
Helmholtz 4/22/2018 2:22 PM
The capacitance of a coil is a surprising subject. The capacitance of a single layer coil is a transmission line effect, not what you might think. A multilayer coil is complicated, and there is certainly some effect from electrostatic coupling between layers. But it might be with certain winding techniques you effectively get a multiple component circuit perhaps involving more than one transmission line (with capacitive reactance).
Thanks, this is exactly the direction/background of my second possible explanation. Now we need a volunteering winder who is willing to produce some extreme samples for measuring and verification purposes. In the meantime I will try to master transformers with varying degrees of coupling in LTSpice (being a beginner still) to simulate differently coupled parts of the PU coil.
 
Antigua 4/22/2018 2:30 PM
Quote Originally Posted by Helmholtz View Post
BTW, the one with almost no anomaly (Fralin) sounds by far best to me. It is the only one that has the great brilliance of a good vintage strat PU. (I own a set of original '59 strat PUs for reference.)
You have the Velleman setup as I do, it would be cool if you could share plots of the 59's you have on hand. I'm also curious to know what the inductance values are.

Someone mentioned the AlNiCo grades varrying in the 50's, I've received some pickups from China that supposedly had AlNiCo 5 pole pieces, but showed a Br value that was about 10% below expected, and yielded a higher Q factor than expected, due to higher resistivity. I don't know what made that Chinese AlNiCo different, be it a composition difference, or a production difference (though I'd think composition on account of the resistivity issue). Maybe the AlNiCo in your 59's has a similar sort of difference. I'd also be curious to know what the flux reading is at the tops of the pole pieces.

Regarding the notion that they sound subjectively better, there is an obvious bias to favor all things vintage, which might owe to nostalgia, or simple scarcity or it's perceived worth to others, or the famous guitarists who promote vintage gear, such Eric Clapton and Keith Richards. This bias impacts nearly every aspect of electric guitar, including caps, pots, hookup wire, body wood, the finish coat, steel hardware, even guitar amps and effects pedals. It seems that guitar manufactures just can't make a great guitar anymore, can they? It's such a pervasive and enduring bias that will power alone can't assure impartiality.
 
Helmholtz 4/22/2018 3:49 PM
You have to account for cognitive bias in order to draw a conclusion with respect to hearing.
I am well aware of possible cognitive bias. My ears and the attached computer are certainly biased by my personal sound preferences. But I have absolutely no desire to fool myself. More than often my listening tests did not confirm my (biased) expectations. Still, what counts in the end is sound and not measurements. I am not trying to persuade anybody to trust my assessments, instead I encourage everyone to do his own listening comparisons/evaluations and not rely on data only. But of course having and understanding measurement data helps to narrow down the variety of candidates.
It would be naive, though, to assume the a PU's total transfer reponse could be completely descibed by a simple linear passive low pass filter composed of lumped elements. A PU is essentialy a non-linear transducer. Sorry for going astray.
 
Helmholtz 4/22/2018 4:01 PM
Someone mentioned the AlNiCo grades varrying in the 50's, I've received some pickups from China that supposedly had AlNiCo 5 pole pieces, but showed a Br value that was about 10% below expected, and yielded a higher Q factor than expected, due to higher resistivity. I don't know what made that Chinese AlNiCo different, be it a composition difference, or a production difference (though I'd think composition on account of the resistivity issue). Maybe the AlNiCo in your 59's has a similar sort of difference. I'd also be curious to know what the flux reading is at the tops of the pole pieces.
Give me some time to look up my notes. Not going to dissect my old strat at the time, so no frequency plots. But I took resistance, inductance, capacitance and B values. Will send PM eventually.
 
Antigua 4/22/2018 4:02 PM
Quote Originally Posted by Helmholtz View Post
It would be naive, though, to assume the a PU's total transfer reponse could be completely descibed by a simple linear passive low pass filter composed of lumped elements.
It's not naive, it's a real possibility. I'd even argue it's the more probable possibility. You're well versed in scientific principles regarding physics, but you're not giving much regard to the necessity of double blind testing.
 
Helmholtz 4/22/2018 5:17 PM
You're well versed in scientific principles regarding physics, but you're not giving much regard to the necessity of double blind testing.
How could you know? I consider this a disrespectful allegation and a personal offense. Of course in the end my pickups have to appeal to me.

I hope you do extensive (double blind) listening tests with trained ears and experienced players- and not only rely on measurements.
 
Antigua 4/22/2018 5:29 PM
Here are extended impedance plots of four pickups I had made at home, which were intentionally wound with low uniformity and varying degrees of tension, including one which is a loose mess. Each is 8,000 turns with a DC resistance right around 6k ohms.

[ATTACH=CONFIG]48587[/ATTACH]


[ATTACH=CONFIG]48588[/ATTACH]

[ATTACH=CONFIG]48589[/ATTACH]

[ATTACH=CONFIG]48590[/ATTACH]

(the big messy pickup)
[ATTACH=CONFIG]48591[/ATTACH]

The earlier theory that a less consistent pickup might exhibit fewer or no anomolous high secondary resonant peaks appears to be squashed, as some of these pickups show several such peaks. The large, bushy Strat pickup which could never fit a cover over the top, has two very prominent secondary resonances, one at 86kHz and another at 153kHz. Two others show a single dominant secondary resonance, with additional peaks that look like ripples. Only one of the four, #3, appears to have no secondary resonance, although there is possibly a subtle knee at 64kHz.

Based on this information, I wondering if there is some sort of event that occurs randomly in the production of a pickup, one that causes multiple, smaller resonant circuits to appear within the coil.

Maybe it has to do with a segment of wire being laid, which comes physically very close to a segment of the coil that is many turns removed. For example, sometimes at the ends of the coil, you get crevices between the coil and the flat work, because the wire doesn't come right up to the edge of the flat work as it traverses back and forth. Then, at some random point, a segment of wire will manage to slip down into the crevice, which would cause that segment of wire that slipped down to be side be side with a portion of the coil that is maybe several hundred turns removed from itself.

While it's true that these higher peaks don't manifest in the audible frequency ranges, I think it's important to figure out what causes them, since it is a feature of the pickup all the same. It might even serve as a clue to indicate how a coil was made, without having to disassemble the coil to look at it directly.
 
Antigua 4/22/2018 5:32 PM
Quote Originally Posted by Helmholtz View Post
How could you know? I consider this a disrespectful allegation and a personal offense. Of course in the end my pickups have to appeal to me.

I hope you do extensive (double blind) listening tests with trained ears and experienced players- and not only rely on measurements.
In the past I had a similar view, that there is something special about pickups, and that my goal is to find it, but some other people pointed out to me that I haven't established with certainty that anything special really exists to find. It might be how I feel about the pickups, some good psychological connotation, that makes me believe there is something special. That's how it remains to this day, I'm not sure that I wasn't imagining the thing I set out to look for.

The easiest way to marry specs with subjective experience would be to, not only have a blind fold and a friend help you conduct an test free of extrinsic factors, but to also have as many specifications as possible about those pickups. Unfortunately, only the DC resistance is readily available, so I'm working towards providing other guitarists more extensive specs for popular pickups, including capacitance measures.
 
Helmholtz 4/23/2018 6:15 AM
Maybe it has to do with a segment of wire being laid, which comes physically very close to a segment of the coil that is many turns removed. For example, sometimes at the ends of the coil, you get crevices between the coil and the flat work, because the wire doesn't come right up to the edge of the flat work as it traverses back and forth. Then, at some random point, a segment of wire will manage to slip down into the crevice, which would cause that segment of wire that slipped down to be side be side with a portion of the coil that is maybe several hundred turns removed from itself.
This is what I ment. I suppose that the effect requires a contiguous portion of the winding with a discontinuity in magnetic coupling and/or distributed capacitance. Magnetic coupling decreases with distance from the center/core. Distributed capacitance changes with the arrangement of the turns.
I tend to think that the effect would show strongly (while at lower frquency) in a bifilar wound coil with the two windings wired in series. But this is just speculation at this point.
 
Helmholtz 4/23/2018 6:29 AM
In the past I had a similar view, that there is something special about pickups, and that my goal is to find it, but some other people pointed out to me that I haven't established with certainty that anything special really exists to find. It might be how I feel about the pickups, some good psychological connotation, that makes me believe there is something special. That's how it remains to this day, I'm not sure that I wasn't imagining the thing I set out to look for.
I don't quite understand what you are talking about. Probably due to my limited command of English. Not sure if I really need to know. Anyway, I prefer technical arguments/discussion.

But it definitely does not sound like an excuse to me.
 
okabass 4/23/2018 7:15 AM
Great info on the thread. Not that witchcraft or caveman level physics, which is so common when you read web pickup forums.
Thanks.
 
Helmholtz 4/23/2018 7:33 AM
My favorite way to measure pickup impedance is to put a resistor (say 2K) in series and drive this series combination with a signal with a broad frequency spectrum, sampling across the series combination, and across the resistor. Then with suitable processing, you can get the voltage across the pickup and the current through it. The ratio is the impedance with essentially no loading effects. You can measure hundreds of frequency points at once, giving a very good measurement of the frequency variation.
This sounds very interesting. But my expertise in signal and system theory is rather limited, so I have no feeling for the power and benefit of such method. What kind of signal do you use? A kind of noise?
But, being a pragmatic, my main question is: How do your results differ from those of standard single frequency point measurements with a current source? Can you show some?
 
Antigua 4/23/2018 8:34 AM
Quote Originally Posted by Mike Sulzer View Post
My favorite way to measure pickup impedance is to put a resistor (say 2K) in series and drive this series combination with a signal with a broad frequency spectrum, sampling across the series combination, and across the resistor. Then with suitable processing, you can get the voltage across the pickup and the current through it. The ratio is the impedance with essentially no loading effects. You can measure hundreds of frequency points at once, giving a very good measurement of the frequency variation.
Quote Originally Posted by Helmholtz View Post
This sounds very interesting. But my expertise in signal and system theory is rather limited, so I have no feeling for the power and benefit of such method. What kind of signal do you use? A kind of noise?
But, being a pragmatic, my main question is: How do your results differ from those of standard single frequency point measurements with a current source? Can you show some?
This sounds like what we've been doing in this thread, using the VElleman bode plot and function generator, similar to what is described here http://www.syscompdesign.com/assets/...ar-pickups.pdf but using a 1meg resistance instead of 56k.

Another option is to use the Velleman's frequency sweep with persistance, which yields the peak like this:

[ATTACH=CONFIG]48592[/ATTACH]

I though there was a way to feed it white noise with the function generator, which would reveal a peak in the FFT view even more quickly, but I'm not seeing the option.



Quote Originally Posted by Helmholtz View Post
This is what I ment. I suppose that the effect requires a contiguous portion of the winding with a discontinuity in magnetic coupling and/or distributed capacitance. Magnetic coupling decreases with distance from the center/core. Distributed capacitance changes with the arrangement of the turns.
I tend to think that the effect would show strongly (while at lower frquency) in a bifilar wound coil with the two windings wired in series. But this is just speculation at this point.
I remember you mentioning this possibility. If this is what is going on, it should be possible to model with LTSpice somehow. I'll work on that more later.
 
bbsailor 4/23/2018 10:22 AM
Quote Originally Posted by Antigua View Post
In the past I had a similar view, that there is something special about pickups, and that my goal is to find it, but some other people pointed out to me that I haven't established with certainty that anything special really exists to find. It might be how I feel about the pickups, some good psychological connotation, that makes me believe there is something special. That's how it remains to this day, I'm not sure that I wasn't imagining the thing I set out to look for.

The easiest way to marry specs with subjective experience would be to, not only have a blind fold and a friend help you conduct an test free of extrinsic factors, but to also have as many specifications as possible about those pickups. Unfortunately, only the DC resistance is readily available, so I'm working towards providing other guitarists more extensive specs for popular pickups, including capacitance measures.
When measuring pickups put a 200K Ohm resistor for single coils or 350K Ohm resistor in parallel for humbucker pickups with about 350pf capacitor (resistor and cap in parallel) across the pickup output to simulate what the pickup looks like loaded by the typical volume pot and the typical coax cable capacitance. These value represent the full load on the pickup when either a 250K Ohm pot or 500K Ohm pot is in parallel to the typical amp input impedance of 1 Meg Ohm. Doing the same measurements with this added load will probably change the upper frequency peaks that you are seeing. The main audio effect will be a slight reduction of the peak resonance near 3KHz to 5KHz due to the fact that the coil impedance is highest at pickup resonance and the pot loading will reduce the peak somewhat. The capacitance loading of the coax cable (350pf simulated load) will also lower the resonant frequency.

Bottom line: Try to do all tests in the same environment that the pickup sees when mounted in the guitar and used in typical situations about 10 ft from the amplifier. This will allow your ears to be in tune better with what you see on the graphs.

Joseph J. Rogowski
 
Helmholtz 4/23/2018 10:51 AM
This sounds like what we've been doing in this thread, using the VElleman bode plot and function generator, similar to what is described here http://www.syscompdesign.com/assets/...ar-pickups.pdf but using a 1meg resistance instead of 56k.
I don't quite agree with some details in this papers. As a consequence the f-responses in figure 5 are wrong. Will elaborate if someone cares.
 
Antigua 4/23/2018 10:59 AM
Quote Originally Posted by Helmholtz View Post
I don't quite agree with some details in this papers. As a consequence the f-responses in figure 5 are wrong. Will elaborate if someone cares.
This PDF is a prominent search result when searching for information on how to measure the response of guitar pickups. Any critique you have would be valuable to anyone who finds both that PDF and this thread.
 
Mike Sulzer 4/23/2018 12:00 PM
Quote Originally Posted by Helmholtz View Post
This sounds very interesting. But my expertise in signal and system theory is rather limited, so I have no feeling for the power and benefit of such method. What kind of signal do you use? A kind of noise?
But, being a pragmatic, my main question is: How do your results differ from those of standard single frequency point measurements with a current source? Can you show some?
The current version uses Golay complementary sequences, that is, a pair of codes that together weight all frequencies equally, or have no side lobes, as a radar person might say. This gives faster more accurate measurements than the noise like signals I have used before. (Since the measurement is a ratio, taken in the frequency domain, equal weighting in the code is not strictly necessary for accuracy, but it does give more uniform signal to noise ratio.)

There is no need for a very high input impedance amplifier to drive the sampler, nor for a good current source. It can be done with a two channel recording interface, cheap these days, something many people already have.

I use an audio package in Python, incorporated into custom software for the signal processing.

Here is an example taken with an earlier version, but it is interesting because it shows two measurements with the same coil, alnico cores and steel.

[ATTACH=CONFIG]48595[/ATTACH]
 
Antigua 4/23/2018 1:07 PM
Quote Originally Posted by bbsailor View Post
When measuring pickups put a 200K Ohm resistor for single coils or 350K Ohm resistor in parallel for humbucker pickups with about 350pf capacitor (resistor and cap in parallel) across the pickup output to simulate what the pickup looks like loaded by the typical volume pot and the typical coax cable capacitance. These value represent the full load on the pickup when either a 250K Ohm pot or 500K Ohm pot is in parallel to the typical amp input impedance of 1 Meg Ohm. Doing the same measurements with this added load will probably change the upper frequency peaks that you are seeing. The main audio effect will be a slight reduction of the peak resonance near 3KHz to 5KHz due to the fact that the coil impedance is highest at pickup resonance and the pot loading will reduce the peak somewhat. The capacitance loading of the coax cable (350pf simulated load) will also lower the resonant frequency.

Bottom line: Try to do all tests in the same environment that the pickup sees when mounted in the guitar and used in typical situations about 10 ft from the amplifier. This will allow your ears to be in tune better with what you see on the graphs.

Joseph J. Rogowski
I use 200k and 470pF as fixed "loaded" values. I use the same values for humbuckers and single coils for the sake of consistency. Myself and someone else had settled on these values, and then I found out later Helmuth Lemme used the exact same values here http://www.planetz.com/wp-content/up..._Technique.pdf , so it seems to be a reasonable in-between standard.
 
Helmholtz 4/23/2018 2:43 PM
I use 200k and 470pF as fixed "loaded" values. I use the same values for humbuckers and single coils for the sake of consistency. Myself and someone else had settled on these values, and then I found out later Helmuth Lemme used the exact same values here http://www.planetz.com/wp-content/up..._Technique.pdf , so it seems to be a reasonable in-between standard.
After extensive testing I have settled with guitar cables having 1000pF to 1200pF, both for my Strats and my Les Pauls. The wiring harness in a typical vintage LP adds 300 to 500pF. A tube amplifier input adds another 150pF typically. I use vintage style PUs.
A realistic load resistance for strats is 100k to 200K(bridge PU) and 200k for LPs. This includes typical amplifier/pedals input resistance.

I don't think any pro player would be comfortable with a stage cable of 10ft or less.
 
Antigua 4/23/2018 2:53 PM
Quote Originally Posted by Helmholtz View Post
As a guitar player I have settled (after extensive testing) with guitar cables having 1000pF to 1200pF, for my Strats as well as my Les Pauls. The wiring harness in a typical LP adds 400+pF.
400pF for the pots and hookups? I measured 70pF per foot for the braided wire used with PAF humbuckers, but 400pF overall seems high.

Fender guitars usually feature non shielded hookups and wiring, so the capacitance there is really low, well under 50pF I'd recon.

Quote Originally Posted by Helmholtz View Post
A tube amplifier input adds another 150pF typically. I use vintage style PUs.
A realistic load resistance for strats is 100K..200K and 200k for LPs. This includes typical amplifier/pedals input resistance.
Since peak freq. will vary from rig to rig, I think it's best to settle on a "center", and then let people shift the frequency up or down mentally, depending on their own rig. So if you know you like 1000pF cables, you know that you will have a peak that is somewhat lower than the standard loaded data points. If the intrinsic L and C are known, then any loaded peak freq. can be solved for, and because inductance factors more prominently, L is is the more valuable metric to have on hand.

Quote Originally Posted by Helmholtz View Post
I don't think any pro player would be comfortable with a stage cable of 10ft or less.
Pros often use wireless units too, which sometimes have selectable capacitance, or a fixed values. I analyzed a Line 6 G10 and found that it imparted about 120pF capacitance http://www.strat-talk.com/threads/th...ay-g10.467237/
 
Helmholtz 4/23/2018 4:08 PM
400pF for the pots and hookups? I measured 70pF per foot for the braided wire used with PAF humbuckers, but 400pF overall seems high.
The neck PU signal in a LP runs through around 3ft of coax wire between PU and jack. My Stew Mac and Allparts coax wires measure over 120pF/foot. But this value strongly increases with ambient humidity in summer months. The values I specified are quite realistic.
The self-capacitance of humbuckers with cloth-insulated coax cable is often dominated by the cable attached .
The capacitance of the guitar cable is the strongest influencer of PU frequency response besides inductivity.
 
Helmholtz 4/23/2018 4:32 PM
The current version uses Golay complementary sequences, that is, a pair of codes that together weight all frequencies equally, or have no side lobes, as a radar person might say. This gives faster more accurate measurements than the noise like signals I have used before. (Since the measurement is a ratio, taken in the frequency domain, equal weighting in the code is not strictly necessary for accuracy, but it does give more uniform signal to noise ratio.)

There is no need for a very high input impedance amplifier to drive the sampler, nor for a good current source. It can be done with a two channel recording interface, cheap these days, something many people already have.

I use an audio package in Python, incorporated into custom software for the signal processing.

Here is an example taken with an earlier version, but it is interesting because it shows two measurements with the same coil, alnico cores and steel.

alandsteelcores.png
Does this alternative method reveal any additional information compared to standard methods? Is there any direct comparison? Of course the effect of a finite source impedance can always be compensated via calculation.
 
Mike Sulzer 4/23/2018 6:04 PM
Quote Originally Posted by Helmholtz View Post
Does this alternative method reveal any additional information compared to standard methods? Is there any direct comparison? Of course the effect of a finite source impedance can always be compensated via calculation.
Well, I suppose there could be no more standard way of measuring impedance than by forming a ratio, as a function of frequency, of the voltage across and the current through the device, using superior inexpensive technology. But let's leave this aside for now. But yes, there is more to do. Consider the capacitance: it is of little importance itself because it is so small compared to other capacitances in the guitar circuit. But it does get in the way because it causes a resonance that makes it hard to see the effects of the metal on the impedance. So the capacitance is found, just for the purpose of removing it from the impedance, by a non-linear least squares fit to a set of samples taken well above the resonance where the capacitance is dominant. The low frequency inductance and the resistance are in the model of the impedance, and the parameters fitted to are the capacitance, and two parameters associated with the coupling to the metal, k, the coupling coefficient, and Rse, the effective resistance of the "secondary" reflected back to the primary. Once the C is found it is taken out of the impedance, and you can see in the plots that the real part increases above the pickup resistance as frequency increases, and the imaginary part decreases below the reactance of the coil inductance.
 
Antigua 4/23/2018 6:32 PM
Quote Originally Posted by Helmholtz View Post
My Stew Mac and Allparts coax wires measure over 120pF/foot. But this value strongly increases with ambient humidity in summer months. The values I specified are quite realistic.
I first read about the cloth shield hookup varying in capacitance by humidity in Manfred Zollner's book Physik der Elektrogitarre, and it's a rather significant claim, because people make hay over the littlest things, but never talk about how their pickups sound different between dry and wet climates. I tried a little experiment where I left shielded cloth hookup wire outside, measured the capacitance, the put it in the oven to dry it out, then measured again, but the capacitance didn't change much. I might give it another try, though, ensuring that it's very damp and then very dry.
 
Antigua 4/24/2018 12:43 AM
I think this model jives somewhat closely with the notion that having the winding fall into the edges puts a capacitance across two distant sections of the coil:

[ATTACH=CONFIG]48611[/ATTACH]

In this model, the series inductance and resistance are broken into three sets, with the middle "RL" set having a capacitance in parallel with it, as would happen if, say, the 3,000th winding somehow managed to come side by side with the 2,500th winding, by falling into a gap at the edge of the coil, or something or that sort. The result of the model is a side by side impedance dip and spike, which isn't 100% like what is seen in the practical plots, which appears to have only an impedance spike, but it has the similar characteristic of a second high frequency resonance. Maybe with a little more refinement the model can duplicate the practical plot completely.
 
freefrog 4/24/2018 1:32 AM
Quote Originally Posted by Helmholtz View Post
The neck PU signal in a LP runs through around 3ft of coax wire between PU and jack. My Stew Mac and Allparts coax wires measure over 120pF/foot. But this value strongly increases with ambient humidity in summer months. The values I specified are quite realistic.
Even humidity aside, my experience agrees with your statements.

I've measured 268pF per meter in normal conditions on the braided shielded that I use.

And there's a serious lenght of cable in a LP... This site recommends to have 5ft at disposal for such a wiring: Six String Supplies ? How to Wire a Les Paul (50s Wiring)

Let's add to it the cables coming from the pickups themselves and the stray capacitance of other components: it forms a highly capacitive inner wiring. Its tonal effect is especially obvious IME when both pickups are selected.

I don't think any pro player would be comfortable with a stage cable of 10ft or less.
BTW, Helmoltz, you have an outstanding brand of cable in Germany: Sommer... Their LLX coax. wire measures 52pF per meter (published value that I've checked with a lab meter). And IME, it's a sturdy cable, whose only relative flaw is its limited flexibility.
Of course, this observation hasn't much interest for you since you use high capacitance cables. But it would be a pity not to share with all potential readers a possibly useful info, while Sommer cables are so unequally known by musicians around the World... :-)
 
Helmholtz 4/24/2018 7:31 AM
BTW, Helmoltz, you have an outstanding brand of cable in Germany: Sommer... Their LLX coax. wire measures 52pF per meter (published value that I've checked with a lab meter). And IME, it's a sturdy cable, whose only relative flaw is its limited flexibility.
Yes, this Sommer cable is fine, as is e.g. Klotz GY 107 ("La Grange"). I use both types with different lenghts. But generally I have no need for extra low specific capacitance, as this forces me to buy and use cables measuring 15m or more in length.
 
Helmholtz 4/24/2018 8:11 AM
Well, I suppose there could be no more standard way of measuring impedance than by forming a ratio, as a function of frequency, of the voltage across and the current through the device, using superior inexpensive technology. But let's leave this aside for now. But yes, there is more to do. Consider the capacitance: it is of little importance itself because it is so small compared to other capacitances in the guitar circuit. But it does get in the way because it causes a resonance that makes it hard to see the effects of the metal on the impedance. So the capacitance is found, just for the purpose of removing it from the impedance, by a non-linear least squares fit to a set of samples taken well above the resonance where the capacitance is dominant. The low frequency inductance and the resistance are in the model of the impedance, and the parameters fitted to are the capacitance, and two parameters associated with the coupling to the metal, k, the coupling coefficient, and Rse, the effective resistance of the "secondary" reflected back to the primary. Once the C is found it is taken out of the impedance, and you can see in the plots that the real part increases above the pickup resistance as frequency increases, and the imaginary part decreases below the reactance of the coil inductance.

Thanks. Seems like a powerful and useful tool. Unfortunately I don't have the time to dig any deeper and make myself familiar with your method at present.
 
Helmholtz 4/24/2018 4:07 PM
Quote Originally Posted by Antigua View Post
This PDF is a prominent search result when searching for information on how to measure the response of guitar pickups. Any critique you have would be valuable to anyone who finds both that PDF and this thread.
I edited my first post and changed it completely as I noticed, that it was not the author's intend to plot the PU's impedance accurately. For the Lissajous method a series resistor of 56k seems fine.

Here are my comments on this http://www.syscompdesign.com/assets/...ar-pickups.pdf paper.

First of all I acknowledge that it gives some useful information regarding measurement methods of PUs' parameters. The description of a magnetic PU as a Variable Reluctance Sensor is perfect. But:

Measuring PU inductance via Lissajous figure
This is an excellent and accurate method to determine the inductance of a parallel resonant circuit like a PU. But it requires the capacitance to be known exactly.

There are two problems associated with measuring the inductance with an LCR meter at a fixed frequency:

1) The meter can only measure apparent inductance. The apparent L of a PU (parallel resonant circuit) increases steadily with increasing frequency below and up to resonance, caused by the effect of capacitance. Apparent L has no practical meaning for PUs and is only a theoretical way to descibe the systematic measuring error of LCR meters. As this error increases with frequency, the value at the lowest measuring frequency is the most meaningful.

2) Eddy current effects in conductive parts (especially in ferromagnetic cores ->magnetic skin effect) reduce the effective L with increasing frequency. Inductors with conductive, ferromagnetic cores do not have a single true inductance. Instead L is a function of frequency. This means that the L value at 100Hz is not per se better or truer than the value at a higher frequency.

What we actually want to know is the (effective) L at or close to the resonant frequency in real life operation. This is where the Lissajous method comes in. Done carefully, it can deliver the correct effective L at the chosen resonant frequency of interest.

As mentioned before, for accurate results the total capacitance Ctot= Cpu+Cadd needs be known exactly. If Ctot is too low by 10%, your calculated L will be too large by 10%.
Cadd can be easily measured with an LCR but also Cpu should be determined beforehand at least approximately.
The method indicated in the article, namely "overpowering" an unknown Cpu by a huge Cadd of several nFs, will give the effective L at a much too low frequency. The result will only be useful for PUs where L does not depend on frequency. But in these cases you may as well use your LCR meter at 100Hz.


And here is the more important part of my comments, dealing with measuring the PU's transfer response:

Measuring PU transfer response requires access to an input port. Inserting a signal voltage source in series with the inductor part as typically done in simulations is not possible in real life. Instead, the well accepted method is to use the PU coil as secondary in a current transformer arrangement. The idea is to inject a current into the PU coil (inductance) via a coupled external coil driven by constant current and measure the resulting voltage across the PU terminals. Mind that driving the external coil directly by a (low impedance) voltage source would load down the PU and change its frequency response.
The induced constant current in the PU coil produces a voltage across its inductance, rising proportionally with frequency and consequently the PU shows a typical bandpass behaviour.
The main requirement for the external primary circuit is that the drive current must stay constant for all frequencies to be measured. This means that not only the self-resonance of the field coil has to lie far above the highest frequency of interest but also that the impedance of the field coil stays negligible compared to the total series resistance (279 Ohms in the example). With the values given in the article the corner frequency for this requirement is around 1.2kHz. Above this frequency the drive current drops with 6dB/octave and distorts the measured frequency response as can be seen in the PU responses of figure 5. The cure is to increase the L/R ratio by a factor of 20 or more.
 
Antigua 4/25/2018 12:54 PM
Quote Originally Posted by Helmholtz View Post
1) The meter can only measure apparent inductance. The apparent L of a PU (parallel resonant circuit) increases steadily with increasing frequency below and up to resonance, caused by the effect of capacitance. Apparent L has no practical meaning for PUs and is only a theoretical way to descibe the systematic measuring error of LCR meters. As this error increases with frequency, the value at the lowest measuring frequency is the most meaningful.

2) Eddy current effects in conductive parts (especially in ferromagnetic cores ->magnetic skin effect) reduce the effective L with increasing frequency. Inductors with conductive, ferromagnetic cores do not have a single true inductance. Instead L is a function of frequency. This means that the L value at 100Hz is not per se better or truer than the value at a higher frequency.

What we actually want to know is the (effective) L at or close to the resonant frequency in real life operation. This is where the Lissajous method comes in. Done carefully, it can deliver the correct effective L at the chosen resonant frequency of interest.

As mentioned before, for accurate results the total capacitance Ctot= Cpu+Cadd needs be known exactly. If Ctot is too low by 10%, your calculated L will be too large by 10%.
Cadd can be easily measured with an LCR but also Cpu should be determined beforehand at least approximately.
The method indicated in the article, namely "overpowering" an unknown Cpu by a huge Cadd of several nFs, will give the effective L at a much too low frequency. The result will only be useful for PUs where L does not depend on frequency. But in these cases you may as well use your LCR meter at 100Hz.
Thanks for the write up. For some reason this forum truncates this URL http://www.syscompdesign.com/assets/...ar-pickups.pdf on your posts.

As for inductance varying with frequency, I've noticed that Fender pickups, with AlNiCo pole pieces and little to no other metal parts, show about the same inductance at 1kHz test freq as they do at 100 or 120 Hz. It's only pickups with steel cores that show incorrect readings. One reason I prefer taking down the loaded and unloaded resonant peaks of pickups is because 1) it's a value that relates more closely to audible performance, and 2) it overcomes errors that might arise from trying to solve for peak freq. from incorrect values for L and C.

Quote Originally Posted by Helmholtz View Post

Measuring PU transfer response requires access to an input port. Inserting a signal voltage source in series with the inductor part as typically done in simulations is not possible in real life. Instead, the well accepted method is to use the PU coil as secondary in a current transformer arrangement. The idea is to inject a current into the PU coil (inductance) via a coupled external coil driven by constant current and measure the resulting voltage across the PU terminals. Mind that driving the external coil directly by a (low impedance) voltage source would load down the PU and change its frequency response.
Can the field coil still load down the pickup even if the coupling factor is very small compared to a traditional transfomer?

Quote Originally Posted by Helmholtz View Post
The induced constant current in the PU coil produces a voltage across its inductance, rising proportionally with frequency and consequently the PU shows a typical bandpass behaviour.
The main requirement for the external primary circuit is that the drive current must stay constant for all frequencies to be measured. This means that not only the self-resonance of the field coil has to lie far above the highest frequency of interest but also that the impedance of the field coil stays negligible compared to the total series resistance (279 Ohms in the example). With the values given in the article the corner frequency for this requirement is around 1.2kHz. Above this frequency the drive current drops with 6dB/octave and distorts the measured frequency response as can be seen in the PU responses of figure 5. The cure is to increase the L/R ratio by a factor of 20 or more.
The PCSU200 shows "output impedance: 50ohm" https://www.velleman.eu/products/view/?id=407512 , so the field coil's impedance would need to be well below 50 ohms, otherwise series resistance must be added?
 
Antigua 4/25/2018 1:00 PM
Also a question, does anyone know these plots typically show a slope that is much lower than 6dB/oct below 200Hz?

[ATTACH=CONFIG]48632[/ATTACH]
 
Mike Sulzer 4/25/2018 1:25 PM
Quote Originally Posted by Antigua View Post
Also a question, does anyone know these plots typically show a slope that is much lower than 6dB/oct below 200Hz?

[ATTACH=CONFIG]48632[/ATTACH]

Without checking all the details of what you are doing, I would guess that it is the effect of the coil resistance.
 
Antigua 4/25/2018 1:32 PM
Quote Originally Posted by Mike Sulzer View Post
Without checking all the details of what you are doing, I would guess that it is the effect of the coil resistance.
This plot is with a 1meg resistor in series with the pickup, then comparing the voltage across the resistor and the pickup, but the same thing happens when using an external inducer coil in a transformer configuration.
 
Mike Sulzer 4/25/2018 1:35 PM
Quote Originally Posted by Helmholtz View Post
And here is the more important part of my comments, dealing with measuring the PU's transfer response:

Measuring PU transfer response requires access to an input port. Inserting a signal voltage source in series with the inductor part as typically done in simulations is not possible in real life. Instead, the well accepted method is to use the PU coil as secondary in a current transformer arrangement. The idea is to inject a current into the PU coil (inductance) via a coupled external coil driven by constant current and measure the resulting voltage across the PU terminals. Mind that driving the external coil directly by a (low impedance) voltage source would load down the PU and change its frequency response.
The induced constant current in the PU coil produces a voltage across its inductance, rising proportionally with frequency and consequently the PU shows a typical bandpass behaviour.
The main requirement for the external primary circuit is that the drive current must stay constant for all frequencies to be measured. This means that not only the self-resonance of the field coil has to lie far above the highest frequency of interest but also that the impedance of the field coil stays negligible compared to the total series resistance (279 Ohms in the example). With the values given in the article the corner frequency for this requirement is around 1.2kHz. Above this frequency the drive current drops with 6dB/octave and distorts the measured frequency response as can be seen in the PU responses of figure 5. The cure is to increase the L/R ratio by a factor of 20 or more.
I use coils with diameter equal to or smaller than a pole piece radius with 3 to 6 turns, driven from an audio amp through an 8 ohm resistor, with a current of about 1 amp. Coupling is very small. Driving a pickup with a pickup size coil, as some otherwise clever people do, seems like asking for trouble.

You can make a response model from the parameters derived from an impedance measurement that works well. I think the only reason for using a driving coil is to include the eddy current loss encountered in passing through an extra thick cover.
 
Mike Sulzer 4/25/2018 1:37 PM
Quote Originally Posted by Antigua View Post
This plot is with a 1meg resistor in series with the pickup, then comparing the voltage across the resistor and the pickup, but the same thing happens when using an external inducer coil in a transformer configuration.
In either case, the coil resistance places a lower limit on the magnitude of the coil impedance.
 
Helmholtz 4/25/2018 2:22 PM
Can the field coil still load down the pickup even if the coupling factor is very small compared to a traditional transfomer?
I have no values for the coupling factor. Generally a coupling factor below 100% introduces additional series inductance in the equivalent circuit. But why don't you just measure and compare? Call it loading down or not, in result the (high) frequency response will change.

The PCSU200 shows "output impedance: 50ohm" https://www.velleman.eu/products/view/?id=407512 , so the field coil's impedance would need to be well below 50 ohms, otherwise series resistance must be added?
You have to add the field coil's resistance to the series resistance for total circuit resistance Rtot. What matters is the Rtot/L ratio. It should be well above 150kOhm/H. In other words the corner frequency should lie well above the frequency range analysed and is given by f=Rtot/(2pi*L). This can be achieved by increasing series and/or coil resistance as well as by decreasing field coil inductance.

Also a question, does anyone know these plots typically show a slope that is much lower than 6dB/oct below 200Hz?

v2adfer.png
My impedance plots of PUs without anomalies show almost perfect -6dB/octave (i.e. capacitive) behaviour above ca. 100kHz. To stay ahead of noise floor I recommend max. generator voltage and automatic voltage scale.
Anomalies indicate that the PU's behaviour is not purely capactive (but disturbed by the interaction with a smaller separated part of the inductance) in the corresponding frequency range, thus no clear -6dB/octave slope.
 
Helmholtz 4/25/2018 2:34 PM
Quote Originally Posted by Mike Sulzer View Post
In either case, the coil resistance places a lower limit on the magnitude of the coil impedance.
But only below resonance. As the whole thing is shunted by the distributed capacitance, impedance tends to 0 for very high frequencies.
 
Mike Sulzer 4/25/2018 3:22 PM
Quote Originally Posted by Helmholtz View Post
But only below resonance. As the whole thing is shunted by the distributed capacitance, impedance tends to 0 for very high frequencies.
I believe Antigua's question was for below 200 Hz where the pickup coil resistance sets a lower limit on the magnitude of the pickup coil impedance.
 
Helmholtz 4/25/2018 3:46 PM
You are right, I am sorry. Did not read carefully. Impedance always starts horizontally with DCR from 0 Hz. But the bandpass transfer response is different and must have 0 signal at 0 Hz.
 
Mike Sulzer 4/25/2018 4:19 PM
Quote Originally Posted by Helmholtz View Post
And here is the more important part of my comments, dealing with measuring the PU's transfer response:

Measuring PU transfer response requires access to an input port. Inserting a signal voltage source in series with the inductor part as typically done in simulations is not possible in real life. Instead, the well accepted method is to use the PU coil as secondary in a current transformer arrangement. The idea is to inject a current into the PU coil (inductance) via a coupled external coil driven by constant current and measure the resulting voltage across the PU terminals. Mind that driving the external coil directly by a (low impedance) voltage source would load down the PU and change its frequency response.
The induced constant current in the PU coil produces a voltage across its inductance, rising proportionally with frequency and consequently the PU shows a typical bandpass behaviour.
The main requirement for the external primary circuit is that the drive current must stay constant for all frequencies to be measured. This means that not only the self-resonance of the field coil has to lie far above the highest frequency of interest but also that the impedance of the field coil stays negligible compared to the total series resistance (279 Ohms in the example). With the values given in the article the corner frequency for this requirement is around 1.2kHz. Above this frequency the drive current drops with 6dB/octave and distorts the measured frequency response as can be seen in the PU responses of figure 5. The cure is to increase the L/R ratio by a factor of 20 or more.
The current in the field or exciter coil creates an ac magnetic field that induces a voltage, not a current, in series with the pickup coil. This follows from Maxwell's equation or the law of magnetic induction. Current flows if there is a load on the coil, such as the coil capacitance or the loading caused by eddy currents in the cores, etc. (A so called current transformer is a tightly coupled transformer driven from a high impedance so that the current in the secondary is related to that in the primary by the turns ratio. The very loosely coupled situation here does not behave that way.)

You describe one way to make the current through the field coil independent of frequency: make the inductive reactance low compared to the dc resistance of the coil across the whole useful frequency range. A better way is to drive the coil with a current source, that is, a circuit with an output impedance much higher than the impedance of the coil at any useful frequency.
 
Antigua 4/26/2018 12:36 AM
Quote Originally Posted by Mike Sulzer View Post
The current in the field or exciter coil creates an ac magnetic field that induces a voltage, not a current, in series with the pickup coil. This follows from Maxwell's equation or the law of magnetic induction. Current flows if there is a load on the coil, such as the coil capacitance or the loading caused by eddy currents in the cores, etc. (A so called current transformer is a tightly coupled transformer driven from a high impedance so that the current in the secondary is related to that in the primary by the turns ratio. The very loosely coupled situation here does not behave that way.)

You describe one way to make the current through the field coil independent of frequency: make the inductive reactance low compared to the dc resistance of the coil across the whole useful frequency range. A better way is to drive the coil with a current source, that is, a circuit with an output impedance much higher than the impedance of the coil at any useful frequency.

Thanks for your last few posts, you cleared up several things that were not clicking for me before, especially about there being a voltage, but not necessarily a current unless a load exists on the pickup. I know that's a basic idea, but I was slow to connect the dots.

And the other point, about the resistance preceding the reactance at low frequencies, I modeled this with LTSpice. The different plot lines indicate three steps of resistance, 10k, 20k and 30k ohms, with the flat, low frequency portion extending further for each increase in step (as well as lowering the Q factor).

[ATTACH=CONFIG]48644[/ATTACH]

Something I'm confused about though, are the circumstances under which +6dB...-6dB/oct slopes emerge, as seen in "raw" bode plots, and the above LTSpice model, but only when the AC voltage source has been placed outside of the pickup.

When a pickup is modeled as with the AC source inside the pickup, as shown below, there is a 0dB/oct line, then the resonant peak, and then a -12dB/oct slope:

[ATTACH=CONFIG]48645[/ATTACH]

In practical testing, both the exciter / field coil method, as well we putting the pickup in series with the function generator, both yield +dB...-6dB/oct slopes, as seen in the first simulation. Driving the pickup with a series voltage obviously puts the voltage source outside of the pickup, but shouldn't the exciter coil method place the voltage inside of the pickup? Why does this testing method not result in a 0dB/oct..-12dB/oct plot, as is seen in the second screen shot?

Another issue with the model which has the voltage source inside the pickup, is as seen in the second screen shot, that increasing the series resistance from 10k to 20k to 30k has no apparent impact on the 0dB/oct slope, as it does with the-6dB slope in the first screen shot.
 
Antigua 4/26/2018 12:58 AM
Quote Originally Posted by Helmholtz View Post
I have no values for the coupling factor. Generally a coupling factor below 100% introduces additional series inductance in the equivalent circuit. But why don't you just measure and compare? Call it loading down or not, in result the (high) frequency response will change.

You have to add the field coil's resistance to the series resistance for total circuit resistance Rtot. What matters is the Rtot/L ratio. It should be well above 150kOhm/H. In other words the corner frequency should lie well above the frequency range analysed and is given by f=Rtot/(2pi*L). This can be achieved by increasing series and/or coil resistance as well as by decreasing field coil inductance.
In that past, I had measured a pickup using an exciter, both with and without a resistor in series with the exciter. It made no difference in the plot lines, but the added resistance made the exciter coil weaker, reducing the S/N ratio. So if the coupling is lower, the series inductance is higher, but apparently not high enough to interfere with measurements.
 
Helmholtz 4/26/2018 5:59 AM
Quote Originally Posted by Antigua View Post
In that past, I had measured a pickup using an exciter, both with and without a resistor in series with the exciter. It made no difference in the plot lines, but the added resistance made the exciter coil weaker, reducing the S/N ratio. So if the coupling is lower, the series inductance is higher, but apparently not high enough to interfere with measurements.
I cannot comment on your measurements. I gave you all the info necessary to make sure you have constant exciter current over frequency. When the exciter current falls with increasing frequency, so will your PU output voltage and thus frequency response will deviate. This effect can be seen in figure 5 of the document, where the responses show a pseudo plateau starting around 1kHz, just as predicted by the formula. In reality the PU bandpass response gets (increasingly) steeper towards resonance.

You may verify the frequency (in)dependance of the drive current by measuring the voltage over a series resistor, via the second channel of the Velleman. The effect of a decreasing drive may be partly masked by the resonance of the PU.
 
Helmholtz 4/26/2018 7:18 AM
The current in the field or exciter coil creates an ac magnetic field that induces a voltage, not a current, in series with the pickup coil. This follows from Maxwell's equation or the law of magnetic induction. Current flows if there is a load on the coil, such as the coil capacitance or the loading caused by eddy currents in the cores, etc. (A so called current transformer is a tightly coupled transformer driven from a high impedance so that the current in the secondary is related to that in the primary by the turns ratio. The very loosely coupled situation here does not behave that way.)

You describe one way to make the current through the field coil independent of frequency: make the inductive reactance low compared to the dc resistance of the coil across the whole useful frequency range. A better way is to drive the coil with a current source, that is, a circuit with an output impedance much higher than the impedance of the coil at any useful frequency.
True, the primary effect of induction is electric field and voltage. But this causality rarely matters in real life situations, where there are current paths and the currents produce counterfields.
Any pair of coupled coils can be descibed as a (non-ideal) transformer. And a transformer is able to transform voltage and current as well as Z,R,L,C. Loose coupling causes reduced voltages and deviations from the ideal turns ratio relations. But the principle works nevertheless.
The idea behind the method is to generate a frequency-independent current in the inductance part of the PU. Constant current through the inductance produces a voltage across the inductance rising linearly with frequency, just like in real PU operation, where the voltage is induced via dPhi/dt.
This constant current is the input test signal and must flow through the load consisting of DCR and capacitance. The output signal is the voltage developed across the capacitance/terminals.
The method is best descibed via the current transformer principle. And if done carefully it works just fine, as can be most easily seen by the straight horizontal line behaviour of the integrated output voltage. My explanations are in line with Lemme and Zollner.

Your are right, generating the constant current via the constant current driven field coil is nothing but a constant current source. And it could be replaced by an (active) wide band CCS if there were direct access to the inductance part of the PU, which is not.
Feeding a constant current to the (output) terminals of the PU inevitably yields the two-pole/two-terminal impedance response. This is different from the quadripole transfer response revealed by the descibed method.
I don't pretend, though, that the transfer response reveals information not available from the impedance response.
 
Helmholtz 4/26/2018 7:33 AM
oabx00z.png

In practical testing, both the exciter / field coil method, as well we putting the pickup in series with the function generator, both yield +dB...-6dB/oct slopes, as seen in the first simulation. Driving the pickup with a series voltage obviously puts the voltage source outside of the pickup, but shouldn't the exciter coil method place the voltage inside of the pickup? Why does this testing method not result in a 0dB/oct..-12dB/oct plot, as is seen in the second screen shot?
Placing a constant voltage source in series with L does not correspond to the exciter method, where a constant current through L is generated. You should use a swept current source in parallel with L instead. This will give you the bandpass transfer response with +/- 6dB slopes and 0 output for 0 Hz.
You did simulate the low-pass transfer response instead.
 
Mike Sulzer 4/26/2018 8:30 AM
Quote Originally Posted by Helmholtz View Post
The idea behind the method is to generate a frequency-independent current in the inductance part of the PU.

That does not happen. The constant current in the exciter coil generates a magnetic field with amplitude independent of frequency. This generates a frequency dependent voltage in the pickup coil. The current that flows is frequency dependent because of that and because the load on the pickup is frequency dependent. At very low frequencies where the reactance of the coil capacitance is very high and the effect of eddy currents is negligible, this current is very small, only that which flows in the input resistance of the amplifier that senses the pickup voltage.

As long as the current in the exciter coil remains constant, the excitation part of the total magnetic field does not change, and so there is no interaction between the pickup coil and the exciter coil, except for the capacitance of the exciter coil. But if you use a tiny exciter coil, this is negligible. In my set up the constant current is a result of the 8 ohm resister, which has an impedance much larger than the tiny coil. Any coil resistance just adds to the resister, and the reactance value of the coil inductance is too small to change the current significantly at audio frequencies.
 
Helmholtz 4/26/2018 8:47 AM
That does not happen. The constant current in the exciter coil generates a magnetic field with amplitude independent of frequency. This generates a frequency dependent voltage in the pickup coil. The current that flows is frequency dependent because of that and because the load on the pickup is frequency dependent. At very low frequencies where the reactance of the coil capacitance is very high and the effect of eddy currents is negligible, this current is very small, only that which flows in the input resistance of the amplifier that senses the pickup voltage.
You are wrong. The current circulating in the PU will be independent of frequency if done correctly. This is a precondition for measuring the true bandpass response of a filter circuit. It can be verfied by simulation.
 
Antigua 4/26/2018 10:50 AM
Quote Originally Posted by Helmholtz View Post
I cannot comment on your measurements. I gave you all the info necessary to make sure you have constant exciter current over frequency. When the exciter current falls with increasing frequency, so will your PU output voltage and thus frequency response will deviate. This effect can be seen in figure 5 of the document, where the responses show a pseudo plateau starting around 1kHz, just as predicted by the formula. In reality the PU bandpass response gets (increasingly) steeper towards resonance.

You may verify the frequency (in)dependance of the drive current by measuring the voltage over a series resistor, via the second channel of the Velleman. The effect of a decreasing drive may be partly masked by the resonance of the PU.
I don't know how the "figure 5" plot came to exist, with the huge "hump", I've never witnessed that myself, except in the case of extreme eddy current losses. For example, it looks suspiciously similar to a Fiolter'tron:

[ATTACH=CONFIG]48650[/ATTACH]

Both my exciter coil and "driven" plots show the same curves, so I think I'm good to go. The only difference, as mentioned earlier, is that the exciter coil will also reveal eddy current losses with respect to the guitar string, where as the "driven" method with a series resistor only shows eddy current losses with respect to the pickup's coil.
 
Mike Sulzer 4/26/2018 11:13 AM
Quote Originally Posted by Helmholtz View Post
You are wrong. The current circulating in the PU will be independent of frequency if done correctly. This is a precondition for measuring the true bandpass response of a filter circuit. It can be verfied by simulation.
Can you describe the physical process by which a constant magnitude ac magnetic field, varying over the required frequency range, can excite a current in a coil independent of the frequency varying load on the coil? Even if you could, it would not be what you want to do. Filter circuits generate the correct bandpass when the source and load have the impedance for which they were designed. For example, rf passive filters might be designed to work from a 50 ohm source into a fifty ohm load.

The field generated by the coil, if small enough, is very similar to the ac field generated by a vibrating string. If you have any doubts that that generates a voltage, read MacDonald (Princeton). Very similar application of Maxwell's equations. So if the string generates a voltage in series with the coil, then we want our test to generate a voltage.
 
Helmholtz 4/26/2018 1:30 PM
Can you describe the physical process by which a constant magnitude ac magnetic field, varying over the required frequency range, can excite a current in a coil independent of the frequency varying load on the coil?
Yes, I can but don't bother to elaborate. No sense discussing with people who don't even consider my arguments.

The field generated by the coil, if small enough, is very similar to the ac field generated by a vibrating string. If you have any doubts that that generates a voltage, read MacDonald (Princeton). Very similar application of Maxwell's equations. So if the string generates a voltage in series with the coil, then we want our test to generate a voltage.
I never denied that the ac field generates a voltage. In fact, the voltage induced across the inductance of the PU images the voltage across the inductance part of the exciter coil (which is not directly accessible, though), namely a voltage rising linearly with frequency. And this means a constant, frequency independent current.
I definitely don't need private lessons on physics by amateurs.

This is my last post on this subject. I have wasted enough time trying to convince people who don't want to learn.

You may try to direct your questions to GITEC e.V.
 
Helmholtz 4/26/2018 1:39 PM
Both my exciter coil and "driven" plots show the same curves, so I think I'm good to go. The only difference, as mentioned earlier, is that the exciter coil will also reveal eddy current losses with respect to the guitar string, where as the "driven" method with a series resistor only shows eddy current losses with respect to the pickup's coil.
Well, if you don't care, I don't either - but will rely on my own measurements. Your "eddy current losses" are only partly real.
 
Mike Sulzer 4/26/2018 1:51 PM
Quote Originally Posted by Helmholtz View Post
Yes, I can but don't bother to elaborate. No sense discussing with people who don't even consider my arguments.



I never denied that the ac field generates a voltage. In fact, the voltage induced across the inductance of the PU images the voltage across the inductance part of the exciter coil (which is not directly accessible, though), namely a voltage rising linearly with frequency. And this means a constant, frequency independent current.
I definitely don't need private lesson on physics by amateurs.

This is my last post on this subject. I have wasted enough time trying to convince people who don't want to learn.

You may try to direct your questions to GITEC e.V.
No, the voltage is generated in series with the pickup coil, and this voltage is not directly accessible. The inductance of the pickup coil is the series leg, and its capacitance is the shunt leg of a voltage divider. This makes a resonant low pass filter. Thus the voltage across the pickup coil is the output of the filter, and it is flat at low frequencies, rises below the resonance and falls above it.

In addition, the voltage rises with frequency because the induced voltage is proportional to the rate of change of the flux through the coil. This has to be accounted for. Finally, the pickup coil couples to metal parts like a transformer with high leakage flux. This places a load across the coil that mostly affects the response in the region of the resonance.

The current that flows through the pickup coil is a function of these loads on it. It is very small at low frequencies since the load is a very high resistance, the preamp input. The current peaks at resonance because of the circulating current in the parallel resonating circuit, and then it falls at higher frequencies.

I am not an amateur.
 
Mike Sulzer 4/26/2018 2:03 PM
Quote Originally Posted by Helmholtz View Post
Well, if you don't care, I don't either - but will rely on my own measurements. Your "eddy current losses" are only partly real.
So are you saying that a thick conductive cover cannot reduce the signal reaching the pickup coil? It can. Both types of eddy current losses can be significant, and that is why it is necessary to make both kinds of measurements if the pickup has a thick cover.
 
Helmholtz 4/26/2018 2:24 PM
So are you saying that a thick conductive cover cannot reduce the signal reaching the pickup coil? It can. Both types of eddy current losses can be significant, and that is why it is necessary to make both kinds of measurements if the pickup has a thick cover.
I can only advise to make sure/control that the exciter current stays constant over the whole frequency range. Otherwise you might find unreal sag below the resonance. But I am repeating myself. This said, some PUs with strong eddy effect do show some real sag. But it is suspicious if this effect depends on the drive circuit.
 
Antigua 4/26/2018 2:32 PM
Quote Originally Posted by Helmholtz View Post
But it is suspicious if this effect depends on the drive circuit.
One example would be the base plate of a pickup, it's very close to the coil itself, so it figures prominently with respect to the coil, but it's very far away from the moving guitar string, so it would have little interaction with the AC magnetic field of the guitar string. A cover would be the other way around; impeding the AC field of the string more so than the AC field of the coil.

Going back to the transformer analogy, wouldn't eddy current losses be greater if the conductive plane was in between the coils, rather than being placed at far side of the transformer?

Within the next week or two I will compare driven versus externally excited plots of pickups with covers and substantial steal cores in order to quantify any difference.
 
Helmholtz 4/26/2018 2:37 PM
Quote Originally Posted by Antigua View Post
One example would be the base plate of a pickup, it's very close to the coil itself, so it figures prominently with respect to the coil, but it's very far away from the moving guitar string, so it would have little interaction with the AC magnetic field of the guitar string. A cover would be the other way around; impeding the AC field of the string more so than the AC field of the coil.

Going back to the transformer analogy, wouldn't eddy current losses be greater if the conductive plane was in between the coils, rather than being placed at far side of the transformer?
Yes, of course. But where is the link to the exciter coil circuit?
 
Helmholtz 4/27/2018 8:08 AM
Within the next week or two I will compare driven versus externally excited plots of pickups with covers and substantial steal cores in order to quantify any difference.
I may have misinterpreted you, as it is not clear to me what you mean by "driven". Both measurements, impedance as well transfer response require a drive (signal).

-The impedance measurement (direct current feed) is sensitive to eddy current effects in cores but much less so to eddy current losses in conductive covers.

-The field drive method for transfer response much better (more realistically) reveals the effect of lossy covers at medium and high frequencies by producing a drecrease (loss) in output voltage. After integration, this typically shows as a depression below resonance.
But this same kind of response with depression can be produced as an artefact, if the exciter current starts decreasing in this frequency range, caused by the increasing impedance/reactance of the exciter coil over frequency. In this case you need to increase the series resistor value and/or reduce exciter coil inductance.
 
Mike Sulzer 4/27/2018 8:37 AM
Quote Originally Posted by Helmholtz View Post
I may have misinterpreted you, as it is not clear to me what you mean by "driven". Both measurements, impedance as well transfer response require a drive (signal).

-The impedance measurement (direct current feed) is sensitive to eddy current effects in cores but much less so to eddy current losses in conductive covers.

-The field drive method for transfer response much better (more realistically) reveals the effect of lossy covers at medium and high frequencies by producing a drecrease (loss) in output voltage. After integration, this typically shows as a depression below resonance.
But this same kind of response with depression can be produced as an artefact, if the exciter current starts decreasing in this frequency range, caused by the increasing impedance/reactance of the exciter coil over frequency. In this case you need to increase the series resistor value and/or reduce exciter coil inductance.
But this kind of artefact would be seen without a cover as well, and so it is easy to determine if it is a serious problem.
 
Helmholtz 4/27/2018 9:23 AM
But this kind of artefact would be seen without a cover as well, and so it is easy to determine if it is a serious problem.
Right, but I am not sure if everbody cares to check. The depression might also hide below the resonance and cause a too low resonance peak (Q) and a high frequency drop-off steeper than -12dB/octave in the integrated response. I would recommend to use a low loss control PU with very high resonance frequency (low/medium impedance type) for this kind of check.
But it is easier to verify that the exciter coil current does not change over the whole frequency range.
 
Antigua 4/27/2018 2:25 PM
Quote Originally Posted by Helmholtz View Post
I may have misinterpreted you, as it is not clear to me what you mean by "driven". Both measurements, impedance as well transfer response require a drive (signal).

-The impedance measurement (direct current feed) is sensitive to eddy current effects in cores but much less so to eddy current losses in conductive covers.

-The field drive method for transfer response much better (more realistically) reveals the effect of lossy covers at medium and high frequencies by producing a drecrease (loss) in output voltage. After integration, this typically shows as a depression below resonance.
But this same kind of response with depression can be produced as an artefact, if the exciter current starts decreasing in this frequency range, caused by the increasing impedance/reactance of the exciter coil over frequency. In this case you need to increase the series resistor value and/or reduce exciter coil inductance.
Right, this is what is at issue. The Filter'tron plot in post #80 shows that depression in dramatic form, but we know it's caused by eddy currents, because other plots of Fender single coils are very flat before they reach resonances, for example:

[ATTACH=CONFIG]48653[/ATTACH]


Also, the plot in this post http://music-electronics-forum.com/t46007-4/#post493813 has a plot that gets to 16kHz without any dips. Also, that plot shows the effects of dramatic eddy current losses when the secondary coil was closed and continuous.


Here's something I don't understand: why can't I get the field drive coil to generate a -6dB/oct slope by putting capacitance in parallel with the field drive coil? Shouldn't the voltage drop with frequency? I tried it and it seems to have no such effect.
 
Helmholtz 4/27/2018 3:44 PM
Here's something I don't understand: why can't I get the field drive coil to generate a -6dB/oct slope by putting capacitance in parallel with the field drive coil? Shouldn't the voltage drop with frequency? I tried it and it seems to have no such effect.
The natural high frequency slope of the field coil drive response (aka bandpass transfer reponse) is -6dB/octave, i.e. capacitive behaviour The integrator transforms this to the low-pass response and adds another -6dB/octave. The result is -12dB/octave.
What is the point of putting a capacitor in parallel with the exciter coil? What is the bandwidth of the integrator?
 
Joe Gwinn 4/28/2018 4:44 PM
Quote Originally Posted by Helmholtz View Post
What are all these needle artifacts? I don't see them in my impedance measurements. Also the high and low frequency slopes appear not to be correct. Did you measure as I proposed without the field coil?
We call them "spurs" in English. They look like external interference to me, probably from clocked digital logic with very short rise and fall times on the edges. Plotting amplitude versus linear (not log) frequency will reveal any harmonic ladders, which may be a clue as to source. Be suspicious of nearby test equipment.


The zigzag anomality is the result of an additional series and parallel resonance with higher resonant frequencies. I could show in simulations that such behaviour can be the result of partially shorted windings. Another explanation could be a very sloppy wind, where the winding is not carefully layered but outer turns are used to fill lower spaces thereby causing an uneven distribution of the distributed capacitance. I have not found a way yet to prove this idea wrong or right, as I am not winding.
One way to dig into this mathematically may be found in the System Identification literature, which is large, and mostly directed at things like modelling chemical plants the better to control them. But we can use their methods in miniature, to deduce likely equivalent circuits. Note that eddy currents will likely need to be modeled directly, as no lumped-component circuit does it justice.
 
Helmholtz 4/28/2018 5:05 PM
The current in the field or exciter coil creates an ac magnetic field that induces a voltage, not a current, in series with the pickup coil.
This is nonsense. Induced voltage always develops across the inductor and not in series. This follows from Maxwell's equations as well as Faraday's law of induction.
 
Helmholtz 4/28/2018 5:09 PM
Here's something I don't understand: why can't I get the field drive coil to generate a -6dB/oct slope by putting capacitance in parallel with the field drive coil? Shouldn't the voltage drop with frequency? I tried it and it seems to have no such effect.
Well, this is easy, if you understand resonant circuits. The exciter coil and the parallel C form a parallel resonant circuit. In the vicinity of its resonant frequency, coil current and coil voltage rise, while the outer current through the series resistor decreases caused by the increased impedance. The normal voltage decreasing effect of the added C only shows above this resonance.

The most probable reason for a high frequency drop-off flatter than -6dB/octace before and -12dB/octave after integration is noise floor of the Velleman. S/N ratio is improved by higher drive current. I use a power amplifier connected to the generator output to drive the coil.

You may also want to check the integrator for errors at low signal levels caused by offset.

BTW, the 6dB/octave slopes below and above resonance prove that the current through the inductance at low frequencies and the current through the capacitance at high frequencies must be constant (independent of frequency). Those who understand filters will know what I mean.
 
Mike Sulzer 4/28/2018 8:57 PM
Quote Originally Posted by Helmholtz View Post
This is nonsense. Induced voltage always develops across the inductor and not in series. This follows from Maxwell's equations as well as Faraday's law of induction.
No. Take the turns one at a time. Integrate the E field around each turn. (The E field has a non-zero curl.) This gives the voltage around each turn. The turns are all in series; so add up the individual voltages to give a single voltage in series with the coil.

If a voltage source appeared in parallel with the coil, it could drive the coil inductance as well as any external load. This is obviously impossible; it would be able to provide an infinite amount of energy.
 
Helmholtz 4/29/2018 7:31 AM
No. Take the turns one at a time. Integrate the E field around each turn. (The E field has a non-zero curl.) This gives the voltage around each turn. The turns are all in series
Correct. In consequence the sum of the voltages appears at/between the terminals of the real coil.

so add up the individual voltages to give a single voltage in series with the coil.
The conception of an ideal inductor in series with a voltage source is one valid model. But this voltage source needs to be controlled to give f-proportional voltage, as does the real inductor. (Simulations using a constant voltage source will not give the real PUs frequency response, which shows an output voltage proportional to string velocity (frequency*amplitude) below resonance.)

I prefer Zollner's method of producing the induced voltage with a constant current source wired in parallel with the ideal inductor.

If done correctly both methods/models give the same simulation results and thus can be considered equivalent.
 
Antigua 4/29/2018 10:40 AM
Quote Originally Posted by Helmholtz View Post
The natural high frequency slope of the field coil drive response (aka bandpass transfer reponse) is -6dB/octave, i.e. capacitive behaviour The integrator transforms this to the low-pass response and adds another -6dB/octave. The result is -12dB/octave.
What is the point of putting a capacitor in parallel with the exciter coil? What is the bandwidth of the integrator?
The point of this is that I'm trying to see if the integrator can be removed from the testing process, by duplicating its effects by passive means, because it there's a way to get a similar plot without specialized equipment, it would make pickup testing more accessible to hobbyists like myself.

Just so you know, the integrator is between the pickup and the oscilloscope, as this is how Ken Willmott designed the device. I understand that Helmuth Lemme put the integrator in between the function generator and field coil.

I'm having partial success using this circuit on the field coil side. You can see from the simulation that I across L1 "field coil" slopes off past 1kHz, so that's good, but it's still far from being great.

[ATTACH=CONFIG]48657[/ATTACH]

Here are actual plots using an SSL-1, using the schematic above on the field coil side. It looks more like an integrated plot as the value of C is increased, but at lower frequencies it's not flat, and the noise increases as it's made flatter with higher values of C.

[ATTACH=CONFIG]48658[/ATTACH]

I notice that if R2 is a higher value, the slope drops off at a lower frequency, but doing that causes the field coil's strength to become too weak.
 
Mike Sulzer 4/29/2018 12:10 PM
Quote Originally Posted by Helmholtz View Post
Correct. In consequence the sum of the voltages appears at/between the terminals of the real coil.



The conception of an ideal inductor in series with a voltage source is one valid model. But this voltage source needs to be controlled to give f-proportional voltage, as does the real inductor. (Simulations using a constant voltage source will not give the real PUs frequency response, which shows an output voltage proportional to string velocity (frequency*amplitude) below resonance.)

I prefer Zollner's method of producing the induced voltage with a constant current source wired in parallel with the ideal inductor.

If done correctly both methods/models give the same simulation results and thus can be considered equivalent.
The sum of the voltages does not appear at the terminals of the real coil, although it can approach it at very low frequencies. The sum of the voltages is modified by the various impedances I mentioned before. In particular at the resonance, the output voltage across the coil is greater than the series voltage at that frequency.

Of course you can always use a current source, properly located. But this is just a distraction; neither of us was talking about this equivalent current source.

Of course the series voltage increases with frequency; that is what the physics tells us to do.
 
Helmholtz 4/29/2018 1:24 PM
Quote Originally Posted by Antigua View Post
The point of this is that I'm trying to see if the integrator can be removed from the testing process, by duplicating its effects by passive means, because it there's a way to do get a similar plot without without specialized equipment, it would make pickup testing more accessible to hobbyists like myself.

Just so you know, the integrator is between the pickup and the oscilloscope, as this is how Ken Willmott designed the device. I understand that Helmuth Lemme put the integrator in between the function generator and field coil.

I'm having partial success using this circuit on the field coil side. You can see from the simulation that I across L1 "field coil" slopes off past 1kHz, so that's good, but it's still far from being great.

[ATTACH=CONFIG]48657[/ATTACH]

Here are actual plots using an SSL-1, using the schematic above on the field coil side. It looks more like an integrated plot as the value of C is increased, but at lower frequencies it's not flat, and the noise increases as it's made flatter with higher values of C.

[ATTACH=CONFIG]48658[/ATTACH]

I notice that if R2 is a higher value, the slope drops off at a lower frequency, but doing that causes the field coil's strength to become too weak.
Did you read my post #96? It explains, why the C across the exciter coil is not a good idea. This method can never replace a real integrator.

The integrator can be placed between PU and scope as well as between generator and exciter coil. The latter arrangement provides a stronger signal for the integrator, which is generally good. But I am not sure, if the integrator is able to directly drive the low impedance exciter coil load. In any case I strongly recommend to drive the coil via a linear power amplifier. I use one channel of a 60W stereo amplifier.

For integration I use a passive integrator after the power amplifier, consisting of a huge air core inductor of around 20mH and a DCR of 2Ohms. This is wired in series with the exciter coil and takes care of the 1/f drive current.
 
Antigua 4/29/2018 2:07 PM
Quote Originally Posted by Helmholtz View Post
Did you read my post #96? It explains, why the C across the exciter coil is not a good idea. This method can never replace a real integrator.

The integrator can be placed between PU and scope as well as between generator and exciter coil. The latter arrangement provides a stronger signal for the integrator, which is generally good. But I am not sure, if the integrator is able to directly drive the low impedance exciter coil load. In any case I strongly recommend to drive the coil via a linear power amplifier. I use one channel of a 60W stereo amplifier.

For integration I use a passive integrator after the power amplifier, consisting of a huge air core inductor of around 20mH and a DCR of 2Ohms. This is wired in series with the exciter coil and takes care of the 1/f drive current.
I did read your post, the -6dB slope comes after the resonance between the cap and the field coil, but I used very high C values, 1uF, so the resonant peak was very low, so that in and of itself didn't appear to be a problem. It appears to me, based on LTSpice modelling, that if R2 is a higher value of resistance, the roll off occurs at a lower frequency, but a more powerful amplification would be required to drive the field coil. The goal is to come up a more simple test procedure, and a power amp would add complexity, but at least it's a much (much much) more common device than an integrator circuit.

It sounds like your series inductor integration past a power amp is an easy setup to create, I'd be interested in seeing plots made with the integration, and pictures of the coil itself. I do keep a power amp on hand for testing purposes, I could give it a try.

Ken Willmott was of the opinion that putting the integrator between the pickup and oscilloscope was less prone to noise, but I don't have a strong opinion one way or the other.
 
Antigua 4/30/2018 12:51 AM
Here is a test showing the result of two different methods of bode plotting the transfer function of a pickup. The first method has the pickup hooked up directly to the function generator and oscilloscope, and treats the pickup as though it were just a low pass filter, a series inductance. The second method treats the pickup like a very poorly coupled transformer, where the inducing coil is a primary connected to the function generator, and the pickup itself is treated as secondary, hookup up to the oscilloscope.

This is what the test setup looks like when being directly driven by a function generator, and having the voltage difference measure across a 1meg resistor in series:

[ATTACH=CONFIG]48689[/ATTACH]


And this is what the external field coil test setup looks like. There is a tiny excitation coil wrapped around a wood stick:

[ATTACH=CONFIG]48688[/ATTACH]


For this test, I used an Epiphone 57CH PAF style humbucker with a brass cover, so that the "worst case scenario" of a brass cover that causes a high amount of eddy currents.

Here are four plots lines comparing difference measurements:

[ATTACH=CONFIG]48687[/ATTACH]

All four plots are made with the integrator in front of the first channel of the Velleman PSCU2000 to get -6dB/oct.

The two taller peaks (green and red) are with the brass cover removed, while the two lower peaks (blue and black) have the brass cover in place.

Of the two taller peaks with the brass cover removed, the taller green peak was made using the external field coil, while the shorter red peak was produced by driving the pickup directly with a function generator. For some reason, these two peaks show fairly different amplitudes, a difference of 3.5dB, with the external coil test method yielding a higher Q.

Then, of the two shorter peaks, for which the cover was in place, the higher blue line is the pickup being driven directly with a function generator, where as the lower black line was made using the external coil.

There are a few things going on. Interestingly the two testing methods show the greatest difference with the brass cover off, instead of on, which is the opposite of what I would have expected. The Q factors of the "cover off" plots are about 3.5dB apart at the peak. Even though there are differences in need of an explanation, they two test methods are more similar than I thought they would be, possibly suggesting that trying to mimic the geometry of a guitar string with an external field coil is not strictly necessary to create useful data plots, eddy currents and all.
 
Helmholtz 4/30/2018 6:58 AM
The sum of the voltages does not appear at the terminals of the real coil, although it can approach it at very low frequencies. The sum of the voltages is modified by the various impedances I mentioned before. In particular at the resonance, the output voltage across the coil is greater than the series voltage at that frequency.
This statement is correct. Mike Sulzer is/was right and I apologize. I was on the wrong track.

The total induced voltage/EMF would show at the inductor's terminals only in an open loop situation without any current. But any real world inductor like a PU is terminated at least by its own distributed capacitance. Just as you said.
 
Helmholtz 4/30/2018 9:26 AM
Is anybody going to wind a bifilar (two wires in parallel) pickup coil and measure its impedance curve in series configuration?
 
bbsailor 4/30/2018 10:39 AM
Quote Originally Posted by Helmholtz View Post
Is anybody going to wind a bifilar (two wires in parallel) pickup coil and measure its impedance curve in series configuration?
Here is a thought experiment to consider and some may want to make this a real experiment.

Consider a pickup with four independent sections with an equal number of turns.

Upper section: Inner coil 1; Outer coil 2
Lower section: Inner coil 3; Outer coil 4

If you consider the inner coils being similar to a Strat type pickup, tall and narrow. Then you consider the Upper or Lower section coils 1 and 2 or 3 and 4 to being a short and flat pickup like a Jazzmaster or P90 type.

If you stimulate the pickup configurations with a drive coil or a vibrating string, would you have the same voltage output with either stimulation way or would the outer coil have less output being farther from the magnets?

If you consider the output of the total upper section compared to the lower section it is like the upper section is closer to the strings than the lower section and the upper section would have a higher output.

Creating a good mental model of a pickup shape supported by some measurements being done with a more realistic vibrating string or a stimulation coil should lead to a more deep understanding of the issues involved in this discussion.

How would you rank the output level from each coil in this experiment?

Joseph J. Rogowski
 
Mike Sulzer 4/30/2018 8:45 PM
Quote Originally Posted by Helmholtz View Post
This statement is correct. Mike Sulzer is/was right and I apologize. I was on the wrong track.

The total induced voltage/EMF would show at the inductor's terminals only in an open loop situation without any current. But any real world inductor like a PU is terminated at least by its own distributed capacitance. Just as you said.

I am glad we have agreement!
 
Helmholtz 5/1/2018 8:38 AM
I need to correct at least this previous statement of mine:

I never denied that the ac field generates a voltage. In fact, the voltage induced across the inductance of the PU images the voltage across the inductance part of the exciter coil (which is not directly accessible, though), namely a voltage rising linearly with frequency.
Instead it should read: ...., the voltage induced in series with the inductance...

Thanks again to Mike Sulzer for the "private lesson".

My alternative explanation with the current transformer isn't as simple as I thought. To make it work, the transformed primary circuit + leakage inductances taking care of loose coupling would have to be included in the complete equivalent circuit, leading to branched currents. This makes it quite complex and unattractive.
 
Helmholtz 5/1/2018 8:51 AM
Here is a picture of my integrator coil(s):

[ATTACH=CONFIG]48710[/ATTACH

And here is a low pass PU transfer measurement using the coil:

[ATTACH]48711[/ATTACH]

A second pair of these coils is used in my magnetizer:

[ATTACH=CONFIG]48712[/ATTACH]
 
Antigua 5/1/2018 1:45 PM
Quote Originally Posted by bbsailor View Post
If you stimulate the pickup configurations with a drive coil or a vibrating string, would you have the same voltage output with either stimulation way or would the outer coil have less output being farther from the magnets?
Here's a plot I created with my home made tapped coil, comparing the inner and outer coils with a small ficoil above the pickup:

[ATTACH=CONFIG]48719[/ATTACH]

(note that the secondary peaks after the primary resonance are irrelevant, but I included them because they're sort of interesting anyway. Thanks again to Helmholtz for the auto voltage scaling tip that allowed these to be seen.)

The outer coil shows about +2dB over the inner coil. Both are 4,000 turns, but the outer coil involves more area and wire length, of course. Ultimately the deciding factor for the sum voltage comes down to which coil catches a greater overall flux change. I'm not sure of the exact geometry or integrations that are at play, but it intuitively stands to reason that bigger net catches more fish.


Quote Originally Posted by bbsailor View Post
If you consider the output of the total upper section compared to the lower section it is like the upper section is closer to the strings than the lower section and the upper section would have a higher output.
That's also very similar to simply raising or lowering a pickup, or measuring the two halves of a stacked humbucker. I've done that before, and documented it on another forum. Needless to say, the lower coil generates quite a bit less voltage.

Quote Originally Posted by bbsailor View Post
Creating a good mental model of a pickup shape supported by some measurements being done with a more realistic vibrating string or a stimulation coil should lead to a more deep understanding of the issues involved in this discussion.
I'd be interested to talk about the difference between an external field coil and a guitar string as far as how similar they are and aren't, and whether there is something peculiar about a guitar string that makes it requisite for thorough pickup testing. The major difference IMO would relate to the transient, because when you first pluck the string it displaces a wide area, and then it settles down to a rather small amount of movement, at which point the geometry is nearly static, where as an external coil's geometry is literally static. I think overall they're more alike than they are different, both put out an EMF from from an nearly fixed point in space.
 
Antigua 5/1/2018 1:57 PM
Quote Originally Posted by Helmholtz View Post
Is anybody going to wind a bifilar (two wires in parallel) pickup coil and measure its impedance curve in series configuration?
Do you have a theory as to what could be expected? Wouldn't this greatly increase the voltage potential across the self-capacitance?
 
Antigua 5/1/2018 1:59 PM
Quote Originally Posted by Helmholtz View Post
Here is a picture of my integrator coil(s):

[ATTACH=CONFIG]48710[/ATTACH

And here is a low pass PU transfer measurement using the coil:

[ATTACH]48711[/ATTACH]

A second pair of these coils is used in my magnetizer:

[ATTACH=CONFIG]48712[/ATTACH]
Thanks for posting the picks. Those coils look pretty large. I'd be interested in trying this method, do you know what the minimum inductance value and voltage rating would be for a suitable coil to be purchased on, say, Mouser? Is your exciter coil in the picture somewhere as well?
 
Helmholtz 5/1/2018 2:51 PM
Quote Originally Posted by Antigua View Post
Do you have a theory as to what could be expected? Wouldn't this greatly increase the voltage potential across the self-capacitance?
I'd expect a strong anomaly.

BTW, could you measure the interwinding capacitance (i.e. capacitance between the separate windings) of your 4-wire tapped coil PU, as this was the OP's question?
 
Antigua 5/1/2018 3:23 PM
Quote Originally Posted by Helmholtz View Post
I'd expect a strong anomaly.
I'd expect that also, but from a time management perspective, it's a fair amount of work to create the pickup to demonstrate something that has no apparent practical value.

Quote Originally Posted by Helmholtz View Post
BTW, could you measure the interwinding capacitance (i.e. capacitance between the separate windings) of your 4-wire tapped coil PU, as this was the OP's question?
Given the issue of reflected capacitance, I'm not exactly sure how this is best accomplished. Which testing procedure would you recommend to get the most accurate capacitance between them?
 
Helmholtz 5/1/2018 3:48 PM
Quote Originally Posted by Antigua View Post
Thanks for posting the picks. Those coils look pretty large. I'd be interested in trying this method, do you know what the minimum inductance value and voltage rating would be for a suitable coil to be purchased on, say, Mouser? Is your exciter coil in the picture somewhere as well?
I recommend a minimum inductance of 10mH and a max. DCR of 2.5Ohms. What matters is the L/R ratio, where R includes the exciter coil's DCR. If this is too low, bass response will suffer. Also important is that the self resonance of the coil lies well above the highest frequency of interest. My coil has a resonant frequency of 45kHz. A too low coil resonance causes the output signal to rise at high frequencies. I am not familiar with the inductors'/chokes' range of any distributor, but I think that a suitable choke doesn't need to be bigger than 3x3x3 cubic inches. I would look for speaker crossover parts.

The exciter coil I used for the measurement consisted of 20 turns wound around a strat PU cover.
 
Helmholtz 5/2/2018 6:51 AM
I'd expect that also, but from a time management perspective, it's a fair amount of work to create the pickup to demonstrate something that has no apparent practical value.
The reason why I am interested in such an experimental PU is that up to now there is no direct proof that the build of the coil can actually produce anomalies. This would be an extreme situation with maximum coupling between the two sections wired in series. If such kind of PU does not show anomalies, I have to assume a different reason, namely partial shorts in the winding. And this should be quite alarming from a QC point of view.


Given the issue of reflected capacitance, I'm not exactly sure how this is best accomplished. Which testing procedure would you recommend to get the most accurate capacitance between them?
The reflected capacitances appear in parallel with each winding but not between separate windings. With a 4 wire PU there are 4 possibilities to connect the LCR meter between the 2 windings. I would measure all 4 of them, the most meaningful being the C value between end of inner coil and start of outer coil.
 
Joe Gwinn 5/2/2018 7:58 AM
Quote Originally Posted by Helmholtz View Post
The reason why I am interested in such an experimental PU is that up to now there is no direct proof that the build of the coil can actually produce anomalies. This would be an extreme situation with maximum coupling between the two sections wired in series. If such kind of PU does not show anomalies, I have to assume a different reason, namely partial shorts in the winding. And this should be quite alarming from a QC point of view.
One can detect partial shorts by looking at the excess of AC resistance (measured with a DE5000 at 1 KHz) over DC resistance, this excess reflection the sum of all power losses. A shorted turn will cause a large increase in loss.

One compares the loss in known good units to units under suspicion to detect the outliers.
 
Helmholtz 5/2/2018 8:57 AM
Quote Originally Posted by Joe Gwinn View Post
One can detect partial shorts by looking at the excess of AC resistance (measured with a DE5000 at 1 KHz) over DC resistance, this excess reflection the sum of all power losses. A shorted turn will cause a large increase in loss.

One compares the loss in known good units to units under suspicion to detect the outliers.
Not necessarily. A single shorted turn won't show at all. Even 100 hard-shorted outer turns change Rs@1kHz by less than 10%. A more sensitive method is to compare the Q values of the resonance in the impedance plots. But verified reference PUs are rarely available.

You may want to read this article by Prof. Manfred Zollner:
https://gitec-forum.de/wp/wp-content...he-winding.pdf

There are several variables that influence the results: number of turns shorted, position of the shorted section within the coil influencing coupling - and contact resistance of the "short". The article only covers perfect coupling.

One may argue that only shorts that noticeably effect the loaded transfer response will be audible. But I would be alarmed by other evidence as well, because this indicates that something is wrong with the wire quality or the winding technique.
 
Antigua 5/2/2018 10:10 AM
I agree that Q factor would reveal internal short circuits well. A very extreme example is the loss in Q (and everything else) when you short one of the windings of a tapped single coil.

I'm very suspicious that shorts exist, though, because it would require some mechanism to have stripped the insulation off, not just a segment of wire, but also another segment of wire that happens to be in physical contact with the first. Suppose insulation was missing from a few feet of magnet wire, through some sort of manufacturing defect which I've never heard of, the total size of that continuous short would be very small compared to the rest of the coil. I can appreciate that the enamel used in the old days might have been prone to failure, I bet the modern polyurethane insulations are probably much less prone to failure.

Based on the accumulation of plots of home made and commercially available pickups, that secondary resonances come from non uniformities across the network of internal capacitances within the coil, due to uneven turn layering. I was able to partially model this concept with LTSpice. The very loose home made coil that was wound with minimal tension showed the most dramatic secondary resonances, and was least likely to have an internal short.
 
Helmholtz 5/2/2018 11:08 AM
Based on the accumulation of plots of home made and commercially available pickups, that secondary resonances come from non uniformities across the network of internal capacitances within the coil, due to uneven turn layering. I was able to partially model this concept with LTSpice. The very loose home made coil that was wound with minimal tension showed the most dramatic secondary resonances, and was least likely to have an internal short.
I agree that the uneven layering of turns is the more probable explanation of secondary resonances, but there is no solid proof yet - only some probability. I have found new strat PUs that show both an unusual low Q as well as a very strong antiresonance-resonance pair. And I am convinced that such secondary resonances could be modelled from the equivalent circuit of partial shorts shown by Zollner if distributed capacitances are included. Fact is that winding shorts do happen but are only likely to be detected if output is noticeably reduced and/or the sound is really dull. So I suspect some dark figure.

Hence my wish for a bifilar wound PU. There were requests from other posters also.
 
Antigua 5/2/2018 11:57 AM
Quote Originally Posted by Helmholtz View Post
I agree that the uneven layering of turns is the more probable explanation of secondary resonances, but there is no solid proof yet - only some probability. I have found new strat PUs that show both an unusual low Q as well as a very strong antiresonance-resonance pair. And I am convinced that such secondary resonances could be modelled from the equivalent circuit of partial shorts shown by Zollner if distributed capacitances are included. Fact is that winding shorts do happen but are only likely to be detected if output is noticeably reduced and/or the sound is really dull. So I suspect some dark figure.

Hence my wish for a bifilar wound PU. There were requests from other posters also.
You say winding shorts "do happen" but that uneven layering resulting in secondary resonances lacks solid proof. Both of these things lack solid proof, IMO.

A bifilar coil wouldn't necessarily prove a relationship between uneven winding and secondary resonances, though. I would be interested in making a pickup that deliberately emphasizes a secondary resonance, in order to better understand existing pickups, but I don't think a bifilar coil truly approximates that, since existing pickups are not bifilar. I personally don't find winding pickup to be an enjoyable process, and it hurts my eyes to guide the wire onto the spinning bobbin, so it's not something I do readily.

I think a better test pickup might be one where the coil is intentionally wound extremely lopsided at first, and then balanced out as the wind is completed, so that the majority of the early winds would be on one side of the coil former, and the latter winds on the other. This would be like a stacked coil, except but without a significant division between the two halves.

I have also found Strat pickups to have unusually high or low Q factors, but that can also be attributed to defective formulations of AlNiCo, which was supported by the fact that the pole pieces had lower residual flux values than was to be expected.
 
Joe Gwinn 5/2/2018 7:44 PM
Quote Originally Posted by Helmholtz View Post
Not necessarily. A single shorted turn won't show at all. Even 100 hard-shorted outer turns change Rs@1kHz by less than 10%. A more sensitive method is to compare the Q values of the resonance in the impedance plots. But verified reference PUs are rarely available.
I'll grant that Q is more sensitive, but it's harder to measure than the excess of AC over DC resistance. For pickup makers in production, they can easily measure the AC and DC resistance of every unit of a given model they make, and then units with too much excess resistance will stick out. And if it doesn't stand out, it's likely to be inaudible as well.


You may want to read this article by Prof. Manfred Zollner:
https://gitec-forum.de/wp/wp-content...he-winding.pdf

There are several variables that influence the results: number of turns shorted, position of the shorted section within the coil influencing coupling - and contact resistance of the "short". The article only covers perfect coupling.

One may argue that only shorts that noticeably effect the loaded transfer response will be audible. But I would be alarmed by other evidence as well, because this indicates that something is wrong with the wire quality or the winding technique.
There are a number of old threads on this very subject. The core problem is that if one winds 4000 turns of thin wire on a form with winding pressure of say 20 grams, the total hoop stress is 4000*20= 80 kilograms, in a very small space, so the unit pressure at the bottom is quite high. This is enough to force perfectly good magnet wire onto the core and/or other strands with sufficient pressure to pierce the insulation layer on the wire - shorts to the magnets are a common problem in strat pickups. It turned out that no kind of varnish film was stiff enough to prevent this (the wires would creep through the varnish film), but taping the core with paper of kapton or the like would work. These are soft enough to give a little, and yet won't keep on creeping. Likewise, if bobbins were not perfectly smooth and deburred near the core, there could be shorts. (Tape also helps with rusty magnets.)
 
Mike Sulzer 5/3/2018 5:10 AM
Quote Originally Posted by Antigua View Post

The outer coil shows about +2dB over the inner coil. Both are 4,000 turns, but the outer coil involves more area and wire length, of course. Ultimately the deciding factor for the sum voltage comes down to which coil catches a greater overall flux change. I'm not sure of the exact geometry or integrations that are at play, but it intuitively stands to reason that bigger net catches more fish.

But as you make this net bigger, some of the fish that swim in, turn around and swim back out. That is, field lines from the vibrating string must return to it, and so as you make the coil very big, flux begins to cancel out. But apparently you have not reached that point yet with your inner and outer coils. I am a bit surprised that the outer has 2db more output. Since downward going flux is not completely confined to the core, I do expect loops right at the core to contribute less than ones somewhat larger, but I would have thought not so much. Maybe some FEMM modeling is required, a cylindrical pole piece with a tiny cylinder above it to act as the string. Or perhaps MacDonald's integration could be used with different limits to look at this.
 
Helmholtz 5/3/2018 7:44 AM
I'll grant that Q is more sensitive, but it's harder to measure than the excess of AC over DC resistance. For pickup makers in production, they can easily measure the AC and DC resistance of every unit of a given model they make, and then units with too much excess resistance will stick out. And if it doesn't stand out, it's likely to be inaudible as well.
Completely agreed. Measuring loss resistance or the Q of L at 1kHz is by far the most convenient way to detect serious defects.


There are a number of old threads on this very subject. The core problem is that if one winds 4000 turns of thin wire on a form with winding pressure of say 20 grams, the total hoop stress is 4000*20= 80 kilograms, in a very small space, so the unit pressure at the bottom is quite high. This is enough to force perfectly good magnet wire onto the core and/or other strands with sufficient pressure to pierce the insulation layer on the wire - shorts to the magnets are a common problem in strat pickups. It turned out that no kind of varnish film was stiff enough to prevent this (the wires would creep through the varnish film), but taping the core with paper of kapton or the like would work. These are soft enough to give a little, and yet won't keep on creeping. Likewise, if bobbins were not perfectly smooth and deburred near the core, there could be shorts. (Tape also helps with rusty magnets.)
Thanks, this makes a lot of sense and essentially confirms what I had in mind. I have seen several Fender strat PUs from the late 60s and early 70s showing this kind of problem. All of them had (period correct) PE insulation. Formvar is a tougher material according to MWS.

Another critical area could be the start wire crossing the bottom of the coil.

Such creepage induced shorts may need some time (and thermal cycles) to develop and show. Thus pre-delivery inspection might not detect them.
 
Helmholtz 5/3/2018 8:17 AM
But as you make this net bigger, some of the fish that swim in, turn around and swim back out. That is, field lines from the vibrating string must return to it, and so as you make the coil very big, flux begins to cancel out. But apparently you have not reached that point yet with your inner and outer coils. I am a bit surprised that the outer has 2db more output. Since downward going flux is not completely confined to the core, I do expect loops right at the core to contribute less than ones somewhat larger, but I would have thought not so much. Maybe some FEMM modeling is required, a cylindrical pole piece with a tiny cylinder above it to act as the string. Or perhaps MacDonald's integration could be used with different limits to look at this.
This is exactly what Zollner explains in his book. Based on simulations of the ac component of B, he shows that the induced V/turn value of a Jazzmaster PU first increases with the radius (coil half-width) up to ca. 7.5mm, but decreases above.
 
Joe Gwinn 5/3/2018 8:25 AM
Quote Originally Posted by Helmholtz View Post
Thanks, this makes a lot of sense and essentially confirms what I had in mind. I have seen several Fender strat PUs from the late 60s and early 70s showing this kind of problem. All of them had (period correct) PE insulation. Formvar is a tougher material according to MWS.
PE isn't all that bad, and while Formvar is tougher, it still can be penetrated.

Another critical area could be the start wire crossing the bottom of the coil.

Such creepage induced shorts may need some time (and thermal cycles) to develop and show. Thus pre-delivery inspection might not detect them.
Yes. As for the start wire, one remedy is to tape over it before winding the full coil. Or a groove or alternate path for the start. And so on.

As for creepage, that has to be solved by design, and metric of success is the absence of warranty repair requests.

We have had reports of bad wire, with brittle enamel that invariably cracked under the stresses of winding (where this wire is stretched). If the enamel doesn't respond to stretching more or less the same as copper, the enamel pops off the copper.
 
Antigua 5/3/2018 9:23 AM
Quote Originally Posted by Mike Sulzer View Post
But as you make this net bigger, some of the fish that swim in, turn around and swim back out. That is, field lines from the vibrating string must return to it, and so as you make the coil very big, flux begins to cancel out. But apparently you have not reached that point yet with your inner and outer coils. I am a bit surprised that the outer has 2db more output. Since downward going flux is not completely confined to the core, I do expect loops right at the core to contribute less than ones somewhat larger, but I would have thought not so much. Maybe some FEMM modeling is required, a cylindrical pole piece with a tiny cylinder above it to act as the string. Or perhaps MacDonald's integration could be used with different limits to look at this.
I didn't mean to imply that if the pickup was infinite in size that you'd get a maximum potential output.

BTW, my testing coil is shaped like the end of a Popsicle stick, which I usually have oriented as though it were a guitar string, but I also tried turning it sideways, and the overall output increased by 1dBV, but the difference between inner and outer coils was still 2dBV:

[ATTACH=CONFIG]48747[/ATTACH]

I suspect that if the coil had steel pole pieces, the difference between the inner and outer coils would be smaller than they are having AlNiCo pole pieces.
 
Antigua 5/3/2018 7:57 PM
I was doing some experiments with the exciter coil over a Stratocaster pickup, so see how the output level drops off as offset distance is added between the exciter and the pickup, and one thing I see is that when the exciter is about two millimeters off the edge of the pickup, there is a dead spot where the voltage output drops, and then it increases again at 3mm, and a little more at 4mm before dropping off again. What is happening is that in this space it's transitioning from the primary path to the return path, and that can see in the phase of the size wave.

Exciter over pickup:
[ATTACH=CONFIG]48757[/ATTACH]

Exciter 2mm off the edge
[ATTACH=CONFIG]48758[/ATTACH]

Exciter 6mm off the edge
[ATTACH=CONFIG]48759[/ATTACH]

The blue line is the voltage from the pickup. The red line is the voltage from the function generator. The frequency happens to be 5kHz, which is below resonance.

You can see that once the exciter is 6mm away, the voltage is a opposite phase from what it is when the exciter is over the coil. This means the pickup coil is generating voltage from the return path of the exciter coil. The dead spot at 2mm must represent the distance at which the primary and return paths cause a perfect cancellation.

The insight I get from this is that there must be some ideal ratio of pole piece to coil width, because the width of the pole piece will determine the width of the magnetic field, and the width of the coil will determine what ratio of primary to return path flux manifests as a voltage. If the coil is too small, the primary flux path will exceed the size of the coil itself, wasting flux. If the coil is wider, it will integrate all of the primary flux, and some non-productive lines of flux which point sideways, but if the coil is a multiple of the pole piece width, say a P-90 or a Jazzamster pickup, it will capture primary flux, unproductive sideways flux, and return path.

Another thing I can see from the oscilloscope is that the return path of flux, though weaker than the primary flux, does not drop off as quickly with distance, for example, here's the reading with the exciter 20mm away:

[ATTACH=CONFIG]48760[/ATTACH]

Even at a larger distance, the return path is still generating a consistent voltage. Assuming the magnetic field off of a guitar string has a similar overall geometry, then as the coil is made wider and wider, the more and more return path flux you end up capturing, causing an increase in phase cancellation.

I'm not sure what the ideal pole piece width to coil width ratio is, but it would also have to account for the distance between the coil and the string, since if the string is further away, that would influence the area of magnetization. My guess should be that the typical geometry of a Strat coil, where the coil's outer radius is about three times the inner radius, or the radius of the pole pieces, is probably close to idea.
 
Helmholtz 5/4/2018 7:42 AM
I was doing some experiments with the exciter coil over a Stratocaster pickup, so see how the output level drops off as offset distance is added between the exciter and the pickup, and one thing I see is that when the exciter is about two millimeters off the edge of the pickup, there is a dead spot where the voltage output drops, and then it increases again at 3mm, and a little more at 4mm before dropping off again. What is happening is that in this space it's transitioning from the primary path to the return path, and that can see in the phase of the size wave.
In this experiment you are using the PU as sensor for the flux distribution of the exciter coil. Thus your results will depend on the shape of the exciter. The real ac field between string and PU has a spatial distribution different from a solenoid (albeit squeezed) and therefore most probably penetrates the PU coil in a somewhat different way.

I generally doubt that such measurement can give precise information about the PU's relative sensitivity (H. Lemme does not recommend his method for sensitivity measurements).

But does anybody really care about a +/- 2dB difference in sensitivity as long as the sound does not change? Anyway, the more effective way to increase sensitivity with the same number of turns is a flatter, wider coil - or simply a pickup structure that allows to position the coil (not necessarily the magnets) closer to the strings.
 
Antigua 5/4/2018 3:36 PM
Quote Originally Posted by Helmholtz View Post
In this experiment you are using the PU as sensor for the flux distribution of the exciter coil. Thus your results will depend on the shape of the exciter. The real ac field between string and PU has a spatial distribution different from a solenoid (albeit squeezed) and therefore most probably penetrates the PU coil in a somewhat different way.

I generally doubt that such measurement can give precise information about the PU's relative sensitivity (H. Lemme does not recommend his method for sensitivity measurements).
Precision was not the goal. The goal is merely to demonstrate that, at a certain width, the coil ceases to be productive, and becomes counter productive. Even though this might be obvious to some, seeing it in a real bode plot proves it, for people who have to see to believe. Since all magnetic fields have a return path, it need only be a small magnetic field, as a string is small, in order to demonstrate the effect.

Quote Originally Posted by Helmholtz View Post
But does anybody really care about a +/- 2dB difference in sensitivity as long as the sound does not change?
Why make a pickup inefficient if you don't have to? The goal is simply to know all there is to know about pickup design, because among guitar players, there's just a lot of whimsy and guessing going on. Whether someone chooses to apply the information in their own pickup designs is for them to decide.
 
Helmholtz 5/4/2018 4:38 PM
The goal is merely to demonstrate that, at a certain width, the coil ceases to be productive, and becomes counter productive.
I don't think that your results can confirm this statement. Even if the local ac flux changes polarity at same distance from the PU's center, this doesn't mean that the outer turns do not contribute positively to output voltage. What matters is the integrated net flux through the loop area of each turn. And this stays positive (i.e constructive) even for the outer turns of a coil having a width well above 1.5''. This means that the outer turns of a fat coil don't diminish output, they just contribute somewhat less than the inner turns.
 
Antigua 5/4/2018 9:02 PM
Quote Originally Posted by Helmholtz View Post
This means that the outer turns of a fat coil don't diminish output, they just contribute somewhat less than the inner turns.
Something doesn't sit right with me; suppose you were to take this to an extreme, you have a pickup with a coil that is one foot in diameter, and it magically fits in the guitar. So you have about six inches of air gap, and the pole pieces are standard size. Will this pickup produce a good output, even though the turns of coil wire are six inches away from the pole pieces?
 
Mike Sulzer 5/5/2018 4:41 AM
Quote Originally Posted by Antigua View Post
Something doesn't sit right with me; suppose you were to take this to an extreme, you have a pickup with a coil that is one foot in diameter, and it magically fits in the guitar. So you have about six inches of air gap, and the pole pieces are standard size. Will this pickup produce a good output, even though the turns of coil wire are six inches away from the pole pieces?
He is not referring to a very wide coil, just a "fat" pickup. All flux cancels out if the loop is big enough, since the lines that leave the string must come back to it.
 
Helmholtz 5/5/2018 6:59 AM
All flux cancels out if the loop is big enough, since the lines that leave the string must come back to it.
Yes, and for this reason very wide loops (turns) produce near zero induced voltage. But as the voltages of all turns add up, the total EMF will not decrease by additional outer turns.
 
Mike Sulzer 5/5/2018 10:30 AM
Quote Originally Posted by Helmholtz View Post
Yes, and for this reason very wide loops (turns) produce near zero induced voltage. But as the voltages of all turns add up, the total EMF will not decrease by additional outer turns.

Yes, you are right, as long as you have the narrow turns, you get something from them, but the wide turns make a low Q low pass filter with a low frequency cutoff, and so as a practical matter, you do not get what you want.
 
Antigua 5/5/2018 11:26 AM
Quote Originally Posted by Helmholtz View Post
Yes, and for this reason very wide loops (turns) produce near zero induced voltage. But as the voltages of all turns add up, the total EMF will not decrease by additional outer turns.
So what creates a sum of voltage: all of the flux through the loop itself, or just the flux through the wire that constitutes the loop? Lets say you have a wide area turn of wire with a little pole piece in the centered and above this large loop. Even though the wide area of the wide loop has both positive and negative flux from the pole piece passing through its wide area, the actual wire of the loop only intersects with the broad return path of the pole piece in the center. Therefore, when the pole piece moves around, it will induce a voltage that is opposite phase from what it would have been had the loop been very small. This reverse-phase voltage would not be zero. Please let me know if I'm missing something.
 
Mike Sulzer 5/5/2018 11:31 AM
Quote Originally Posted by Antigua View Post
So what creates a sum of voltage: all of the flux through the loop itself, or just the flux through the wire that constitutes the loop? Lets say you have a wide area turn of wire with a little pole piece in the centered and above this large loop. Even though the wide area of the wide loop has both positive and negative flux from the pole piece passing through its wide area, the actual wire of the loop only intersects with the broad return path of the pole piece in the center. Therefore, when the pole piece moves around, it will induce a voltage that is opposite phase from what it would have been had the loop been very small. This reverse-phase voltage would not be zero. Please let me know if I'm missing something.
All the flux through the loop. The voltage exists around the path of the wire even if the wire is not there.
 
Joe Gwinn 5/6/2018 5:12 PM
Quote Originally Posted by Mike Sulzer View Post
All the flux through the loop. The voltage exists around the path of the wire even if the wire is not there.
I'd rethink that a bit. Specifically, each turn stands alone, as does each line of flux, so the voltage in each turn is the flux change in that turn. A coil of finite size in a non-uniform field will have different amounts of flux in every turn. The closer the coil is in physical shape to a single thin turn, the less variation there will be, but pickup coils are physically pretty thick and short.
 
Antigua 5/6/2018 5:35 PM
Quote Originally Posted by Mike Sulzer View Post
All the flux through the loop. The voltage exists around the path of the wire even if the wire is not there.
You're right, total flux change would increasingly approach zero, but would not go negative if the source of flux change is over the loop boundary.

The mistake I made was thinking flux "through" the wire caused a voltage and pushed current, but it's actually flux change "beside" the wire that causes voltage and then current. And so when there's a loop, it ensures that all of the flux of a given polarity beside the wire will push the current in one particular direction, thereby doing something useful. The problem with a wide loop of say, a P-90, is that instead of a "given polarity", there are two polarities, primary and return path, and though the primary flux will always be the denser of the two, as the loop becomes wider it catch more of the return flux, coming ever closer to zero net change.
 
Mike Sulzer 5/6/2018 7:56 PM
Quote Originally Posted by Joe Gwinn View Post
I'd rethink that a bit. Specifically, each turn stands alone, as does each line of flux, so the voltage in each turn is the flux change in that turn. A coil of finite size in a non-uniform field will have different amounts of flux in every turn. The closer the coil is in physical shape to a single thin turn, the less variation there will be, but pickup coils are physically pretty thick and short.
I think given the choice of "all the flux through a loop" or just the "flux trough the wire", I answered correctly. (I realize that there are some potentially confusing cases [https://en.wikipedia.org/wiki/Farada..._of_induction] and what I wrote does not cover all possibilities, but I think it is OK for what we are discussing here.)
 
Helmholtz 5/7/2018 6:08 AM
The problem with a wide loop of say, a P-90, is that instead of a "given polarity", there are two polarities, primary and return path, and though the primary flux will always be the denser of the two, as the loop becomes wider it catch more of the return flux, coming ever closer to zero net change.
Zollner's results show that the outer turns of a Jazzmaster PU on average produce as much volts per turn as the most inner windings (both 1.7V/turn) with a maximum of 2.3V/turn somewhere in the middle of the coil (numbers are for reference, actual values depend on several variables).
The JM PU produced about twice the output of a vintage strat PU. This is mainly due to the wider and flatter coil. The flatter coil allows the majority of the turns to be closer to the strings, thus increasing efficiency. EMF yield per turn decreases strongly with distance from the strings.

This also demonstrates that the flux returning from the string is not confined within the aperture but spreads out over an area considerably wider. Consequently a coil can be made pretty wide (wider than JM and P-90 coils ) before the outer turns cease to contribute to output noticeably and merely increase L and DCR.

I can't see a "problem" with JM or P-90 shaped coils.
 
Antigua 5/7/2018 8:34 AM
Quote Originally Posted by Helmholtz View Post
Zollner's results show that the outer turns of a Jazzmaster PU on average produce as much volts per turn as the most inner windings (both 1.7V/turn) with a maximum of 2.3V/turn somewhere in the middle of the coil (numbers are for reference, actual values depend on several variables).
The JM PU produced about twice the output of a vintage strat PU. This is mainly due to the wider and flatter coil. The flatter coil allows the majority of the turns to be closer to the strings, thus increasing efficiency. EMF yield per turn decreases strongly with distance from the strings.

This also demonstrates that the flux returning from the string is not confined within the aperture but spreads out over an area considerably wider. Consequently a coil can be made pretty wide (wider than JM and P-90 coils ) before the outer turns cease to contribute to output noticeably and merely increase L and DCR.

I can't see a "problem" with JM or P-90 shaped coils.
Do you remember which resource from Zollner included the Jazzmaster pickup coil analysis? Was is somewhere in PotEG?

I think that even if the primary-path flux contribution covers a wide area of a wide coil, that it would be important to note that the harmonic make up of the flux change, as caused by the harmonically moving guitar string, would be based on a smaller aperture, one that is closer in size to the pole piece.
 
Helmholtz 5/7/2018 9:28 AM
Do you remember which resource from Zollner included the Jazzmaster pickup coil analysis? Was is somewhere in PotEG?
This information is contained in the coloured illustrations of page 5-31, German book version of PotEG.

I think that even if the primary-path flux contribution covers a wide area of a wide coil, that it would be important to note that the harmonic make up of the flux change, as caused by the harmonically moving guitar string, would be based on a smaller aperture, one that is closer in size to the pole piece.
Sorry, this sentence is too complicated for me. But sounds like pure speculation without reasoning. Aperture corresponds to the magnetized string length. It is roughly given by twice the pole diameter of a SC (as measured by Zollner). Extension of primary flux close to string must be smaller than aperture. I have not seen any evidence of a frequency-dependance of the aperture up to now.
 
bbsailor 5/7/2018 9:47 AM
Quote Originally Posted by Helmholtz View Post
Zollner's results show that the outer turns of a Jazzmaster PU on average produce as much volts per turn as the most inner windings (both 1.7V/turn) with a maximum of 2.3V/turn somewhere in the middle of the coil (numbers are for reference, actual values depend on several variables).
The JM PU produced about twice the output of a vintage strat PU. This is mainly due to the wider and flatter coil. The flatter coil allows the majority of the turns to be closer to the strings, thus increasing efficiency. EMF yield per turn decreases strongly with distance from the strings.

This also demonstrates that the flux returning from the string is not confined within the aperture but spreads out over an area considerably wider. Consequently a coil can be made pretty wide (wider than JM and P-90 coils ) before the outer turns cease to contribute to output noticeably and merely increase L and DCR.

I can't see a "problem" with JM or P-90 shaped coils.
Yes, there are many variables to consider. Jazzmaster pickups with their short magnets project a different magnetic field compared to a Stratocaster type pickup with longer magnets. Then there is increasing magnetic strength but at the expense of potentially damping the string vibration. Look at the mechanical design of a P90 type pickup. https://courses.physics.illinois.edu...eport_Sp10.pdf

Two rectangular bar type magnets with the same magnetic pole facing the metal bar which holds the string pole pieces spreads the magnetic field differently than the magnetic field in a Jazzmaster with short round magnets.

Given that the initial design of the guitar pickup was to generate enough voltage to drive a high impedance tube-based amplifier input, all early passive pickups used 5,000 to 10,000 turns of AWG 42 to AWG 44 wire with different sonic consequences. Adding more turns typically increases the output but changes the sound. The human ear is very sensitive to the initial 30 to 50 milliseconds of the pickup attack transient and when playing with other instruments allows the guitar transient to provide accents to the sound. Short wide pickups like a P90 of Jazzmaster keep more wire closer to the string up to the point where adding more wire has a diminishing return not only in output level but in length of the string that is generating voltage with some upper harmonics being out of phase with very wide pickup designs.

It all gets down to understanding the desired effect of what we do to alter the pickup design with the technical understanding that simply adding more turns to increase the output has other consequences as well. Our ears are the best test device for what sounds well while the technical analysis has evolved to better understand the variables such as:
1. coil turn numbers
2. coil shape (tall and narrow versus short and wide)
3. magnet size tall or short
4. magnet type
5. amount of ferrous metal in the pickup design
6. coil wire size, insulation dielectric
7. winding style, machine wind (tightly wound) versus hand wind (more scatter)
8. distance from strings
9. loading by passive controls
10. the effect of cable capacitance and amplifier input impedance

When you listen to an acoustic guitar with a non magnetic pickup typically mounted in the bridge, you hear a much different sound than any magnetic type guitar as any magnetic type pickup has the typical "electric guitar sound". This is because at low listening levels the human ear is most sensitive between about 3 to 5 kHz where most electric guitar pickups have some resonant peak. Look up Fletcher-Munson Curve to see how the sensitive region of human hearing typically matches the resonant area of most guitar pickups. Granted, magnetic guitar pickups will never pickup any acoustic qualities but there is an "electric guitar" sound.

Magnetic pickups can be made to sound less electric or more acoustic by not putting the resonant point in the 3 to 5kHZ range by winding about one tenth the amount of turns (500 to 600 turns) and then use active electronics to boost the signal or carefully select a mic matching transformer mounted at the amp end of the cable to eliminate the cable capacitance effect or just target the input impedance of a mic mixer with an actual input impedance of 2400 ohms which properly loads a low impedance microphone rated at 150 ohms but with an actual range of about 100 to 250 ohms.

After market pickups evolved to target famous guitar player sounds with all types of marketing terms such as Eric Clapton "woman tone". Then, when we collectively like a certain type of sound, we turn to technology such as Zollner's work to attempt to decompose the pickup design to better understand all the variables involved.

Joseph J. Rogowski
 
Antigua 5/7/2018 10:04 AM
Quote Originally Posted by Helmholtz View Post
This information is contained in the coloured illustrations of page 5-31, German book version of PotEG.
Thanks, I'll take a look.

Quote Originally Posted by Helmholtz View Post
Sorry, this sentence is too complicated for me. But sounds like pure speculation without reasoning.
Wouldn't it make sense to ask for clarification before passing judgement?

Quote Originally Posted by Helmholtz View Post
Aperture corresponds to the magnetized string length. It is roughly given by twice the pole diameter of a SC (as measured by Zollner). Extension of primary flux close to string must be smaller than aperture. I have not seen any evidence of a frequency-dependance of the aperture up to now.
J Donald Tillman has authored a few web pages on the subject Response Effects of Guitar Pickup Position and Width and Guitar Pickup Response Demonstration . Even though it's based on theory and doesn't contain empirical evidence, the reasoning is sound: if the physical length of the harmonic oscillation is close to half that of the aperture width, then you will have two opposite phase instances of that harmonics within the aperture, causing a cancellation of that harmonic. For nearly all pickups on the market, the pole piece is small enough that the harmonic cancellations would effect very high frequencies.
 
Antigua 5/7/2018 10:41 AM
Quote Originally Posted by bbsailor View Post
Short wide pickups like a P90 of Jazzmaster keep more wire closer to the string up to the point where adding more wire has a diminishing return not only in output level ...
I'm skeptical as to how much output is produced by the outer winds of a Jazzmaster pickup in particular, because it is especially flat, but very similar to a Stratocaster pickup otherwise, and yet I don't suspect that a Jazzmaster pickup is correspondingly louder than a Strat pickup, having both types of guitar and using them often. If we were to assume the outer winds were on par with the inner winds in terms of voltage, then a JM pickup should be loud, louder than a Filter'tron, nearly as loud as PAF.

I'm willing to put this to the test, though. I'll find one of each with similar electrical values and compare them.

Quote Originally Posted by bbsailor View Post
... but in length of the string that is generating voltage with some upper harmonics being out of phase with very wide pickup designs.
Assuming we're talking about the Tillman comb filtering I linked above, it's a given the flux is most dense above the pole piece, and it that spreads outwards over the pickup, towards the string. At this point, the string is as magnetized as much as it can be, and making the coil wider doesn't change this area of magnetization. Therefore, the coil plays the role of observing that which has already been determined, and so it neither increases nor decreases Tillman-described harmonic cancellations.

I agree with you that the shape of the magnetic field differs with the more stubby AlNiCo pole piece, compared to the taller pole piece of a Strat/Tele/Jaguar pickup, due to the lower coercive force of AlNiCo causing it's field shape to be more subject to the geometry of the magnetic itself, but referencing Tillman again, the difference in aperture width, of say two or three millimeters, would be too trivial to make an audible difference.
 
Helmholtz 5/7/2018 10:45 AM
J Donald Tillman has authored a few web pages on the subject Response Effects of Guitar Pickup Position and Width and Guitar Pickup Response Demonstration . Even though it's based on theory and doesn't contain empirical evidence, the reasoning is sound: if the physical length of the harmonic oscillation is close to half that of the aperture width, then you will have two opposite phase instances of that harmonics within the aperture, causing a cancellation of that harmonic. For nearly all pickups on the market, the pole piece is small enough that the harmonic cancellations would effect very high frequencies.
Yes, this is the well known comb filter effect caused by a finite aperture. The calculation is based on a constant aperture. The author's statement that the aperture is given by the width of the PU is wrong. The shape of the coil doesn't influence the aperture. The measured aperture of a P-90 (10..13mm) is only marginally wider than that of a strat PU (8..10mm).
 
Antigua 5/7/2018 10:49 AM
Quote Originally Posted by Helmholtz View Post
Yes, this is the well known comb filter effect caused by a finite aperture. The calculation is based on a constant aperture. The author's statement that the aperture is given by the width of the PU is wrong. The measured aperture of a P-90 (10..13mm) is only marginally wider than that of a strat PU (8..10mm).
You had said "I have not seen any evidence of a frequency-dependance of the aperture up to now.", maybe this is a simple miscommunication, but I would describe comb filtering as a frequency dependence.
 
Helmholtz 5/7/2018 10:56 AM
You had said "I have not seen any evidence of a frequency-dependance of the aperture up to now.", maybe this is a simple miscommunication, but I would describe comb filtering as a frequency dependence.
It is not a frequency-dependance that is influenced by the shape of the coil. The returning flux has the same frequency-dependance as the primary flux.
 
Mike Sulzer 5/7/2018 11:21 AM
Quote Originally Posted by Helmholtz View Post
This information is contained in the coloured illustrations of page 5-31, German book version of PotEG.



Sorry, this sentence is too complicated for me. But sounds like pure speculation without reasoning. Aperture corresponds to the magnetized string length. It is roughly given by twice the pole diameter of a SC (as measured by Zollner). Extension of primary flux close to string must be smaller than aperture. I have not seen any evidence of a frequency-dependance of the aperture up to now.

There is another component to the aperture, more important for steel pole pieces than alnico. A high permeability pole piece increases the flux from the vibrating string most effectively when the source of the flux is right over it. You can see this by using a very small exciter coil and making measurements in various positions. This is not a big effect, but because of it, the aperture from the two effects is somewhat smaller than from the magnetization alone.
 
Antigua 5/7/2018 11:22 AM
Quote Originally Posted by Helmholtz View Post
It is not a frequency-dependance that is influenced by the shape of the coil.
It sounds like you're defining the coil as the aperture. I suppose there is a discussion to have as to what constitutes the aperture as a whole, but I'd rule out considering only the coil alone.

Quote Originally Posted by Helmholtz View Post
The returning flux has the same frequency-dependance as the primary flux.
Exactly, any size or shape of coil is going to receive the same information, it's just a question of amplitude.
 
Mike Sulzer 5/7/2018 11:29 AM
Quote Originally Posted by bbsailor View Post
Two rectangular bar type magnets with the same magnetic pole facing the metal bar which holds the string pole pieces spreads the magnetic field differently than the magnetic field in a Jazzmaster with short round magnets.
I think the pole pieces (near the strings) affect the shape of the field at the strings more than the magnets below the coil. What do you get if you take out the pole pieces while holding the magnets in place?
 
bbsailor 5/7/2018 11:54 AM
Quote Originally Posted by Mike Sulzer View Post
I think the pole pieces (near the strings) affect the shape of the field at the strings more than the magnets below the coil. What do you get if you take out the pole pieces while holding the magnets in place?
The pole pieces simply put the magnetic field up over the top of the bobbin and closer to the string than if they were removed. I would suspect that even with the pole pieces removed there would be less output from the pickup since the lower bar in the center of like pole magnets would still project some magnetic field up to the strings. I would guess that observing the FEMM models of the magnetic field of various style pickups:

Tall cylinder magnets and types
Short cylinder magnets and types
Metal plates under the pickup
P90 with center focused like poles under the bobbin

would provide a visual indication of what coil shape would produce more voltage from either the inner winding, outer winding or the upper winding compared to the lower winding. This is from a string rather than a stimulation coil that may not accurately represent the voltage distribution in various sections of a pickup coil (using a stimulation coil).

Joseph J. Rogowski
 
Helmholtz 5/7/2018 12:51 PM
It sounds like you're defining the coil as the aperture. I suppose there is a discussion to have as to what constitutes the aperture as a whole, but I'd rule out considering only the coil alone.
Definitely not. Please read my posts carefully. The shape of the coil has no influence on aperture. But I thought we were talking about the effects of wide and/or fat coils. I only mentioned aperture because it kind of determines the width of the source of the returning ac flux.
 
Mike Sulzer 5/7/2018 1:12 PM
Quote Originally Posted by bbsailor View Post
The pole pieces simply put the magnetic field up over the top of the bobbin and closer to the string than if they were removed. I would suspect that even with the pole pieces removed there would be less output from the pickup since the lower bar in the center of like pole magnets would still project some magnetic field up to the strings. I would guess that observing the FEMM models of the magnetic field of various style pickups:

Tall cylinder magnets and types
Short cylinder magnets and types
Metal plates under the pickup
P90 with center focused like poles under the bobbin

would provide a visual indication of what coil shape would produce more voltage from either the inner winding, outer winding or the upper winding compared to the lower winding. This is from a string rather than a stimulation coil that may not accurately represent the voltage distribution in various sections of a pickup coil (using a stimulation coil).

Joseph J. Rogowski
If you remove the pole pieces, the source of the magnetic field is farther from the strings. This makes the field at the strings weaker, but also less focused so that the field that remains is more spread out along the strings. Thus the aperture is wider.
 
Antigua 5/7/2018 1:29 PM
Quote Originally Posted by bbsailor View Post
The pole pieces simply put the magnetic field up over the top of the bobbin and closer to the string than if they were removed. I would suspect that even with the pole pieces removed there would be less output from the pickup since the lower bar in the center of like pole magnets would still project some magnetic field up to the strings. I would guess that observing the FEMM models of the magnetic field of various style pickups:
Regarding the contribution of the AlNiCo bars in a fully assembled P-90, I'm still in the process of learning, but my understanding is that the voltage generated through the loop of wire depends on a change in flux density through the area of the loop, and the more more perpendicular the lines of flux are through the area, the more voltage you get. It seems to me that if you "stretch" the magnetic field so that it fans out over a wider area, with AlNiCo bars or whatever, you decrease the tendency for lines of flux to be perpendicular with the loops of wire in the coil, and you cause them to become more sideways. My feeling is that at best you get a net zero change, if not a loss.

The big caveat is that AlNiCo has a rather low permeability, and so it will not draw the magnetic field out as well as two steel bars would in place of the AlNiCo bars.
 
Antigua 5/7/2018 3:19 PM
Quote Originally Posted by Mike Sulzer View Post
If you remove the pole pieces, the source of the magnetic field is farther from the strings. This makes the field at the strings weaker, but also less focused so that the field that remains is more spread out along the strings. Thus the aperture is wider.
This could be a means of creating a very wide aperture, for the sake of demonstration: take guitar with a P-90 pickup, remove the pole piece from under a string and it's immediate neighbors. Then maybe see how that plucked string sounds as is, probably very weak. Then, hold a bar magnet over the pickup, perpendicular to the strings, which should create a very narrow aperture. After that, hold the bar parallel to the string, which, assuming the magnetism along the length of the bar magnet is relatively uniform, should create an aperture with a width of about 2 1/2 inches. The fact that the magnetized segment of string exceeds the boundaries of the coil would cause some cancellation, but not total cancellation. Harmonics lengths that receive uniform magnetization (or close to it) should cancel, and they should be of a low enough frequency to be audible. I'll give this a try later and see what happens.
 
Helmholtz 5/7/2018 3:35 PM
There is another component to the aperture, more important for steel pole pieces than alnico. A high permeability pole piece increases the flux from the vibrating string most effectively when the source of the flux is right over it. You can see this by using a very small exciter coil and making measurements in various positions. This is not a big effect, but because of it, the aperture from the two effects is somewhat smaller than from the magnetization alone.
Lacking a better definition, I tend to see the aperture as the effective length of string the PU can read and over which string motion is averaged. While high pole pieces better focus ac flux and thus influence flux distribution in the coil, I don't see how this could change the string length being sensed, even if the returning flux gets somewhat squeezed. After all the aperture of the P-90 was measured with these influences taking effect.
 
bbsailor 5/7/2018 4:11 PM
Quote Originally Posted by Antigua View Post
Regarding the contribution of the AlNiCo bars in a fully assembled P-90, I'm still in the process of learning, but my understanding is that the voltage generated through the loop of wire depends on a change in flux density through the area of the loop, and the more more perpendicular the lines of flux are through the area, the more voltage you get. It seems to me that if you "stretch" the magnetic field so that it fans out over a wider area, with AlNiCo bars or whatever, you decrease the tendency for lines of flux to be perpendicular with the loops of wire in the coil, and you cause them to become more sideways. My feeling is that at best you get a net zero change, if not a loss.

The big caveat is that AlNiCo has a rather low permeability, and so it will not draw the magnetic field out as well as two steel bars would in place of the AlNiCo bars.
Antigua,

To further your research, look for a MEF thread labeled “Moving coil pickup for the technically curious”. All you need is a small audio transformer with an 8 ohm or 4 ohm low side to anywhere from 10K to 20K on the high side. Simply alligator clip the transformer low side from behind the nut to behind the bridge on an acoustic guitar or an electric guitar where the strings are not electrically connected in parallel with the test string. Attach the high impedance side to the amp input. Take a long rectangular magnet and place it perpendicular under the string and measure or listen to the plucked string output. Now, rotate the magnet to be parallel with the string so a longer magnet window is stimulating the string and listen or measure a higher output.

With this setup you want the wires feeding the transformer to be one tenth the resistance of the string under test to minimize resistive losses. The 8 ohm winding acts as a bridging input about 7 to 10 times higher than the impedance/resistance of the string to produce the highest output. If you measure the raw voltage with a scope probe across the test string you will see a raw voltage in the low millivolt range. The turns ratio of the audio transformer will boost the output from 25 to 50 times depending on its turns ratio. To find the approximate turns ratio of any transformer, divide the impedance of the low primary side into the value of the high secondary side and take the square root of that number. Example 20000/8 equals 2500. The square root of 2500 is 50 for a turns ratio of 1 to 50.

Tinkering with this stuff is a great way to learn things that give you insight into pickup design and performance.


Joseph J. Rogowski
 
Mike Sulzer 5/8/2018 4:59 AM
Quote Originally Posted by Helmholtz View Post
Lacking a better definition, I tend to see the aperture as the effective length of string the PU can read and over which string motion is averaged. While high pole pieces better focus ac flux and thus influence flux distribution in the coil, I don't see how this could change the string length being sensed, even if the returning flux gets somewhat squeezed. After all the aperture of the P-90 was measured with these influences taking effect.

Yes, but it is this statement that needs some modification: "Aperture corresponds to the magnetized string length." Magnetize a short section of the string half a meter from the pickup, and it contributes very little to the signal. Move the magnetized section closer, and it contributes more. The coil and the pole piece determine how much, and so they are involved in this component of the aperture, although this is not a big effect for magnetization over the coil.
 
Helmholtz 5/8/2018 7:32 AM
Quote Originally Posted by Mike Sulzer View Post
Yes, but it is this statement that needs some modification: "Aperture corresponds to the magnetized string length." Magnetize a short section of the string half a meter from the pickup, and it contributes very little to the signal. Move the magnetized section closer, and it contributes more. The coil and the pole piece determine how much, and so they are involved in this component of the aperture, although this is not a big effect for magnetization over the coil.
Do you mean that PUs with different cores differently weight the point contributions along the aperture length and thus produce differently shaped aperture functions?
While such effect is conceivable, it would show especially in different slopes of the aperture functions. However the measured aperture functions of the JM pu and the P-90 are extremely similar in shape.
From this I conclude that such aperture shaping effects must be very small in typical PU applications. My description (not definition) of the aperture as "magnetized string length" may not be completely precise but is quite useful in practice.
 
Mike Sulzer 5/8/2018 8:26 AM
Quote Originally Posted by Helmholtz View Post
Do you mean that PUs with different cores differently weight the point contributions along the aperture length and thus produce differently shaped aperture functions?
While such effect is conceivable, it would show especially in different slopes of the aperture functions. However the measured aperture functions of the JM pu and the P-90 are extremely similar in shape.
From this I conclude that such aperture shaping effects must be very small in typical PU applications. My description (not definition) of the aperture as "magnetized string length" may not be completely precise but is quite useful in practice.
The effect is more than conceivable, it is significant, if not large. Since the aperture function is not rectangular, it is subject to a definition, such the distance between half power points in the direction along the string. Therefore, the aperture width is a function of everything that affects the aperture function.
 
David King 5/8/2018 9:39 AM
Like the "Unknown unknowables"?
 
Helmholtz 5/8/2018 9:51 AM
The effect is more than conceivable, it is significant, if not large.
This is an unsubstantiated claim.
I gave numbers and arguments that support my evaluation of the effect. You are free to do your own aperture measurements and demonstrate the influence of different cores. I am willing to give this another thought when there is valid evidence in realistic situations.
For reference, the shape of the aperture function of SCs is similar to a Gaussian (no matter if alnico or low carbon steel cores) and I used the -16dB points for the aperture lengths I specified.
 
Mike Sulzer 5/8/2018 11:06 AM
Quote Originally Posted by Helmholtz View Post
This is an unsubstantiated claim.
I gave numbers and arguments that support my evaluation of the effect. You are free to do your own aperture measurements and demonstrate the influence of different cores. I am willing to give this another thought when there is valid evidence in realistic situations.
For reference, the shape of the aperture function of SCs is similar to a Gaussian (no matter if alnico or low carbon steel cores) and I used the -16dB points for the aperture lengths I specified.

Lots of things give approximately Gaussian shapes. Two factors that have Gaussian shapes multiply together to give another Gaussian. So I do not think that you have shown that the string magnetization alone sets the aperture. The measurements that I have made with a small exciter coil show that the sensitivity drops off as the coil is moved away from an initially centered location.
 
Antigua 5/9/2018 12:12 AM
The difference of coercive force between AlNiCo and steel would cause the shape of the magnetic field around them to differ, because when the coercivity is low, the material more readily gives way to the shape of the magnetic field of the aggregated magnetic circuit, whereas if the coercivity is high, the flux contributed by the pole piece will be less influenced by the external fields, and more a function of it's own physical shape. An example would be a PAF that has it's slug replaced with AlNiCo pole pieces. The slugs would have a magnetic field that is influenced by the other magnetized parts of the humbucker, but the AlNiCo's magnetic field would be relatively independent of them. That being said, since the aperture is too narrow to cause audible comb filtering in either case, it probably doesn't matter.

Another thing occurred to me, and it probably explains why the outer tapped coil generated more voltage than the inner tapped coil; if the aperture is wider than the wire loop, then the flux that is outside of the loop will have a cancelling effect. You get voltage when flux of a given polarity passes through the loop, but if that same polarity is also outside of the loop, it generates opposite phase voltage because it's pushing into the return path of the loop. Therefore, the most productive turns of wire might not be the ones that are closest to the core, but would be those that are most optimally sized for the aperture, which also changes in size as the string is nearer or further from the string. So the most productive coil loops would not only change as the pickup is raised and lowered, but as the string moves around. This could also mean that a more optimal pickup would have a buffer between the pole pieces and the coil, which plastic bobbins have by default, while Fender style AlNiCo pickups don't.
 
Helmholtz 5/9/2018 7:33 AM
The difference of coercive force between AlNiCo and steel would cause the shape of the magnetic field around them to differ, because when the coercivity is low, the material more readily gives way to the shape of the magnetic field of the aggregated magnetic circuit, whereas if the coercivity is high, the flux contributed by the pole piece will be less influenced by the external fields, and more a function of it's own physical shape.
Your conception of the coercive force (Hc) seems completely wrong. Hc has absolutely nothing to do with the PU's magnetic field. What matters is flux distribution and ac . Hc is a constant, a specific parameter of the ferromagnetic material. It does not show in magnetic circuits.


Another thing occurred to me, and it probably explains why the outer tapped coil generated more voltage than the inner tapped coil; if the aperture is wider than the wire loop, then the flux that is outside of the loop will have a cancelling effect. You get voltage when flux of a given polarity passes through the loop, but if that same polarity is also outside of the loop, it generates opposite phase voltage because it's pushing into the return path of the loop. Therefore, the most productive turns of wire might not be the ones that are closest to the core, but would be those that are most optimally sized for the aperture, which also changes in size as the string is nearer or further from the string. So the most productive coil loops would not only change as the pickup is raised and lowered, but as the string moves around. This could also mean that a more optimal pickup would have a buffer between the pole pieces and the coil, which plastic bobbins have by default, while Fender style AlNiCo pickups don't.
As already mentioned, the measurements of Zollner show that the signal yield of turns increases with the width of the turns within the first millimeters from the core. As a consequence, the outer turns of a strat PU contribute somewhat more than the inner ones. The local ac flux density was found to remain positive up to a distance of around 6mm from the magnet, from where it changes polarity. As long as the inner flux density does not change polarity, wider loops see more total flux and produce more voltage.
 
Antigua 5/9/2018 12:13 PM
Quote Originally Posted by Helmholtz View Post
Your conception of the coercive force (Hc) seems completely wrong. Hc has absolutely nothing to do with the PU's magnetic field.
I'm just curious, why do you feel it necessary to use hyperbolic adjectives like "completely wrong" and "absolutely nothing"? It makes this discourse less pleasant.

My understanding stems from this:

https://www.duramag.com/techtalk/aln...from-alnico-5/
One obvious question that may be asked is why does an Alnico 5 magnet have a similar Residual Induction as some Rare Earth magnets like Neodymium Iron Boron, but are apparently weaker than the Rare Earth magnets? This is because the Rare Earth magnets have a much higher Coercive Force. As described above, their self-demagnetization impact is much lower when the alloy has a high Coercive Force. Large magnetic fields can be generated with Alncio 5, but the magnet must be very magnetically long and use iron / steel elements to help reduce the effects of self-demagnetization.
That I'm seeing in this explanation, is that the lack of coercivity of AlNiCo causes it to be weaker because it magnetically interacts with itself, resulting in a lower sum of flux density out of the top of the magnet's polar faces. But that Br flux has to go somewhere else instead, I would suppose it must fan outward, away from the polar axis. It appears to me that they generally construct AlNiCo so that the magnet's length will be much longer than it's width in order to mitigate the consequences of a low Hc.


Quote Originally Posted by Helmholtz View Post
As already mentioned, the measurements of Zollner show that the signal yield of turns increases with the width of the turns within the first millimeters from the core. As a consequence, the outer turns of a strat PU contribute somewhat more than the inner ones. The local ac flux density was found to remain positive up to a distance of around 6mm from the magnet, from where it changes polarity. As long as the inner flux density does not change polarity, wider loops see more total flux and produce more voltage.
Maybe so, I'm just corroborating the observation with reasoning. Maybe I'm just talking to myself.
 
Helmholtz 5/10/2018 7:56 AM
Quote Originally Posted by Mike Sulzer View Post
Yes, but it is this statement that needs some modification: "Aperture corresponds to the magnetized string length." Magnetize a short section of the string half a meter from the pickup, and it contributes very little to the signal. Move the magnetized section closer, and it contributes more. The coil and the pole piece determine how much, and so they are involved in this component of the aperture, although this is not a big effect for magnetization over the coil.
Our discussion made me curious. So I did some radial flux density measurements along the (PU facing) under side of the strings above the PUs of a strat and a P-90 equipped LP. I used a Gaussmeter and also checked with a flux detector foil, which shows the areas where the flux runs parallel to the string.

The results were:
- The aperture lenghts as measured by Zollner roughly correspond to the extension of the primary flux as determined from the points of zero radial flux density.
- The returning flux, having much lower B values than the primary flux, extends much farther and can be detected positively up to distances of over 1 inch from the center of the poles on each side.

I have to realize that „magnetized string length“ is not a useful measure for the aperture, but I am glad I did these measurements. They provide another piece of the puzzle and learning something always makes me glad.

I understand your point that the aperture (as defined by the length of string the PU can read) must be influenced by the PU's directional sensitivity characteristic. But it seems that coil width and core material have no strong influence in real guitar use at least between the PU types measured, while magnet strength and PU/string distance do.

My original point was that aperture is not given by the width dimension of the PU.

I need to mention that I measured DC (static) flux. The paths and distribution of the AC (alternating) flux content may differ.
 
Antigua 5/10/2018 2:34 PM
Here's a practical question: based on the information at hand, will an 8,000 turn Stratocaster pickup produce a higher voltage if a spacer is placed between the pole pieces and coil? This way, the first wind is not directly against the pole piece, but instead a couple millimeters away from the pole piece. If so, this could constitute a genuine improvement over existing products. 43AWG or a more carefuly wind pattern might be required in order to still fit the cover over the pickup, due to a wider coil diameter.
 
Helmholtz 5/10/2018 3:07 PM
I would say yes, if you think around 2dB more output justifies the effort. But I guess that making the coil shorter with the magnets protruding from the bottom by 2 or 3mm or so is a more effective way to increase output with the same number of turns. Alternatively you could use double or triple bottoms.
 
Antigua 5/10/2018 4:04 PM
Quote Originally Posted by Helmholtz View Post
I would say yes, if you think around 2dB more output justifies the effort.
It certainly does, if you consider that something like a Texas Special is billed as being a hotter pickup, but with only perhaps 500 more turns of wire, just barely realizes a ~2dB difference. This would achieve the same boost without also making the pickup darker (so long as the inductance doesn't climb too much as a result of the wider loop areas).
 
rjb 5/10/2018 6:17 PM
Quote Originally Posted by Antigua View Post
...Texas Special is billed as being a hotter pickup, but ... barely realizes a ~2dB difference.
This would achieve the same boost without also making the pickup darker...
How much output do you need?
If I'm not mistaken, simple height adjustment can typically realize a >2dB change....

I would wager that, whether they realize it or not, most "players" <snark>(versus "physics weenies")</snark> who opt for "hotter" pickups actually choose them for their "warmer" or "raunchier" (AKA "darker") tone- not for a potential 2dB increase in output level.

-rb
 
Antigua 5/10/2018 9:18 PM
Quote Originally Posted by rjb View Post
How much output do you need?
If I'm not mistaken, simple height adjustment can typically realize a >2dB change....

I would wager that, whether they realize it or not, most "players" <snark>(versus "physics weenies")</snark> who opt for "hotter" pickups actually choose them for their "warmer" or "raunchier" (AKA "darker") tone- not for a potential 2dB increase in output level.

-rb
I agree completely, it really is just about getting a darker tone. But if the customer, in their customerly wisdom, says they want output, let them have their output, I say.
 
Antigua 5/17/2018 11:09 PM
Here is a bode plot comparison between an SD SSL-1, Lollar Special S Middle and a Fender Jazzmaster pickup. The Jazzmaster has a wide and flat coil, while the SSL-1 and the Special S have an average Strat shaped coils. In this test, an external coil drives the pickups, with the coil placed dead center above the bobbin, where there is a whole in the fiber board to accommodate the winding machine's mandrel, so that the vertical distance between the coils is about the same in each case.

[ATTACH=CONFIG]48868[/ATTACH]

The Jazzmaster pickup achieves a higher inductance for a lower DC resistance than the Strat pickups, presumably due to the wider coil area. The Jazzmaster and the SSL-1 are comparable in terms of DC resistances, both being around 6.5k, while the Jazzmaster and Special S are comparable in terms of inductance, both being about 3.1H. The Special-S has a higher DC resistance than either the Jazzmaster or the SSL-1, at 7.5k ohms.

Based on the test, the Special-S only appears to generate 0.9dBV greater output than the SSL-1 at 1kHz, while the Jazzmaster pickup generates 3.1dBV greater voltage than the SSL-1, or 2.2dBV greater than the Special-S. So it seems that when either DC resistance or inductance are matched, the wide and flat Jazzmaster coil still generates more voltage output than the Strat pickup in either case. This makes sense, given that more turns of the Strat coil are located further from the alternative magnetic source, but despite it maybe being obvious, at least this represents a quantifiable demonstration.

Aside from the difference in output, an interesting aspect of the Jazzmaster pickup is that it has a much higher resonant peak, despite also having a higher inductance, which means the intrinsic capacitance is a lot lower. The SSL-1's capacitance calculates out to about 100pF, the Lollar about 90pF, while the Jazzmaster's only comes out to only 30pF. I'm not sure why the capacitance is so much lower. Is a tall, thin coil conducive to capacitance, while a short, fat coil is not?
 
Helmholtz 5/18/2018 7:54 AM
Is a tall, thin coil conducive to capacitance, while a short, fat coil is not?
Yes, this is a general principle. Short, fat coils have more layers than thinner, longer coils with a similar number of turns. This keeps inner and outer windings farther apart thus reducing distributed capacitance.
 
Antigua 5/18/2018 9:03 AM
Quote Originally Posted by Helmholtz View Post
Yes, this is a general principle. Short, fat coils have more layers than thinner, longer coils with a similar number of turns. As a consequence the partial voltage between layers is lower and the total voltage spreads over more layers. Another way to view this is that short, flat coils keep inner and outer windings farther apart thus reducing distributed capacitance.
So greater distances between portions of coil lowers capacitance; a tall, thin coil also puts a lot of distance between the top-most and bottom-most winds, so are you saying that you get less capacitance with the short-flat coil because the starting winds and the ending winds are set farther apart, as opposed to placing the winds-per-layer farther apart?
 
Helmholtz 5/18/2018 9:58 AM
So greater distances between portions of coil lowers capacitance; a tall, thin coil also puts a lot of distance between the top-most and bottom-most winds, so are you saying that you get less capacitance with the short-flat coil because the starting winds and the ending winds are set farther apart, as opposed to placing the winds-per-layer farther apart?
Yes, it can be shown that the partial capacitances between adjacent turns having larger voltage difference contribute most. The direct neighboring turns within the same layer almost have the same voltage, so their contribution is small. The contribution of the capacitances between neighboring layers is much stronger.