Hout 2/14/2018 4:42 AM
Switchable additional winds
Hi all, first post here

I have wound a few pickups for my own basses and I was wondering about the the possibilities of adding an additional circuit to a single coil to get more tonal options.
I am not completely new to this, but by no means any where near an expert, ore like a trial and error diy-er. I did a search and couldn't find anything on it, but since I'm not so familiar with the terminology I might have missed it, so apologies in advance if this has been discussed before.

So here's my idea:
When winding a pickup you stop at a certain point, cut the wire, and lead it somewhere it is fixed and not in the way, and then continue with a new wire, until you end up with an "overwound" pickup. These two coils are then hooked up through a dptp switch so you can switch between running the normal (or underwound) coil to the amp and normal+extra wounds to the amp. I made this following schematic to hopefully explain some more.
[ATTACH=CONFIG]47092[/ATTACH]

As I tried to depict, the coils are wound and wired in the same direction, so there will be no hum cancelling of course. I figure there would be a difference in volume from the two options, so maybe I can fiddle around to get an inline resistor or a trimpot to even out the output.

So what do think? Has anyone tried this? Would it work? Am I overlooking something?
 
darkfenriz 2/14/2018 5:11 AM
Cool idea, why not.
 
LtKojak 2/14/2018 6:09 AM
Quote Originally Posted by Hout View Post
Has anyone tried this?
Your idea is called a "coil tap", and it's been around for ages.
 
Hout 2/14/2018 7:43 AM
Quote Originally Posted by LtKojak View Post
Your idea is called a "coil tap", and it's been around for ages.
There you go So thát's the term
So the wiring schematic, is that correct? Or am I inducing a lot of trouble by using an additional resistor?

EDIT:
As I now know this is called a coil tap, do I actually need to use dpdt switching, or would it suffice to solder a "tap" wire somewhere mid way and just use that one coming out the middle of coil, and one out of the end? Then I could use a normal ON-ON switch for this.
 
J M Fahey 2/14/2018 10:34 PM
Not sure what you mean by ON-ON , what you need is a SPDT .
Ground is always connected and output is taken either from the full winding or from the tap.
I would not rush to compensate for overwound higher output, user will *expect* it to be louder than the less wound one, otherwise what´s the point?
 
Chuck H 2/14/2018 10:42 PM
It's also unfortunate that the term "coil tap" is used in novice circles to indicate splitting humbucker coils and running them as singles. Which, of course, is a coil split. But some bigger name manufacturers make the same error in the lit sometimes. I can only assume this is an attempt to communicate effectively with the electronically illiterate guitar players they cater to. At least I hope so. But misuse of the term makes it hard to know what someone is selling.
 
epizootics 2/14/2018 10:55 PM
That's one of the cool things when diving into the wonderful world of guitar pickups - you end up reinventing the wheel every other week, which is comforting in a way because it puts the process back into a human perspective. The very fact you reach the same conclusions as pickup makers back in the day should tell you you're on the right track!

You can indeed use an ON-ON SPDT switch for your wiring, simply ditch the resistor. But you can make any number of taps - as many as you can fit eyelets on the side of your flatwork! You can then tap your coil from the 'in' wire to any of the taps you want, or from any tap to any subsequent tap. This makes for a wide tonal flexibility. Some people use a rotary switch for this.
 
Hout 2/15/2018 9:18 AM
Thanks for all the advice! Very interesting world indeed, inventing and engineering your own sound! SPDT ON-ON was what I was trying to say with normal ON-ON, the correct abbreviation didn't come to mind.
Just for clarification, as you all seem to oppose the resistor, increasing the winds changes the tonal characteristics as well as the output volume, right? I'm not fond of changing my bass' output volume much as I find it messes with my pedal chain, so that's the reason for the resistor. Or will adding a resistor sort of undo both volume and tonal characteristics of the extra winds?
 
Chuck H 2/15/2018 8:00 PM
I should probably let the pickup guys handle this, but...

I think a resistor of any value that would significantly stave LF output would also roll down the resonant peak like turning down the volume control on your guitar. Making the overall signal balance favor LF for the relative amount of output.

Maybe a capacitor instead?
 
Antigua 2/18/2018 7:31 PM
Quote Originally Posted by Hout View Post
Hi all, first post here

I have wound a few pickups for my own basses and I was wondering about the the possibilities of adding an additional circuit to a single coil to get more tonal options.
I am not completely new to this, but by no means any where near an expert, ore like a trial and error diy-er. I did a search and couldn't find anything on it, but since I'm not so familiar with the terminology I might have missed it, so apologies in advance if this has been discussed before.

So here's my idea:
When winding a pickup you stop at a certain point, cut the wire, and lead it somewhere it is fixed and not in the way, and then continue with a new wire, until you end up with an "overwound" pickup. These two coils are then hooked up through a dptp switch so you can switch between running the normal (or underwound) coil to the amp and normal+extra wounds to the amp. I made this following schematic to hopefully explain some more.
[ATTACH=CONFIG]47092[/ATTACH]

As I tried to depict, the coils are wound and wired in the same direction, so there will be no hum cancelling of course. I figure there would be a difference in volume from the two options, so maybe I can fiddle around to get an inline resistor or a trimpot to even out the output.

So what do think? Has anyone tried this? Would it work? Am I overlooking something?
An under appreciated aspect of coil tapping is that, suppose you wind it to 7,000 turns, then tap it, then wind it another 2,000 turns for a total of 9,000, when you use the 7,000 tap, it will not sound the same as a coil that was wound to 7,000 turns without the tap, because those extra 2,000 turns of wire remain in the circuit as a capacitive coupling. The amount of capacitance added by that extra bulk of coil is audibly significant.
 
Jason Rodgers 2/18/2018 8:09 PM
If the layers affect one another, can you use this to create some tonal variation? Here’s a scenario: say you wind 5000 turns, tap it, wind 5000 more, put a 4-wire lead on the pickup, then hook it up to a 3-way toggle. The toggle gives you inside coil, full series, outside coil. Would the inside and outside taps sound different enough to be worth it?
 
Antigua 2/18/2018 11:35 PM
Quote Originally Posted by Jason Rodgers View Post
If the layers affect one another, can you use this to create some tonal variation? Here’s a scenario: say you wind 5000 turns, tap it, wind 5000 more, put a 4-wire lead on the pickup, then hook it up to a 3-way toggle. The toggle gives you inside coil, full series, outside coil. Would the inside and outside taps sound different enough to be worth it?
If both halves are 5000 turns, both halves will perform near identically, regardless of whether the coil is on the inside or the outside.
 
copperheadroads 2/18/2018 11:55 PM
The coil on the outside is quite larger than the inside coil ,I wouldn't consider it half or even
 
J M Fahey 2/19/2018 9:07 AM
Inner and outer halves will sound slightly different, not sure this will merit the switching complication, maybe you can hear something playing alone in a basement at low volume and *focusing* on it, but I guess onstage, loud, and with other musicians around any difference will be lost.

Physically: output level and inductance remain the same, resistance will be somewhat higher ... but main component of pickup impedance is inductance, not resistance; capacitance will be slightly higher, although difference may be swamped merely by choice of a different guitar cable, so...

But in any case you are asking here things you should be experimenting
 
Jason Rodgers 2/19/2018 12:05 PM
Oh, for sure! I have a guitar in mind that will have one pickup and very simple controls (maybe just volume with push-pull for tap or coil cut, maybe just a toggle for full/cut/off) and this thread came up at the perfect time.
 
rjb 2/19/2018 1:47 PM

Quote Originally Posted by Jason Rodgers View Post
I have a guitar in mind that will have one pickup and very simple controls (maybe just volume with push-pull for tap or coil cut, maybe just a toggle for full/cut/off) and this thread came up at the perfect time.
I'm considering doing the same thing with a lap steel. I was thinking a fairly hot humbucker with an on-on-on dpdt for series/cut/parallel. The tones would be selected in order from raunchy to clean. And with a "twin dual-rail" [EDIT: AKA "quad rail"] pickup (each side a humbucker), it would be hum-free.

-rb
 
Steve A. 2/21/2018 3:24 PM
Quote Originally Posted by J M Fahey View Post
Inner and outer halves will sound slightly different, not sure this will merit the switching complication, maybe you can hear something playing alone in a basement at low volume and *focusing* on it, but I guess onstage, loud, and with other musicians around any difference will be lost.
That could be said for a lot of the tweaks we might do to guitars, amps and pedals but there are another factors besides sound, such as the response to your playing. When I think of a guitar rig being "hot" it is more than volume, tone or gain — it is how everything responds to my playing. So even though a listener might not be able to hear an audible difference the tweaks made to my equipment might make me feel more inspired which would improve my playing. Just a thought...

Steve A.
 
David King 2/27/2018 6:15 PM
My experience with tapped coils is that they only sound good when the whole coil is engaged. As mentioned above the unused portions are dead weight that will have a significant capacitance. No different from having shorted turns inside a coil. I consider them a waste of time but some guys think they gotta have everything in one pickup. If you could somehow peel the extra turns away when you weren't using them that would be handy.
 
Chuck H 2/27/2018 9:01 PM
I gave a like because this is an excellent observation to the discussion. Still, I will mention that you CAN'T peel then away the unused coil (well, maybe you can, more in a sec). So the burden of the extra winds is offset by the versatility for a given design. At least that's the ideal I think. So... Probably more trouble than it's worth, but if you could switch open both ends of an additional wind (yes, complicates switching horribly) you shouldn't suffer any capacitance or mutual inductance (relative to the end that isn't open if one is left connected). Perhaps some bending of the EMF due to what amounts to a copper case surrounding the inner coil though.?. Just thinking out loud.
 
Antigua 2/27/2018 9:33 PM
Quote Originally Posted by Chuck H View Post
I gave a like because this is an excellent observation to the discussion. Still, I will mention that you CAN'T peel then away the unused coil (well, maybe you can, more in a sec). So the burden of the extra winds is offset by the versatility for a given design. At least that's the ideal I think. So... Probably more trouble than it's worth, but if you could switch open both ends of an additional wind (yes, complicates switching horribly) you shouldn't suffer any capacitance or mutual inductance (relative to the end that isn't open if one is left connected). Perhaps some bending of the EMF due to what amounts to a copper case surrounding the inner coil though.?. Just thinking out loud.
Yes, if both ends of the tap coil are disconnected then you can truly have your cake and eat it to, and I wish a commercial pickup maker would exploit this fact. If both ends of the tap coil are disconnected, it still remains in circuit, because there is still capacitive coupling along the length of the tap coil and the primary, and you see the same issue occur when you split a humbucker when you fully disconnect the secondary as opposed to simply shunting to ground, but the effect of that capacitive coupling will be small, compared to the usual practice of keeping one end of the coil physically connected to the circuit.
 
Hout 2/28/2018 12:49 AM
Okay, so that means it would be most effective to use dtdp switching like the diagram in the first post? Essentially having two separate coils around the same polepieces. With some more elaborate switching, you would be able to use 3 different coil "sounds".
 
Antigua 2/28/2018 10:39 AM
Quote Originally Posted by Hout View Post
Okay, so that means it would be most effective to use dtdp switching like the diagram in the first post? Essentially having two separate coils around the same polepieces. With some more elaborate switching, you would be able to use 3 different coil "sounds".
I don't really understand the OP diagram, but you could do it like this:

[img]https://i.imgur.com/hJ0cWyY.png[/img]

When the DPDT on-on switch is down, the inner portion of the coil is in circuit and the outer portion is disconnected at both ends. When the switch is up, the inner and outer coils are in series, as if it were just one big coil.

Some tapped coils on the market are wound with equal turns inner and outer, and presumably the reason they do this is because the high capacitance of industry standard wiring causes the tapped coil to have a very low resonance, so if you end up using this DPDT scheme, you could have the inner coil wound with a substantially higher wind count than the outer portion, and still get a good effect. If I were doing this, I'd try 7,000 turns on the inner coil, and then add another 2,000 turns to the outer coil. That should cause the inductance to switch from about 2 henries tapped, up to 3 henries in full series mode, or "vintage" to "hot".
 
David Schwab 3/23/2018 12:51 PM
Quote Originally Posted by Antigua View Post
An under appreciated aspect of coil tapping is that, suppose you wind it to 7,000 turns, then tap it, then wind it another 2,000 turns for a total of 9,000, when you use the 7,000 tap, it will not sound the same as a coil that was wound to 7,000 turns without the tap, because those extra 2,000 turns of wire remain in the circuit as a capacitive coupling. The amount of capacitance added by that extra bulk of coil is audibly significant.
Not if you leave the other end of the extra winds open. If you form a closed loop, eddy currents will change the tone. It's not really the capacitance, but inductive coupling.
 
Antigua 3/23/2018 2:06 PM
Quote Originally Posted by David Schwab View Post
Not if you leave the other end of the extra winds open. If you form a closed loop, eddy currents will change the tone. It's not really the capacitance, but inductive coupling.
That's what I'm saying, you want both ends to be open, neither end to be connected to anything. If either side is connected to anything else (itself or the circuit), you get unwanted L and/or C. Popular coil tapping wiring schemes with three lead wires require that one end be in-circuit, and there's almost nothing that can be done about it, unfortunately.
 
David Schwab 3/26/2018 7:26 AM
Quote Originally Posted by Antigua View Post
That's what I'm saying, you want both ends to be open, neither end to be connected to anything. If either side is connected to anything else (itself or the circuit), you get unwanted L and/or C. Popular coil tapping wiring schemes with three lead wires require that one end be in-circuit, and there's almost nothing that can be done about it, unfortunately.
If one end is open that's the same as not being connected. No current is going to flow.
 
Chuck H 3/26/2018 7:32 AM
Quote Originally Posted by David Schwab View Post
If one end is open that's the same as not being connected. No current is going to flow.
Except that it's still a copper shield in close proximity to an inductor connected to ??? and forms a capacitor of sorts.
 
Antigua 3/26/2018 10:57 AM
Quote Originally Posted by David Schwab View Post
If one end is open that's the same as not being connected. No current is going to flow.
Like Chuck says, because it's an AC voltage, a capacitive reactance occurs between the primary and secondary coil, and because of the large area and extremely close proximity of the two, the capacitance is rather substantial, unlike a humbucker, where the coils are farther apart, and the surface area between them a lot smaller.
 
David Schwab 3/27/2018 9:46 AM
Quote Originally Posted by Antigua View Post
Like Chuck says, because it's an AC voltage, a capacitive reactance occurs between the primary and secondary coil, and because of the large area and extremely close proximity of the two, the capacitance is rather substantial, unlike a humbucker, where the coils are farther apart, and the surface area between them a lot smaller.
If the secondary coil is open, it's just some metal near the primary coil. Capacitance implies two conductors separated by an insulator. But in this case the secondary coil is not connected. So you can't have mutual-capacitance, since no current is flowing in the secondary coil. Maybe it can have self-capacitance? But again, no current is flowing, as the secondary coil is open. So what's the potential of an open coil compared to the primary coil? Anything at all?

Sorry, I'm just not seeing it.
 
David Schwab 3/27/2018 9:48 AM
Quote Originally Posted by Chuck H View Post
Except that it's still a copper shield in close proximity to an inductor connected to ??? and forms a capacitor of sorts.
Not it's it's open. A capacitor has each conductor connected in the circuit, and the dielectric between them. If you lift one leg on a cap, say in a tone control circuit, what do you get? Nothing.
 
David King 3/27/2018 11:14 AM
Seems like this could be settled with a quick experiment. Then again that might short circuit a lot of discussion based on not totally well understood science. Carry on!
 
Antigua 3/27/2018 11:25 AM
Quote Originally Posted by David Schwab View Post
Not it's it's open. A capacitor has each conductor connected in the circuit, and the dielectric between them. If you lift one leg on a cap, say in a tone control circuit, what do you get? Nothing.
The other end of the cap is the primary coil. You know a metal cover capacitively couples with the coil, no dispute there. Now consider that the secondary relates to the primary exactly as a metal cover does, with the double jeopardy of being much closer to the primary than any cover, (save for copper tape wrapped coils, a common practice in low priced Chinese pickups).
 
Antigua 3/27/2018 11:28 AM
Quote Originally Posted by David King View Post
Seems like this could be settled with a quick experiment. Then again that might short circuit a lot of discussion based on not totally well understood science. Carry on!
I do have experimental evidence, with a tappable SSL-3, SSL-4, SSL-5 and a Stra-Bro 90. You just take the resonant peak and the inductance of the tap, from that you can determine the capacitance.
 
David Schwab 3/27/2018 11:48 AM
Quote Originally Posted by Antigua View Post
The other end of the cap is the primary coil. You know a metal cover capacitively couples with the coil, no dispute there. Now consider that the secondary relates to the primary exactly as a metal cover does, with the double jeopardy of being much closer to the primary than any cover, (save for copper tape wrapped coils, a common practice in low priced Chinese pickups).
Right, but primary coil is a complete circuit. Current flows though it. Just like if you wrap copper tape around a coil, you don't want to close the loop, or current flows and you get eddy currents. Disconnect the ends and no current flows. A metal cover is also a closed loop because it surrounds the coil. The "H" cutout on Filter'Trons is intended to break that up a bit.

Bill Lawrence had a patent showing a closed secondary coil around the primary to change the tone. But the coil has to have its ends connected. There's also a patent with two coaxial coils with opposite ends disconnected. But one end of each coil is in the circuit, i.e., one is ground, and the other is hot.

So I'm saying if the secondary coil has both ends not connected to anything, how would it effect the primary coil?

Tests need to be done!
 
Antigua 3/27/2018 11:53 AM
Quote Originally Posted by David Schwab View Post
Right, but primary coil is a complete circuit. Current flows though it. Just like if you wrap copper tape around a coil, you don't want to close the loop, or current flows and you get eddy currents. Disconnect the ends and no current flows. A metal cover is also a closed loop because it surrounds the coil. The "H" cutout on Filter'Trons is intended to break that up a bit.
Eddy currents are separate and distinct from capacitive coupling. You can have one and not the other, and vice versa.


Quote Originally Posted by David Schwab View Post
So I'm saying if the secondary coil has both ends not connected to anything, how would it effect the primary coil?

Tests need to be done!
Think about a "capacitive plate", you have a large area that is only connected in one spot, usually the center of the "plate". The whole secondary coil is a plate, it need only be connected at one spot.

I've done tests, here's a SSL-3T example:


DC Resistance:
full: 15.53K
tapped: 7.80K

Inductance:
full: 7.842H
tapped: 2.173H

Resonant Peak:
full: dV: 10.7dB f: 4.84kHz (black)
tapped: dV: 9.4dB f: 5.00kHz (red)

Loaded Resonant Peak (200K ohms & 470pF):
full: dV: 2.8dB f: 2.03kHz (green)
tapped: dV: 4.8dB f: 3.51kHz (gray)

Calculated C:
full: 128pF (138-10)
tapped: 456pF (466-10)

[ATTACH=CONFIG]47856[/ATTACH]
 
David Schwab 3/27/2018 12:50 PM
Quote Originally Posted by Antigua View Post
The whole secondary coil is a plate, it need only be connected at one spot.
I've said several times...

"The secondary coil has both ends not connected to anything..."

Now do the tests.
 
Antigua 3/27/2018 1:13 PM
Quote Originally Posted by David Schwab View Post
I've said several times...

"The secondary coil has both ends not connected to anything..."

Now do the tests.
I missed that, but when you have a three conductor coil tap, it is not the case that the secondary is disconnected at both ends, and so this is what I am suggesting to the OP, use four conductor cable and wire it as shown in that diagram I posted above in order to fully disconnect the tapped coil.

With the secondary coil open at both ends, the coil will still be slightly reactive, because the secondary will capacitively couple along its length, but that effects is a lot smaller than when the coil is connected at one end, based on similar testing with humbucker coils.
 
Chuck H 3/27/2018 10:07 PM
Yep. One end connected makes it a capacitor. Same effect you get with shielded cable. The shield presents a capacitance to ground because of it's proximity to the conductor. I don't think anyone would argue that or the similarity in the circuits. Both ends disconnected from the secondary coil eliminates the problem BUT FOR ONE THING!!! If you shield an inductor with copper (grounded or not!) it changes the performance. I can't say why because I don't have the tech chops, but I can assure you that it does because of my own experience shielding pickup coils with copper tape. Copper has a unique ability to bend EMF and I have to assume it's this property that is responsible.
 
Antigua 3/28/2018 12:21 AM
Quote Originally Posted by Chuck H View Post
Yep. One end connected makes it a capacitor. Same effect you get with shielded cable. The shield presents a capacitance to ground because of it's proximity to the conductor. I don't think anyone would argue that or the similarity in the circuits. Both ends disconnected from the secondary coil eliminates the problem BUT FOR ONE THING!!! If you shield an inductor with copper (grounded or not!) it changes the performance. I can't say why because I don't have the tech chops, but I can assure you that it does because of my own experience shielding pickup coils with copper tape. Copper has a unique ability to bend EMF and I have to assume it's this property that is responsible.
Probably eddy currents http://kenwillmott.com/blog/wp-conte...r_Geometry.pdf conductive areas that intersect the lines of flux (covers, pole pieces, copper tape) form eddy currents which manifest as increased series resistance.

If you were to cut the copper tape so that it doesn't have continuity around the coil, it will prevent the eddy current loop, but then if it's not grounded, it won't block electrostatic noise, but if it is grounded it will put capacitance across the coil, so copper tape shielding coils is a lose-lose proposition, in my estimation.
 
Helmholtz 3/28/2018 8:55 AM
DC Resistance:
full: 15.53K
tapped: 7.80K

Inductance:
full: 7.842H
tapped: 2.173H

Resonant Peak:
full: dV: 10.7dB f: 4.84kHz (black)
tapped: dV: 9.4dB f: 5.00kHz (red)

Loaded Resonant Peak (200K ohms & 470pF):
full: dV: 2.8dB f: 2.03kHz (green)
tapped: dV: 4.8dB f: 3.51kHz (gray)

Calculated C:
full: 128pF (138-10)
tapped: 456pF (466-10)


This is an interesting discussion. Thanks for the measurements. (How do you measure the frequency response? By field injection via transmitter coil and integration of the output signal?)

Such a big increase of capacitance (over 300pF) cannot be explained by capacitive coupling.

My explanation is the following:

When using the tap as output, the disengaged part of the winding is still there and remains to be inductively coupled with the rest of the coil. It becomes the secondary of a transformer. This secondary appears to be open but is actually terminated by its own self-capacitance. The latter gets transformed/reflected to the primary side (the active part of the coil). And thus the reflected capacitance of the secondary adds to the capacitance of the primary - as long as you don't physically remove the disengaged part of the winding from the pickup.

This effect does not depend on electrical connection between the coil parts.
 
Antigua 3/28/2018 11:08 AM
Quote Originally Posted by Helmholtz View Post
DC Resistance:
full: 15.53K
tapped: 7.80K

Inductance:
full: 7.842H
tapped: 2.173H

Resonant Peak:
full: dV: 10.7dB f: 4.84kHz (black)
tapped: dV: 9.4dB f: 5.00kHz (red)

Loaded Resonant Peak (200K ohms & 470pF):
full: dV: 2.8dB f: 2.03kHz (green)
tapped: dV: 4.8dB f: 3.51kHz (gray)

Calculated C:
full: 128pF (138-10)
tapped: 456pF (466-10)


This is an interesting discussion. Thanks for the measurements. (How do you measure the frequency response? By field injection via transmitter coil and integration of the output signal?)

I use a Velleman pcsu200 USB oscilloscope + function generator. With a small exciter coil hooked up to it's function generator, the impedance can be plotted with the bode plotter (or seen by viewing the phase by frequency). The inductance can be determined this way by measuring the resonance with a larger known capacitance, but that's tedious to do, so I use an Extech, which seems to work fine.

Quote Originally Posted by Helmholtz View Post
Such a big increase of capacitance (over 300pF) cannot be explained by capacitive coupling.

My explanation is the following:

When using the tap as output, the disengaged part of the winding is still there and remains to be inductively coupled with the rest of the coil. It becomes the secondary of a transformer. This secondary appears to be open but is actually terminated by its own self-capacitance. The latter gets transformed/reflected to the primary side (the active part of the coil). And thus the reflected capacitance of the secondary adds to the capacitance of the primary - as long as you don't physically remove the disengaged part of the winding from the pickup.

This effect does not depend on electrical connection between the coil parts.
You might be right. It's a certainty that the unused coil will still be in circuit when disconnected at both ends, since both capacitive and inductive coupling are still in play. That can be observed with humbuckers too, and the effect is very small, though of course they're not going to couple as well as two co-axial coils. I'll see if I can model this with LTSpice, but if that doesn't give a clear picture, I'll probably have to make a four conductor tapped single coil to test out.

Make note that copper tape around a coil does add a lot of capacitance as well. This pickup in the image below shows to have about 275pF capacitance. A typical Stratocaster pickup of this sort is usually around 120pF, +/- 20pF, so the copper shield is adding something like 150pF capacitance, which is somewhat less than what is calculated for a tapped single coil, despite being a geometrically similar situation.

[IMG]https://i.imgur.com/GepwAja.jpg[/IMG]
 
Helmholtz 3/28/2018 2:45 PM
>I use a Velleman pcsu200 USB oscilloscope + function generator. With a small exciter coil hooked up to it's function generator, the impedance can be plotted with the bode plotter (or seen by viewing the phase by frequency). The inductance can be determined this way by measuring the resonance with a larger known capacitance, but that's tedious to do, so I use an Extech, which seems to work fine.<

This is what I thought and described above, the method of my german physicist colleague Lemme. I have been using this for many years. But there are different possibilities to take care of the necessary 1/f (-6dB/octave) compensation: A constant current source for the excitation coil with 1/f sweep amplitude or integration of the output signal. I use a power amp driven by the PCSU 200 signal generator and a big air core choke in series with the low impedance exciter coil to provide a current source with 1/f characteristic. If you don't use a constant current source with 1/f characteristic or integrate the output signal afterwards, you will not get the true PU's frequency response as the measured frequency response will be distorted by the frequency dependance of the impedance of the exciter coil and the non-ideal coupling. An exciter coil driven by a low impedance (50 Ohms) source will also load the PU and reduce the resonance peak.

>Make note that copper tape around a coil does add a lot of capacitance as well. This pickup in the image below shows to have about 275pF capacitance. A typical Stratocaster pickup of this sort is usually around 120pF, +/- 20pF, so the copper shield is adding something like 150pF capacitance, which is somewhat less than what is calculated for a tapped single coil, despite being a geometrically similar situation<

A closed copper loop around a pickup coil is not a good idea, as this is represents a shorted secondary coil in a transformer, loading down the PU. Consequently output signal, inductance and Q will be lowered. Connecting the loop to ground will strongly increase the PU's capacitance to ground in addition.

Do you have numbers for the capacitance increase with a non-closed and non-grounded copper foil? This would be a better capacitive equivalent to the disengaged windings in a tapped coil.
 
Antigua 3/28/2018 6:27 PM
Quote Originally Posted by Helmholtz View Post
>I use a Velleman pcsu200 USB oscilloscope + function generator. With a small exciter coil hooked up to it's function generator, the impedance can be plotted with the bode plotter (or seen by viewing the phase by frequency). The inductance can be determined this way by measuring the resonance with a larger known capacitance, but that's tedious to do, so I use an Extech, which seems to work fine.<

This is what I thought and described above, the method of my german physicist colleague Lemme. I have been using this for many years. But there are different possibilities to take care of the necessary 1/f (-6dB/octave) compensation: A constant current source for the excitation coil with 1/f sweep amplitude or integration of the output signal. I use a power amp driven by the PCSU 200 signal generator and a big air core choke in series with the low impedance exciter coil to provide a current source with 1/f characteristic. If you don't use a constant current source with 1/f characteristic or integrate the output signal afterwards, you will not get the true PU's frequency response as the measured frequency response will be distorted by the frequency dependance of the impedance of the exciter coil and the non-ideal coupling. An exciter coil driven by a low impedance (50 Ohms) source will also load the PU and reduce the resonance peak.
For the integration I use an integrator amp circuit designed by Ken Willmott Electric Guitar Pickup Measurements | kenwillmott.com , which is why you don't see a +6dB/oct curve in the pickup plot on the previous page. He offers the schematic for free, but if you contact him he might send you a printed circuit board, as he had a handful of them produced, if you're interested.

As for the coupling between the exciter and the pickup, I've found that the coupling between them is so low that it's almost impossible to get the exciter to interfere with the impedance of the tested pickup, at least with the Velleman bode plotters. Maybe in other test setups it's more of an issue, though.

Quote Originally Posted by Helmholtz View Post
>Make note that copper tape around a coil does add a lot of capacitance as well. This pickup in the image below shows to have about 275pF capacitance. A typical Stratocaster pickup of this sort is usually around 120pF, +/- 20pF, so the copper shield is adding something like 150pF capacitance, which is somewhat less than what is calculated for a tapped single coil, despite being a geometrically similar situation<

A closed copper loop around a pickup coil is not a good idea, as this is represents a shorted secondary coil in a transformer, loading down the PU. Consequently output signal, inductance and Q will be lowered. Connecting the loop to ground will strongly increase the PU's capacitance to ground in addition.

Do you have numbers for the capacitance increase with a non-closed and non-grounded copper foil? This would be a better capacitive equivalent to the disengaged windings in a tapped coil.
I agree, I'd advice people with copper taped coils to carefully remove it, if at all possible, or at the very least snip the ground connection. I don't have other values with/without, but I can gather this info in the next week or two and send you a private message when I get it. If you don't visit this forum to often, private message me your email address and I'll send you additional test results that way.
 
Helmholtz 3/29/2018 8:45 AM
Thanks for your replies.

I think I will stick to my trusted measuring setup as I have verfied its linearity to over 20kHz. It doesn't need the integrator because the 6dB/octave behaviour is already implemented in the drive current for the exciter coil.
If you use e.g. a 3mH exciter coil in series with a 100 Ohm resistor driven by a voltage source, the drive current will decrease above 5kHz by 6dB/octave caused by the increasing reactance of the coil, respectively the RL time constant. Consequently the PU's output will show a reduced high frequency response. Of course this can be improved by a lower inductance and higher value series resistor.

It is easy to verify my "capacitance transformer theory" indirectly by the following experiments, if you have a tapped single coil PU (which I do not):

1) Connect an external capacitor of e.g. 100pF to the tap and the outer end of the coil (i.e. in parallel with the disengaged part of the coil).
Measure the respective increase of capacitance in the active coil.

2)Short-circuit the disengaged part of the coil and note the corresponding decrease of capacitance in the active coil. In this case other parameters (L,Q) of the tapped PU may change as well because of the "eddy current effect".


I plan to keep track of interesting threads regarding pickups and tube amplifiers.
 
Helmholtz 3/29/2018 10:56 AM
I just did the experiment with copper foil wrapped around a strat pickup. I used 2 wraps of back insulated foil, ends not shorted to avoid eddy currents.
The results are:

Without foil Cp=87pF, with foil (not connected) Cp=88pF, foil connected to output Cp=89pF and with foil connected to ground Cp=123pF.
The tapped coil situation would correspond to "foil connected to output".
I think the results show clearly that it does not matter, if the disengaged windings remain connected to the tap or not. It also shows that a dramatic increase of capacitance in the active coil of over 300pF cannot be the result of capacitive coupling.
 
Antigua 3/29/2018 11:09 AM
Quote Originally Posted by Helmholtz View Post
Thanks for your replies.

I think I will stick to my trusted measuring setup as I have verfied its linearity to over 20kHz. It doesn't need the integrator because the 6dB/octave behaviour is already implemented in the drive current for the exciter coil.
If you use e.g. a 3mH exciter coil in series with a 100 Ohm resistor driven by a voltage source, the drive current will decrease above 5kHz by 6dB/octave caused by the increasing reactance of the coil, respectively the RL time constant. Consequently the PU's output will show a reduced high frequency response. Of course this can be improved by a lower inductance and higher value series resistor.

The Velleman PCSU has an output impedence of 50 ohms. I'll get specs on the exciter coil, it's about 100 turns of 44AWG, here's a pic http://i.imgur.com/iJ4ejoR.jpg

I haven't come across this problem myself. For example, here is a P-Bass pickup that measured well up to 14kHz
[img]https://images.talkbass.com/attachments/a4ve0dl-png.2610888/[/img]


Quote Originally Posted by Helmholtz View Post
It is easy to verify my "capacitance transformer theory" indirectly by the following experiments, if you have a tapped single coil PU (which I do not):

1) Connect an external capacitor of e.g. 100pF to the tap and the outer end of the coil (i.e. in parallel with the disengaged part of the coil).
Measure the respective increase of capacitance in the active coil.

2)Short-circuit the disengaged part of the coil and note the corresponding decrease of capacitance in the active coil. In this case other parameters (L,Q) of the tapped PU may change as well because of the "eddy current effect".


I plan to keep track of interesting threads regarding pickups and tube amplifiers.

I can do that, though it might be a week or two before I can get to it. I think this is a high value area of exploration since it means endowing a single coil pickup with multiple usable electrical values, without the drawbacks of what is currently available on the market.
 
Helmholtz 4/6/2018 2:30 PM
<My explanation is the following:

When using the tap as output, the disengaged part of the winding is still there and remains to be inductively coupled with the rest of the coil. It becomes the secondary of a transformer. This secondary appears to be open but is actually terminated by its own self-capacitance. The latter gets transformed/reflected to the primary side (the active part of the coil). And thus the reflected capacitance of the secondary adds to the capacitance of the primary - as long as you don't physically remove the disengaged part of the winding from the pickup.

This effect does not depend on electrical connection between the coil parts.>
Here
http://www.datatronics.com/pdf/distr...ance_paper.pdf
is a good description of the summing-up of distributed capacitances of coupled windings.
 
Antigua 4/7/2018 1:04 AM
Quote Originally Posted by Helmholtz View Post
Here
http://www.datatronics.com/pdf/distr...ance_paper.pdf
is a good description of the summing-up of distributed capacitances of coupled windings.
Thanks for the link, I'll have to look over that carefully to see if it's equations might apply to transformers that have coils with a common axis and plane.
 
Helmholtz 4/7/2018 6:40 AM
Quote Originally Posted by Antigua View Post
Thanks for the link, I'll have to look over that carefully to see if it's equations might apply to transformers that have coils with a common axis and plane.
The effect only requires sufficient magnetic coupling. Good coupling means that the windings share most of the same magnetic flux. Concentric arrangement of the windings on a common ferromagnetic core provides good preconditions for strong magnetic coupling. The coupling factor can be determined from the ratio of primary inductance with secondary shorted (Lps) to primary inductance with secondary open (Lpo). Perfect coupling (100%) means Lps/Lpo = 0 and no coupling means Lps/Lpo = 1.
In case of weaker coupling, the transformed secondary self-capacitance will appear on the primary side in series with the transformed secondary leakage inductance, constituting an additional series resonance circuit. As a consequence the pickup's frequency response will show additional peaks and dips above the main primary resonance frequency.
 
okabass 4/13/2018 2:23 PM
HI
I tried the wiring you suggested on a 51 type P bass single coil. Coils are 6,7 kΩ / 2,6 H (tapped) 7,8 kΩ / 3,4 H (full). It works very nicely. The sound difference is subtle but useful. It could be more so I'll try something like 5,7...6,5 kΩ and 7,8 kΩ.
 
Jason Rodgers 4/13/2018 8:56 PM
Quote Originally Posted by okabass View Post
HI
I tried the wiring you suggested on a 51 type P bass single coil. Coils are 6,7 kΩ / 2,6 H (tapped) 7,8 kΩ / 3,4 H (full). It works very nicely. The sound difference is subtle but useful. It could be more so I'll try something like 5,7...6,5 kΩ and 7,8 kΩ.
To which wiring diagram do you refer? The one in the original post, or Antigua’s of Feb 28?
 
okabass 4/13/2018 9:49 PM
To Antigua's.
As a matter of fact I just wind a 5,7 kΩ + 1,9 kΩ (7,8 kΩ) coils to an old Duncan quarter pounder frame. But there's some short in coils (reads 50Ω when it should be ∞Ω). Didn't have time to check more closely. Must perhaps rewind that.
 
Antigua 4/14/2018 10:16 PM
I'm glad to hear that worked out. I'll have to make a four lead tapped single coil and try it myself. I think the difference between two pickups tends to be pretty subtle, but a big deal is made of it, because it's the sound you have to live with. Put a switch in there and you might demand a bigger shift. I figured that was why tapped single coils presently on the market had such radical tap points, until I found out that the capacitance, and possibly inductive coupling, made it somewhat necessary, too.
 
okabass 4/15/2018 4:19 PM
Wind 5,7 kΩ + 1,9 kΩ (7,8 kΩ) coils to normal 51 PU frame (ca. 5 mm magnets). I found that Quarter pounder magnets made it too thick sounding. There's volume difference, but it ain't bad. I like the most that you get a 50's and a modern fuller sound without character change.:thumbsup:
 
Joe Gwinn 4/15/2018 8:04 PM
In a sense, you are all correct - all these effects do happen, independently of one another, so it's a matter of relative magnitude, and experiment is the only way to sort it out. The floating open-circuited coils gets both electrostatic and magnetic coupling. The floating coil increases the self-capacitance of the nearby connected coil. This happens even if the connected coil is only connected at one end, so no metallic currents flow. What does flow through the capacitances are displacement currents due to changing electric fields. Currents through the connected coil will induce voltages in the floating coil, which is capable of resonance with its own self-capacitance, and this effect will be reflected back into the connected coil. It can get pretty complicated. How to reduce the effects? Two main remedies. First, lay down a few layers of mylar or waxed paper tape on top of the inner winding, before starting the outer winding. This will decrease the capacitance between coils, but will not have much effect on the transformer coupling between the coils. One can suppress a lot of the effect by loading the floating coil with a fixed resistor, thus reducing the effects of resonance. The best way to find the correct resistor value is a pot and a golden ear.
 
okabass 4/16/2018 12:30 AM
Thank you for the post. Mylar plastic. Do you mean clear mylar only or with metal foil (like space sheet)? How about this? [url]https://www.ballisticproducts.com/Thin-Mylar-protective-shotshell-wraps-003/productinfo/MYTHIN/[/url] Fixed resistor? Parallel to the floating coil? What value pot would be a good starting point if the floating coil is around 2 kΩ ? The other coil is 5,8 kΩ, total 7,8 kΩ.
 
Helmholtz 4/16/2018 7:01 AM
[QUOTE=Joe Gwinn;493109]In a sense, you are all correct - all these effects do happen, independently of one another, so it's a matter of relative magnitude, and experiment is the only way to sort it out. The floating open-circuited coils gets both electrostatic and magnetic coupling. The floating coil increases the self-capacitance of the nearby connected coil. This happens even if the connected coil is only connected at one end, so no metallic currents flow. What does flow through the capacitances are displacement currents due to changing electric fields. Currents through the connected coil will induce voltages in the floating coil, which is capable of resonance with its own self-capacitance, and this effect will be reflected back into the connected coil. It can get pretty complicated. How to reduce the effects? Two main remedies. First, lay down a few layers of mylar or waxed paper tape on top of the inner winding, before starting the outer winding. This will decrease the capacitance between coils, but will not have much effect on the transformer coupling between the coils. One can suppress a lot of the effect by loading the floating coil with a fixed resistor, thus reducing the effects of resonance. The best way to find the correct resistor value is a pot and a golden ear.[/QUOTE] While the mylar foil might help to better separate the windings (even though its epsilon is around 3) and thus reduce interwinding capacitance, this will have neglegible effect on the active coil's resonance. The reason is that interwinding capacitance is not in the circuit as long as the outer coil is floating (and not shielded by a grounded outer copper foil). The primary (active) coil resonance only changes if its effective capacitance to ground changes. Loading the floating coil with a resistor will as well load the active coil with the reflected resistance (and - depending on coupling - a leakage inductance), thus reducing Q and resonance peak. This might not be desirable.
 
Joe Gwinn 4/16/2018 7:51 AM
[QUOTE=okabass;493115]Thank you for the post. Mylar plastic. Do you mean clear mylar only or with metal foil (like space sheet)? How about this? [url]https://www.ballisticproducts.com/Thin-Mylar-protective-shotshell-wraps-003/productinfo/MYTHIN/[/url][/quote] Yes, clear mylar. Actually, kapton would work just as well. Or scotch tape. [quote]Fixed resistor? Parallel to the floating coil? What value pot would be a good starting point if the floating coil is around 2 kΩ ? The other coil is 5,8 kΩ, total 7,8 kΩ.[/QUOTE] Connecting the ends of the floating coil, to compete the circuit. I have no idea what value is best. It may be that shorting the ends together is best, given the large DC resistance of the floating coil. Start with a 10 K pot and fiddle.
 
Joe Gwinn 4/16/2018 7:57 AM
[QUOTE=Helmholtz;493128]While the mylar foil might help to better separate the windings (even though its epsilon is around 3) and thus reduce interwinding capacitance, this will have neglegible effect on the active coil's resonance. [/quote] Agree. [quote]The reason is that interwinding capacitance is not in the circuit as long as the outer coil is floating (and not shielded by a grounded outer copper foil). The primary (active) coil resonance only changes if its effective capacitance to ground changes.[/quote] Not true. There is always some effect, and adding the interwinding layer reduces that effect. Interwinding layers are standard practice in transformer building. As are interwinding shields, for which one would can use metallized mylar. The shield metal must be grounded. [quote]Loading the floating coil with a resistor will as well load the active coil with the reflected resistance (and - depending on coupling - a leakage inductance), thus reducing Q and resonance peak. This might not be desirable.[/QUOTE] It's true that the loss of the connected coil will increase. The intent is to make it flatter, to reduce coloration. This must be assessed by ear - instruments are not much help here.
 
Helmholtz 4/16/2018 8:46 AM
>Not true. There is always some effect, and adding the interwinding layer reduces that effect. Interwinding layers are standard practice in transformer building. As are interwinding shields, for which one would can use metallized mylar. The shield metal must be grounded.< Not convincing. In high frequency transformer applications open (non-terminated) floating windings are typically avoided. A grounded interwinding shield effectively reduces interwinding capacitance (capacitive coupling between windings) but inevitably increases ground capacitance for both windings, which seems counterproductive in PUs. Can you show measurements and/or an equivalent circuit? BTW: I generally assume the inner start of the active pickup winding to be grounded. >It's true that the loss of the connected coil will increase. The intent is to make it flatter, to reduce coloration. This must be assessed by ear - instruments are not much help here.< In my experience most guitar players prefer some coloration i.e. resonance peak. Otherwise they could as well back the the tone control a bit to get a similar result.
 
Mark Hammer 4/16/2018 2:21 PM
Interesting thread, but here's something I never usually see discussed. We tend to think of coil-tapping in terms of "additional winds". That is, wind enough to make a viable coil, then add some more turns to make a hotter coil, but retain the selectable connection to the point where the added turns begin. Because the additional turns are on the outside, the added circumference makes for greater resistance per turn. But how does that compare to making the additional turns on the [B]inside[/B]? For example, put 1500 turns snuggled up against the polepieces, run a lead out to a solder terminal, then add another, say, 6500 turns. I won't pretend to understand Eddy currents or magnetism in general, nearly as well as my colleagues here. But at the very least, the scenario I describe moves the "start" of the default coil outwards, and obviously changes the circumference of the turns. So what I'm wondering is: how does the relocation of the additional turns to the "inside" change things with respect to inductance, resonant peaks, etc., and which would be the preferred arrangement - added coils on the outside, or the inside?
 
Antigua 4/16/2018 2:31 PM
[QUOTE=Mark Hammer;493162]Interesting thread, but here's something I never usually see discussed. We tend to think of coil-tapping in terms of "additional winds". That is, wind enough to make a viable coil, then add some more turns to make a hotter coil, but retain the selectable connection to the point where the added turns begin. Because the additional turns are on the outside, the added circumference makes for greater resistance per turn. But how does that compare to making the additional turns on the [B]inside[/B]? For example, put 1500 turns snuggled up against the polepieces, run a lead out to a solder terminal, then add another, say, 6500 turns. I won't pretend to understand Eddy currents or magnetism in general, nearly as well as my colleagues here. But at the very least, the scenario I describe moves the "start" of the default coil outwards, and obviously changes the circumference of the turns. So what I'm wondering is: how does the relocation of the additional turns to the "inside" change things with respect to inductance, resonant peaks, etc., and which would be the preferred arrangement - added coils on the outside, or the inside?[/QUOTE] I can't answer this question directly, but I can tell you that when the inner and outer coils are the same turn count, the plots overlap almost identically: [img]http://i.imgur.com/ts0rGtF.png[/img] Based on this, I'd think it wouldn't make much difference if you tapped inward versus tapping outward.
 
Helmholtz 4/16/2018 3:22 PM
[QUOTE=Antigua;493165]I can't answer this question directly, but I can tell you that when the inner and outer coils are the same turn count, the plots overlap almost identically: [img]http://i.imgur.com/ts0rGtF.png[/img] Based on this, I'd think it wouldn't make much difference if you tapped inward versus tapping outward.[/QUOTE] Inductance of the windings is essentially determined by the square of the turns (with a small additional influence of dimensions, forget resistance). Inductive coupling forces the resonant frequencies of both coils to be identical (see link on coupled windings) even if their inductances differ. @Antigua, you can improve S/N ratio of your plots by choosing the Automatic Voltage Scale option in the bode plotter of the PCSU 200.
 
Mark Hammer 4/16/2018 3:56 PM
Not to be too much of a pest, but would you expect those plots to be similar if we were talking about a flatter coil, like a Jazzmaster, as opposed to a coil where inner and outwer turns have very similar circumference? I ask because the overall shape of the coil can have an influence on tone, so naturally I'm curious about whether coil shape/dimensions also have an impact on the inner/outer turns aspect. Like I say, I don't know enough about this stuff to have any opinion. I'm just going on superficial aspects of pickups.
 
Steve A. 4/16/2018 4:12 PM
3 wire vs. 4 wire tapped pickups
Perhaps this has been mentioned already but using the lower resistance tap with a pickup using 3 wires instead of 4 would leave the outer winds connected to the hot signal output but not terminated by a path to ground, a situation which I suspect would make the pickup more susceptible to noise as the unterminated winding would act as an antenna. Which would be another reason to use 4 eyelets and leads instead of 3. Correct me if I am mistaken about that. Thanks! Steve A.
 
Joe Gwinn 4/16/2018 7:58 PM
[QUOTE=Helmholtz;493136]>Not true. There is always some effect, and adding the interwinding layer reduces that effect. Interwinding layers are standard practice in transformer building. As are interwinding shields, for which one would can use metallized mylar. The shield metal must be grounded.< Not convincing. In high frequency transformer applications open (non-terminated) floating windings are typically avoided.[/quote] We are talking audio transformers here, not high-frequency (like RF) here. And in RF floating windings are used, such as for feedback in oscillators. [quote]A grounded interwinding shield effectively reduces interwinding capacitance (capacitive coupling between windings) but inevitably increases ground capacitance for both windings, which seems counterproductive in PUs.[/quote] True. People add some insulation between to reduce capacitance to ground (versus the other coil) if it's a problem. [quote]Can you show measurements and/or an equivalent circuit? BTW: I generally assume the inner start of the active pickup winding to be grounded.[/quote] No, I haven't built this recently. But what I'm saying is straight out of books on designing audio transformers. It's actually a complicated bunch of balanced compromises. [quote]>It's true that the loss of the connected coil will increase. The intent is to make it flatter, to reduce coloration. This must be assessed by ear - instruments are not much help here.< In my experience most guitar players prefer some coloration i.e. resonance peak. Otherwise they could as well back the the tone control a bit to get a similar result.[/QUOTE] Then use a larger resistor, to allow some coloration from the floating coil through. As I said, the correct instrument here is a golden ear.
 
David King 4/16/2018 11:40 PM
Mark Hammer, I can tell you that some pedal steel pickups designs leave an airgap of .015-.020" between the magnets and the inside (start) of the coil to kill off some highs. I'd imagine that switching out the inner coil tap would have the same effect. Never tried it but it might have a use on an otherwise too bright guitar.
 
Helmholtz 4/17/2018 7:44 AM
We are talking audio transformers here, not high-frequency (like RF) here. And in RF floating windings are used, such as for feedback in oscillators.
No new arguments here. I am familiar with scientific literature on transformers. And I know and understand the detailed equivalent circuit, which is the same for RF and LF applications. My assessments are based on this knowledge as well as basic physics.

If a winding is used for feedback purposes, it has to be connected/terminated. This creates a situation that is not comparable to a tapped PU with an open winding.

I repeat: I do not see how the interwinding capacitance (which is rather low for a cleanly wound PU anyway) could influence the actice coil's resonance. It is not in the circuit when the secondary is floating. And it is essentially shorted when the two windings are connected at the tap (3 wire type).

My general resume regarding tapped singlecoils, based on theory and the measurements of Antigua, is that tapping inevitably introduces unwanted additional capacitance in the active coil and thus tends to prevent getting really different sounds (i.e. resonant frequencies) from different configurations (full vs tapped).
If you just want an additional sound with a lower resonant frequency it is much more efficient (and easier) to hardwire a suitable capacitor across a non-tapped PU. And I would start out with a lower wind non-tapped PU to allow for a wider range of useful resonances.
 
Helmholtz 4/17/2018 8:12 AM
Quote Originally Posted by Mark Hammer View Post
Not to be too much of a pest, but would you expect those plots to be similar if we were talking about a flatter coil, like a Jazzmaster, as opposed to a coil where inner and outwer turns have very similar circumference?

I ask because the overall shape of the coil can have an influence on tone, so naturally I'm curious about whether coil shape/dimensions also have an impact on the inner/outer turns aspect. Like I say, I don't know enough about this stuff to have any opinion. I'm just going on superficial aspects of pickups.
You state that the shape of the coil can have an influence on sound. The question is why. Is it really the shape of the coil or just caused by different resonances?
The sound difference between a Jazzmaster and Strat PU can be explained by different resonances (frequency and Q). I you want to study the single influence of coil shape, all other influencing factors - especially resonance - need to be equal for the two PUs. This can be achieved by varying the number of turns and - if necessary - additional capacitors and resistors.
 
Joe Gwinn 4/17/2018 8:14 PM
Quote Originally Posted by Helmholtz View Post
No new arguments here. I am familiar with scientific literature on transformers. And I know and understand the detailed equivalent circuit, which is the same for RF and LF applications. My assessments are based on this knowledge as well as basic physics.

If a winding is used for feedback purposes, it has to be connected/terminated. This creates a situation that is not comparable to a tapped PU with an open winding.

I repeat: I do not see how the interwinding capacitance (which is rather low for a cleanly wound PU anyway) could influence the actice coil's resonance. It is not in the circuit when the secondary is floating. And it is essentially shorted when the two windings are connected at the tap (3 wire type).

My general resume regarding tapped singlecoils, based on theory and the measurements of Antigua, is that tapping inevitably introduces unwanted additional capacitance in the active coil and thus tends to prevent getting really different sounds (i.e. resonant frequencies) from different configurations (full vs tapped).
If you just want an additional sound with a lower resonant frequency it is much more efficient (and easier) to hardwire a suitable capacitor across a non-tapped PU. And I would start out with a lower wind non-tapped PU to allow for a wider range of useful resonances.
Well, as I said upthread, all these effects are present to one degree or another, simultaneously. Various ways to increase or decrease this or that effect was discussed. So, one listens, and uses these tricks to steer the pickup towards a desired sound. The role of the theory is to give us clues as to what alterations are likely to move things toward or away from a desired direction.
 
Helmholtz 4/18/2018 6:11 AM
Well, as I said upthread, all these effects are present to one degree or another, simultaneously. Various ways to increase or decrease this or that effect was discussed. So, one listens, and uses these tricks to steer the pickup towards a desired sound. The role of the theory is to give us clues as to what alterations are likely to move things toward or away from a desired direction.
Yes, Sir. Right you are.

I love and support experimenting and listening tests. Experience and theoretical analysis of the dependencies may help to concentrate on the more promising experiments.
The easiest way to study the influence of the interwinding capacitance and other parameters of the tapped SC would be to simulate the appropriate equivalent circuit in LTSpice.
 
Antigua 4/18/2018 10:08 AM
Quote Originally Posted by Helmholtz View Post
The easiest way to study the influence of the interwinding capacitance and other parameters of the tapped SC would be to simulate the appropriate equivalent circuit in LTSpice.
I use LTSpice quite a bit. If you could propose an equivalent circuit for the apparent high capacitance of a tapped coil I would certainly give it a go. I did read the PDF you linked to discussing transformer capacitance, but I'm not 100% clear on how to translate that to the tapped coil model.

You said further up that you tested foil tape around a single coil, and did not get as high of a capacitance as a was apparent with the tapped coil, but isn't a secondary winding of a tapped pickup extremely close to the primary coil? Even foil tape will contain a little more distance between the windings than will the continued winding of the secondary coil. Being able to model this in LTSpice and see the "apparent" high capacitance without actually assigning a high capacitance would be very informative.
 
Antigua 4/18/2018 10:23 AM
Quote Originally Posted by Mark Hammer View Post
Not to be too much of a pest, but would you expect those plots to be similar if we were talking about a flatter coil, like a Jazzmaster, as opposed to a coil where inner and outwer turns have very similar circumference?

I ask because the overall shape of the coil can have an influence on tone, so naturally I'm curious about whether coil shape/dimensions also have an impact on the inner/outer turns aspect. Like I say, I don't know enough about this stuff to have any opinion. I'm just going on superficial aspects of pickups.
The belief that the shape of the coil effects the tone isn't well founded. The coil merely translates flux change into a voltage. If anything about the geometry of a single coil were to effect the tone, it would have to be the pole piece, because that along with the string determine the flux change, and while guitar strings are geometrically simple and uniform, pole pieces can be had in a variety of shapes and sizes. For example, if the pole piece is a long "blade", even harmonics caused by side to side movement are suppressed. If the pole piece is very narrow, they're increased. If the pole piece were very wide enough,like half an inch or more, comb filtering would cancel out higher harmonics, when the field is about twice the width of those harmonic divisions along the string. Humbuckers do this, insofar as having two rows of small pole pieces is similar to one very wide pole piece.
 
Antigua 4/18/2018 10:43 AM
Quote Originally Posted by Helmholtz View Post
If you just want an additional sound with a lower resonant frequency it is much more efficient (and easier) to hardwire a suitable capacitor across a non-tapped PU. And I would start out with a lower wind non-tapped PU to allow for a wider range of useful resonances.
The thing about that is that if you use a cap to get the "full" sound, you wouldn't get an associated voltage boost for having more turns of wire in series. Maybe an inductor across the pickup can be used to "unwind" it, but the issue with that is that the load decreases the resonance. Maybe if you used a 1meg volume and no-load tone pot, you could retain enough resonance to allow a parallel inductor to work effectively, but then you'd need to replace it with parallel resistance when you switched the inductor out of the circuit, which isn't too much to ask. The harder task is finding a suitable inductor. The Bill Lawrence "Q Filter" is an inductor that supposedly works well, but I don't know too much about it.

OTOH, even with just a cap, I think you can get really close to P-90 sound with a Strat if you pick a cap value that brings the resonant peak down to about 2kHz. I suspect the reason that's not often done is that people associate tone with the guitar as a whole, they don't expect a Strat with three little pickups to have a big, warm P-90 tone. Similarly, you rarely see Les Pauls fitted with Fender single coils.
 
Helmholtz 4/18/2018 1:48 PM
I am well aware of all this and if the loudness step is what you are after, my proposal won't do.

My point is, that the unavoidable inductive coupling between the winding parts prevents a tappable overwound SC from convincingly reproducing the brighter sounds a a lower or "standard" wound PU, no matter how you split the coil or separate the windings, as the unused part of the coil always increases the effectice capacitance across the active coil by a considerable amount. As a result the two sounds available won't differ much - except when using very long guitar cables. But find out yourself.

The parallel capacitors method, instead, allows selective tuning of the resonance and thus for really different sounds without loudness steps, which I consider favorable.

In my experience the sound and dynamics of a good P-90 cannot be emulated by a Strat type design with alnico cores. The passive filter characteristics of a PU cannot explain the total response of a PU, even if the aperture is equal. But this would be a completely different topic.
 
Mark Hammer 4/18/2018 2:08 PM
I'm the last one to impart magical properties to shape. However, coil shape is a "marker" for other things that can be predicted to affect the electrical properties, as in Helmholtz' example of resonance. Certainly the properties of the magnet or polepieces (or slogs) play a significant role, however their role is not in spite of the coil properties, but in concert with them.

I am assuming that if I used normal Strat polepieces, and a conventional number of turns with very conventional wire and tension, but inserted a spacer between the polepieces and coil, such that the coil really only "started" 1/8" or 3/16" out from the polepieces (which, of course, would not fit inside the usual plastic pickup cover), it would likely end up sounding different, if only because of the distance of so many turns from the polepieces themselves. If a pickup has 800 additional turns that are switch-selectable, I would not expect that pickup to sound much different when those turns are added from the location closest to the polepieces vs the outside of the overall coil, simply because so few turns creates a negligible difference in distance from the polepieces.

But when one has a flatter pickup, like a Jazzmaster or even P90, it doesn't take as many turns to create distance from the polepieces (or center bar and slugs). And I would think that where perhaps it might not make that much difference if additional turns were situated on the inside or the outside of a Strat-type pickup, they just might for a flatter wider profile. Again, taking into consideration that there are more, and less, consequential numbers of additional turns.
 
Helmholtz 4/18/2018 2:19 PM
You said further up that you tested foil tape around a single coil, and did not get as high of a capacitance as a was apparent with the tapped coil, but isn't a secondary winding of a tapped pickup extremely close to the primary coil? Even foil tape will contain a little more distance between the windings than will the continued winding of the secondary coil. Being able to model this in LTSpice and see the "apparent" high capacitance without actually assigning a high capacitance would be very informative.
My measurements with the foil (I have measured several PUs) show that it has no noticeable effect on the resonance of the active winding, as long as the foil is not grounded (or shorted). While the interwinding capacitance between the two winding parts will be somewhat higher than the capacitance between coil and foil (my guess: around 50pF, can be easily measured in a 4 wire PU; in a 3 wire PU it is shorted anyway), there will be no influence on the active coil's resonance as well, as it acts like a capacitor with one lead connected and the other lead up in the air. The active coil's resonance can only be influenced by capacitances that take effect to ground.
Please note that in the pdf article on coupled windings the interwinding capacitance is not even mentioned, even though there is some also in toroidal transformers with separated windings. The reason is that it has absolutely no effect as long as not more than one of the windings are grounded.
IMHO the increased effective capacitance across the active coil is not caused by the interwinding capacitance but by reflected capacitance.

I may assist with the LTSpice model but fear that this would mean hijacking this thread. We seem to have lost the OP a while ago.
 
bbsailor 4/18/2018 3:07 PM
Quote Originally Posted by Mark Hammer View Post
I'm the last one to impart magical properties to shape. However, coil shape is a "marker" for other things that can be predicted to affect the electrical properties, as in Helmholtz' example of resonance. Certainly the properties of the magnet or polepieces (or slogs) play a significant role, however their role is not in spite of the coil properties, but in concert with them.

I am assuming that if I used normal Strat polepieces, and a conventional number of turns with very conventional wire and tension, but inserted a spacer between the polepieces and coil, such that the coil really only "started" 1/8" or 3/16" out from the polepieces (which, of course, would not fit inside the usual plastic pickup cover), it would likely end up sounding different, if only because of the distance of so many turns from the polepieces themselves. If a pickup has 800 additional turns that are switch-selectable, I would not expect that pickup to sound much different when those turns are added from the location closest to the polepieces vs the outside of the overall coil, simply because so few turns creates a negligible difference in distance from the polepieces.

But when one has a flatter pickup, like a Jazzmaster or even P90, it doesn't take as many turns to create distance from the polepieces (or center bar and slugs). And I would think that where perhaps it might not make that much difference if additional turns were situated on the inside or the outside of a Strat-type pickup, they just might for a flatter wider profile. Again, taking into consideration that there are more, and less, consequential numbers of additional turns.
Mark,
Do not forget to take into account the strength of the magnetic field on the inner turns versus the outer turns voltage generated. Using a metal plate to help spread the magnetic field under the coil is one way to make the outer winds more efficient voltage generating turns.

The ultimate test is to wind Jazzmaster or P90 type pickup with an inner coil with independent start an ending leads or connections and then add an outer coil with the same number of winds with independent start and ending leads. With no metal plate under the coils attach the inner and outer coils to a dual trace oscilloscope and visualize the output levels of each independent coil. Now add a metal plate to help spread the magnetic field and do the same thing to see the output levels of each coil change due the this added metal plate variable being observed.

Wider coils detect a wider string length window and will have a different harmonic component than a narrow coil with a shorter string length window due to harmonic cancellations of certain frequencies over the pickup coil area.

If you do this experiment you will see results and add to a scientific answer to this question.

Joseph J. Rogowski
 
Helmholtz 4/18/2018 3:55 PM
Quote Originally Posted by Mark Hammer View Post
I'm the last one to impart magical properties to shape. However, coil shape is a "marker" for other things that can be predicted to affect the electrical properties, as in Helmholtz' example of resonance. Certainly the properties of the magnet or polepieces (or slogs) play a significant role, however their role is not in spite of the coil properties, but in concert with them.

I am assuming that if I used normal Strat polepieces, and a conventional number of turns with very conventional wire and tension, but inserted a spacer between the polepieces and coil, such that the coil really only "started" 1/8" or 3/16" out from the polepieces (which, of course, would not fit inside the usual plastic pickup cover), it would likely end up sounding different, if only because of the distance of so many turns from the polepieces themselves. If a pickup has 800 additional turns that are switch-selectable, I would not expect that pickup to sound much different when those turns are added from the location closest to the polepieces vs the outside of the overall coil, simply because so few turns creates a negligible difference in distance from the polepieces.

But when one has a flatter pickup, like a Jazzmaster or even P90, it doesn't take as many turns to create distance from the polepieces (or center bar and slugs). And I would think that where perhaps it might not make that much difference if additional turns were situated on the inside or the outside of a Strat-type pickup, they just might for a flatter wider profile. Again, taking into consideration that there are more, and less, consequential numbers of additional turns.
I don't want to question your findings regarding the coil to polepiece distance. But from the point of physics the only things that change with a wider "blind" core are an increased inductance because of greater mean circumference of the coil for a given number of turns, resulting in a lower resonant frequency and a slightly lower resonance peak because of the increased resistance. Altogether this will make the sound somewhat mellower. But there is another good reason to not let the wire touch the polepieces, i.e. prevention of chemical corrosion.
Again, different PU designs should be compared via samples having identical inductance/resonance (and not identical resistance!) to avoid that one only hears the sound difference caused by different resonance. Resonance/inductance of most designs can be easily adapted by the number of turns.
 
Mark Hammer 4/18/2018 8:10 PM
That's pretty much what I was suggesting, though perhaps in a manner that wasn't as clear as it needed to be. In essence, you have confirmed my initially query: that if one placed the additional turns closest to the polepieces - without any other modifications to compensate (as Joseph suggested) - there could be a change in the inductance and resonance of the default/basic coil, compared to if the basic coil was on the inside, by virtue any difference in the circumference of the turns in that basic coil.

Again, all of this is qualified by whether the number of turns making up the "additional" part yields any sort of appreciable difference to the circumference of the majority of turns on the basic coil. I say "majority of turns" because, even if the additional turns constituted 2000 out of 8000, and were situated on the inside rather than outside, many of the 6000 turns would be the same distance from the polepieces, and have the same circumference they would have had if the coil were only 6000 turns. Some, however, would be farther away from the polepieces than they might have been without those inner 2000.

Joseph's point about the influence of polepiece proximity suggests that placing additional turns on the inside mght result in a more noticeable difference in tone if one switch-selects those additional turns, compared to if the switch-selected addition was on the outside. Or have I misunderstood?
 
Antigua 4/19/2018 12:43 AM
Quote Originally Posted by Helmholtz View Post
I am well aware of all this and if the loudness step is what you are after, my proposal won't do.

My point is, that the unavoidable inductive coupling between the winding parts prevents a tappable overwound SC from convincingly reproducing the brighter sounds a a lower or "standard" wound PU, no matter how you split the coil or separate the windings, as the unused part of the coil always increases the effectice capacitance across the active coil by a considerable amount. As a result the two sounds available won't differ much - except when using very long guitar cables. But find out yourself.
I dunno, you know you stuff, I won't deny that, but I'm getting pretty close to winding a four conductor tapped single coil to see for myself. I only wind pickups for testing, so it's an ordeal to get everything set up.

Quote Originally Posted by Helmholtz View Post
In my experience the sound and dynamics of a good P-90 cannot be emulated by a Strat type design with alnico cores. The passive filter characteristics of a PU cannot explain the total response of a PU, even if the aperture is equal. But this would be a completely different topic.
I'm willing to entertain that topic. The AlNiCo poles have strong magnet pull, but a lower permeability. The lower permeability means the magnetic reluctance is higher, despite the stronger pull of the magnetic pole pieces. I don't believe magnetic reluctance really effects the tone, I don't see any plausible cause for that to be the case, but it is a fact that stronger magnetic pull upon the string interferes with how the string vibrates. I think the key to making the Strat pickup sound like a P-90 is, aside from matching the RLC characteristics, is to set it very low, so that the magnetic pull between the Strat pickup and strings are similarly weak. The overall output might be lower, but that should only effect the S/N ratio, and not the transfer function. A complication with testing this notion is that few Strats have P-90s, and few Gibsons have Strat pickups in them, so it's hard to compare apples to apples.
 
Antigua 4/19/2018 12:59 AM
Quote Originally Posted by bbsailor View Post
Wider coils detect a wider string length window and will have a different harmonic component than a narrow coil with a shorter string length window due to harmonic cancellations of certain frequencies over the pickup coil area.
The coil can't be the window, because the coil doesn't determine what flux change occurs. The coil size and shape only matters for how efficiently it sees flux change that is cast off by the interaction between the pole piece and the guitar string. If a wide section of the string is magnetized, it will cancel higher harmonics as you said, but the coil doesn't determine that, it's merely a receives what has already been determined, with varying degrees of efficiency.

For example, with a coil as wide as the Jazzmaster pickup, it's not only seeing flux change from from the primary magnetic path between the guitar string and pole piece, but the wider loops of the coil are also receiving a flux change from the return path of that same magnetic field. Though the flux density of the return path is weaker than the primary path, the return path is opposite polarity, so it represents a cancellation of signal, and so the wide coil is just an inefficiency, and little more. The job of the coil is just to take the flux change and turn it into a voltage. The question is just how efficiently it does the job.
 
Antigua 4/19/2018 1:39 AM
Quote Originally Posted by Mark Hammer View Post
That's pretty much what I was suggesting, though perhaps in a manner that wasn't as clear as it needed to be. In essence, you have confirmed my initially query: that if one placed the additional turns closest to the polepieces - without any other modifications to compensate (as Joseph suggested) - there could be a change in the inductance and resonance of the default/basic coil, compared to if the basic coil was on the inside, by virtue any difference in the circumference of the turns in that basic coil.

Again, all of this is qualified by whether the number of turns making up the "additional" part yields any sort of appreciable difference to the circumference of the majority of turns on the basic coil. I say "majority of turns" because, even if the additional turns constituted 2000 out of 8000, and were situated on the inside rather than outside, many of the 6000 turns would be the same distance from the polepieces, and have the same circumference they would have had if the coil were only 6000 turns. Some, however, would be farther away from the polepieces than they might have been without those inner 2000.
I just tested a tappable SSL-4 with the DE-5000, and got these values from the two halves of the pickups (Ls at 100Hz, Cp at 100kHz). Note that this is a three conductor tapped pickup, so the both coils are always connected at at least one end.

white to blk (whole coil) DCR: 13.62k Ls: 6.833H Cp: 179pF
white to red (inner coil) DCR: _6.57k Ls: 1.675H Cp: 384pF
black to red (outer coil) DCR: _7.05k Ls: 1.992H Cp: 310pF


As for how the inner verses outer coil geometry impacts the inductance, the outer coil has both a higher DC resistance and a higher inductance, but the inductance is not that much different, considering.

Regarding the capacitance, the DE-5000 is showing different values from what I calculated from the f and L using an oscilloscope, but it is still showing a higher capacitance for either individual half, compared to the whole.
 
Mark Hammer 4/19/2018 6:27 AM
Thanks for your diligence. The extra mile is much appreciated.

True, the differences are not dramatic. But then I suspect the differences existing between pickups X, Y, and Z, in those same parameters, that lead a player to prefer one over the others may be similarly "undramatic". Sometimes, what looks small on the measurement devices is a nuance that attracts a player. It may be small, but if they hear it (reliably), and it makes a difference to them, then I suppose we can say that it matters. Maybe more than it should, but it matters.
 
Helmholtz 4/19/2018 6:35 AM
Quote Originally Posted by Antigua View Post
The coil can't be the window, because the coil doesn't determine what flux change occurs. The coil size and shape only matters for how efficiently it sees flux change that is cast off by the interaction between the pole piece and the guitar string. If a wide section of the string is magnetized, it will cancel higher harmonics as you said, but the coil doesn't determine that, it's merely a receives what has already been determined, with varying degrees of efficiency.
This is correct. German electroacoustics professor Manfred Zollner could show by lab measurements that the string lenght window (aka aperture) only depends on the distribution and strength (to a lesser extend) of the PU's magnetic field. No influence of the coil's shape. The effective aperture length of a single coil is around 0.5''max. Humbuckers have two discrete windows, one over each coil. Humbuckers are "blind" right in the middle over the coils, because in this region the string saturates and its a.c. permeability drops to 1.
He also showed that the inner and especially the upper windings (closest to the strings) contribute most to the signal voltage, as the a.c. magnetic field is strongest in and around the upper part of the poles. The alternating part of the magnetic flux takes paths different from the permanent flux. The reason for this is that the permeabilities and consequently the magnetic conductivities of ferromagnetics (strings, pole pieces) differ vastly between a.c. and d.c. fields, a.c. permeabilities beeing generally much lower.
 
Helmholtz 4/19/2018 7:33 AM
Regarding the capacitance, the DE-5000 is showing different values from what I calculated from the f and L using an oscilloscope, but it is still showing a higher capacitance for either individual half, compared to the whole.
As mentioned earlier, calculating the capacitance from the resonance peak of a PU's impedance is not very precise.

1)There are additional parasitic capacitances from the setup: Wiring, input capacitances of scope/probe/meter etc.
2)The peak frequency of the impedance of a parallel resonant circuit is always somewhat lower than the theoretical resonant frequency given by 2*pi*fres = (L*C)^(- 0.5), depending on damping. If you use the peak of the transfer response instead of the impedance response, the error will be even greater.
3)The inductance drops with increasing frequency and is always lower at resonance than at 100Hz. This effect depends on core materials and eddy current paths. It is typically small for strat type PUs with alnico cores not having metal covers.

But - apart from these more academical considerations - I don't think an error of 10% or +/- 20pF really matters. A difference of more than 100pF might be audible, though.

(Did you try the "Automatic Voltage Scale" option? It will allow you to see the straight high frequency -12dB/octave slope in the transfer response (resp. -6dB/octave in the impedance response) which is masked by the noise floor in your plots.)
 
Helmholtz 4/19/2018 8:22 AM
The ultimate test is to wind Jazzmaster or P90 type pickup with an inner coil with independent start an ending leads or connections and then add an outer coil with the same number of winds with independent start and ending leads. With no metal plate under the coils attach the inner and outer coils to a dual trace oscilloscope and visualize the output levels of each independent coil. Now add a metal plate to help spread the magnetic field and do the same thing to see the output levels of each coil change due the this added metal plate variable being observed.

Can you post scope pictures?
I don't have tappable SCs, but I am curious because lab measurements (Zollner) have shown that the a.c. magnetic field at the bottom of a PU is only a very small percentage of the a.c. field at the top, which means that there is not much to be distributed. The baseplate of a tele bridge PU increases inductivity by 6% but the output is increased only by 0.6dB, which is close to nothing.
 
Helmholtz 4/19/2018 8:52 AM
I'm willing to entertain that topic. The AlNiCo poles have strong magnet pull, but a lower permeability. The lower permeability means the magnetic reluctance is higher, despite the stronger pull of the magnetic pole pieces. I don't believe magnetic reluctance really effects the tone, I don't see any plausible cause for that to be the case, but it is a fact that stronger magnetic pull upon the string interferes with how the string vibrates. I think the key to making the Strat pickup sound like a P-90 is, aside from matching the RLC characteristics, is to set it very low, so that the magnetic pull between the Strat pickup and strings are similarly weak. The overall output might be lower, but that should only effect the S/N ratio, and not the transfer function. A complication with testing this notion is that few Strats have P-90s, and few Gibsons have Strat pickups in them, so it's hard to compare apples to apples.
I agree, a true comparison is hard to do.
As an experienced player I notice major differences in the attack response between different core materials, even with equal B values at the strings and equal resonances. I think the clue is that the effective a.c. permeability of steel is not a constant but depends strongly on a.c. and d.c. field strenghts as well as on frequency. I have been doing measurements for some time that seem to support my theory. Don't want to go into more detail at the time.
 
Antigua 4/19/2018 10:58 AM
Quote Originally Posted by Helmholtz View Post
I agree, a true comparison is hard to do.
As an experienced player I notice major differences in the attack response between different core materials, even with equal B values at the strings and equal resonances. I think the clue is that the effective a.c. permeability of steel is not a constant but depends strongly on a.c. and d.c. field strenghts as well as on frequency. I have been doing measurements for some time that seem to support my theory. Don't want to go into more detail at the time.
Attack response would generally correspond to transient signals. You know a lot about transformer issues, is there any analogous consideration is transformer design, especially when you choose to use steel core versus ferrite? Modeling the guitar pickup and string as a loosely coupled transformer seems to draw an endless number of parallels otherwise.
 
Antigua 4/19/2018 11:06 AM
Quote Originally Posted by Helmholtz View Post
Can you post scope pictures?
I don't have tappable SCs, but I am curious because lab measurements (Zollner) have shown that the a.c. magnetic field at the bottom of a PU is only a very small percentage of the a.c. field at the top, which means that there is not much to be distributed. The baseplate of a tele bridge PU increases inductivity by 6% but the output is increased only by 0.6dB, which is close to nothing.
I've done similar tests with Tele base plates using the Vellemen and an inducer coil over head, and found the different with and without the base plate to be in the area of 1dB, give or take. One thing I did not try, whoch I suspect would make a difference, is if the pole pieces were to be replaced with steel instead of AlNiCo, there would be a lower magnetic reluctance between the strings and base plate, and so it might be realistic to see the output climb higher when the pole pieces are steel.

Regarding the magnetic path though, the metal base of the P-90 is either brass or nickel silver, which are not magnetic / permeable, so it does not have much effect to speak of on the magnetic circuit anyhow, and a Jazzmaster pickup of course uses fiberboard typical of Fenders.
 
Antigua 4/19/2018 11:20 AM
Quote Originally Posted by Helmholtz View Post
As mentioned earlier, calculating the capacitance from the resonance peak of a PU's impedance is not very precise.

1)There are additional parasitic capacitances from the setup: Wiring, input capacitances of scope/probe/meter etc.
2)The peak frequency of the impedance of a parallel resonant circuit is always somewhat lower than the theoretical resonant frequency given by 2*pi*fres = (L*C)^(- 0.5), depending on damping. If you use the peak of the transfer response instead of the impedance response, the error will be even greater.
3)The inductance drops with increasing frequency and is always lower at resonance than at 100Hz. This effect depends on core materials and eddy current paths. It is typically small for strat type PUs with alnico cores not having metal covers.

But - apart from these more academical considerations - I don't think an error of 10% or +/- 20pF really matters. A difference of more than 100pF might be audible, though.

(Did you try the "Automatic Voltage Scale" option? It will allow you to see the straight high frequency -12dB/octave slope in the transfer response (resp. -6dB/octave in the impedance response) which is masked by the noise floor in your plots.)
Regarding point #1, I determined the rig capacitance to be 10pF, so I subtract that from the final value.

I haven't set the Velleman up again yet, but I will try automatic scale when I do. I'd like to create a four lead tapped pickup this weekend, and do it all at once.
 
Helmholtz 4/19/2018 2:09 PM
Attack response would generally correspond to transient signals. You know a lot about transformer issues, is there any analogous consideration is transformer design, especially when you choose to use steel core versus ferrite? Modeling the guitar pickup and string as a loosely coupled transformer seems to draw an endless number of parallels otherwise.
Requirements for transformer and PU core materials are vastly different. In PUs technically inferior materials often give preferable sound results. A PU has more and different functions. I don't consider PUs as transformers. Both use induction, but so do other technical devices.
 
catalin gramada 4/19/2018 2:31 PM
Are the coils stacked or winded with two wires in the same time ?
 
Antigua 4/20/2018 1:15 AM
Quote Originally Posted by Helmholtz View Post
As mentioned earlier, calculating the capacitance from the resonance peak of a PU's impedance is not very precise.

1)There are additional parasitic capacitances from the setup: Wiring, input capacitances of scope/probe/meter etc.
2)The peak frequency of the impedance of a parallel resonant circuit is always somewhat lower than the theoretical resonant frequency given by 2*pi*fres = (L*C)^(- 0.5), depending on damping. If you use the peak of the transfer response instead of the impedance response, the error will be even greater.
3)The inductance drops with increasing frequency and is always lower at resonance than at 100Hz. This effect depends on core materials and eddy current paths. It is typically small for strat type PUs with alnico cores not having metal covers.
I've been measuring the capacitance of more Strat pickups with the DE-5000, and for some pickups, I'm getting very different results than I had based on the measured inductance and the resonant peak, like twice as little, or twice as much. Do you believe the DE-5000's measurements are more likely to be accurate? You mentioned that the inductance measure at 100Hz might not be accurate at the resonant frequency, though especially in the case of Strat pickup where eddy currents are low, the inductance tends to read the same value whether testing at 100Hz or 1kHz.

One example, I have a Fat 50 for which I measured 148pF from 2.4H inductance and a 7.9kHz resonance. The DE-5000 in Cp @ 100Hz is giving me back 66pF. This disparity is alarming because it means one method or the other is liable to be rather off.

One fun thing I was able to do with the DE-5000 was, while measuring capacitance, I pressed in on the coil with my fingers and I could see the capacitance climb by about 2pF when squeezed.
 
Antigua 4/20/2018 1:26 AM
Quote Originally Posted by Helmholtz View Post
Requirements for transformer and PU core materials are vastly different. In PUs technically inferior materials often give preferable sound results. A PU has more and different functions. I don't consider PUs as transformers. Both use induction, but so do other technical devices.
I'm not suggesting the pickup is a transformer by itself, but some others who I've talked to about this describe the pickup as a primary and the guitar string as being like a secondary, with poor coupling between them. In fact I think the idea has been discussed on this forum once or twice. Where has a real transformer's coils would have a mutual inductance near unity, the pickup and the string would have a linkage that is a small fraction of unity, due to large air gap separation. Based on your broader understanding of transformers, do you think this analogy is useful, or not similar enough to draw a comparison?
 
Helmholtz 4/20/2018 6:17 AM
Quote Originally Posted by Antigua View Post
I'm not suggesting the pickup is a transformer by itself, but some others who I've talked to about this describe the pickup as a primary and the guitar string as being like a secondary, with poor coupling between them. In fact I think the idea has been discussed on this forum once or twice. Where has a real transformer's coils would have a mutual inductance near unity, the pickup and the string would have a linkage that is a small fraction of unity, due to large air gap separation. Based on your broader understanding of transformers, do you think this analogy is useful, or not similar enough to draw a comparison?
No, a magnetic PU is a transducer. It is actually an electrical machine (including the strings), an electrical power generator followed by an integrated lowpass filter.
 
Helmholtz 4/20/2018 6:54 AM
One example, I have a Fat 50 for which I measured 148pF from 2.4H inductance and a 7.9kHz resonance. The DE-5000 in Cp @ 100Hz (??) is giving me back 66pF. This disparity is alarming because it means one method or the other is liable to be rather off.
I have seen this effect in strat PUs. The reason was that the second condition for use of the LCR meter at 100kHz not beeing fulfilled, namely a smooth -6dB slope at and in the vicinity of 100kHz. You may verify by measuring the impedance plot up to 100kHz or better 1MHz. This measurement requires using "Automatic Voltage Scale". If you see additional peaks and dips, this means that the PU shows multiresonant behavior, which indicates that something is wrong with it. This effect can be caused by partial winding shorts.

One fun thing I was able to do with the DE-5000 was, while measuring capacitance, I pressed in on the coil with my fingers and I could see the capacitance climb by about 2pF when squeezed.
Two reasons:
1) Pressing on the coil compresses the winding and thus increases distributed capacitance.
2) You body is a big capacitor to ground. Touching the winding or hot leads creates a second capacitor wired in series with your body, the touched part beeing the second "plate" of the capacitor. In result total capitance (to ground) increases.

I feel that this topic better belongs to the thread "Measuring capacitance..."
 
big_teee 4/20/2018 8:02 AM
Maybe you guys could consider a Private Message, PM.
Since You're going over, & over, the same ground?
 
Chuck H 4/20/2018 8:13 AM
Quote Originally Posted by big_teee View Post
Maybe you guys could consider a Private Message, PM.
Since You're going over, & over, the same ground?
Thank you Terry! I thought the same thing and even unsubscribed to the thread since it was looping for so long and I got tired of it popping up in my box. Not that some great stuff isn't being covered. What doesn't go over my head has been hashed and rehashed.
 
Antigua 4/20/2018 9:52 AM
Quote Originally Posted by Helmholtz View Post
No, a magnetic PU is a transducer. It is actually an electrical machine (including the strings), an electrical power generator followed by an integrated lowpass filter.
The thing is you can't model mechanics with a modeler like LTSpice, and since the interaction between string and pickup is electromagnetic, I think there is value is being able to abstract the relationship in terms that can be electrically modeled.
 
Antigua 4/20/2018 9:53 AM
Quote Originally Posted by big_teee View Post
Maybe you guys could consider a Private Message, PM.
Since You're going over, & over, the same ground?
I've found a ton of useful info on MEF, and I mean lots, through Google searches. If this is a PM, the info will not be accessible to anyone else who pursues this subject later.
 
Joe Gwinn 4/20/2018 10:53 AM
Quote Originally Posted by Helmholtz View Post
I have seen this effect in strat PUs. The reason was that the second condition for use of the LCR meter at 100kHz not beeing fulfilled, namely a smooth -6dB slope at and in the vicinity of 100kHz. You may verify by measuring the impedance plot up to 100kHz or better 1MHz. This measurement requires using "Automatic Voltage Scale". If you see additional peaks and dips, this means that the PU shows multiresonant behavior, which indicates that something is wrong with it. This effect can be caused by partial winding shorts.

Two reasons:
1) Pressing on the coil compresses the winding and thus increases distributed capacitance.
2) You body is a big capacitor to ground. Touching the winding or hot leads creates a second capacitor wired in series with your body, the touched part beeing the second "plate" of the capacitor. In result total capitance (to ground) increases.
This in a nutshell is why it is not considered a good idea to attempt to measure coil capacitance a at ten times the self-resonant frequency. Too fiddly. Also, all manner of effects that are irrelevant at the normal operating frequency range can become significant, causing errors. The standard approach since the 1920s (and documented in Terman) has been to measure the self-resonant frequencies with a number of film capacitors connected across the coil, and solve for the fixed capacitance.
 
big_teee 4/20/2018 10:53 AM
I see very little pickup winding relevance!
I'll go back to my yard mowing!
T
 
Antigua 4/20/2018 1:59 PM
Quote Originally Posted by Joe Gwinn View Post
This in a nutshell is why it is not considered a good idea to attempt to measure coil capacitance a at ten times the self-resonant frequency. Too fiddly. Also, all manner of effects that are irrelevant at the normal operating frequency range can become significant, causing errors. The standard approach since the 1920s (and documented in Terman) has been to measure the self-resonant frequencies with a number of film capacitors connected across the coil, and solve for the fixed capacitance.
Quote Originally Posted by Helmholtz View Post
I have seen this effect in strat PUs. The reason was that the second condition for use of the LCR meter at 100kHz not beeing fulfilled, namely a smooth -6dB slope at and in the vicinity of 100kHz. You may verify by measuring the impedance plot up to 100kHz or better 1MHz. This measurement requires using "Automatic Voltage Scale". If you see additional peaks and dips, this means that the PU shows multiresonant behavior, which indicates that something is wrong with it. This effect can be caused by partial winding shorts.

Supposing even 100kHz is prone to error due to secondary resonances in that range, what if a meter was able to test at 1 MHz? It looks like such meters cost well over a thousand dollars, but maybe a hand held that can test 1MHz will hit the market eventually.

Here are plots using the auto voltage scaling, and yes, they fix the noise floor issue amazingly well. Thanks again!

These first two are without the integrator amp, just the pickup into the Velleman PSU200:

[img]https://i.imgur.com/Y7StQQS.png[/img]
closer detail
[img]https://i.imgur.com/7WfojvA.png[/img]

This plot is with the integrator amp designed by Ken Willmott in between the pickup and the Velleman

[img]https://i.imgur.com/WDcLkvi.png[/img]

It appears that there are three impedance anamolies past the resonance, a small one at 48kHz, and a taller one at 93kHz, and another at 308kHz. I'm guessing that second resonance at 93kHz is that of the 100 turn exciter coil connected to the integrated function generator, due to it's prominence. I'm not so sure what accounts for the other smaller peaks beyond the resonance.


Quote Originally Posted by Helmholtz View Post
Two reasons:
1) Pressing on the coil compresses the winding and thus increases distributed capacitance.
2) You body is a big capacitor to ground. Touching the winding or hot leads creates a second capacitor wired in series with your body, the touched part beeing the second "plate" of the capacitor. In result total capitance (to ground) increases.

I feel that this topic better belongs to the thread "Measuring capacitance..."
I believe it's #1, because if it were #2, simply touching the pickup should induce the difference, but the increase corresponded to pressure. It demonstrates in real time the relationship between tension and capacitance, which is really cool.
 
Helmholtz 4/20/2018 4:06 PM
The standard approach since the 1920s (and documented in Terman) has been to measure the self-resonant frequencies with a number of film capacitors connected across the coil, and solve for the fixed capacitance.
"Terman's method" is fine, at least as long as inductance does not change with frequency, one takes care of parasitic capacitances of the measuring setup and the system (PU) has only one resonance. Maybe a little time-consuming. The theory behind is simple. Always nice to have different options. But each method has its drawbacks. I am not going to revive an old, fruitless discussion, though. Prefer being pragmatical instead, as PU capacitance is not a primary influencer of sound.
 
Helmholtz 4/20/2018 4:23 PM
The easiest and more accurate way to plot a PU's impedance response is to directly drive the PU from the generator with a series resistor of 100k to 1M ( the higher, the better). The idea is measuring the voltage across an impedance driven by a constant current source.
 
okabass 4/20/2018 8:11 PM
No PM, please. High level info here.
 
Joe Gwinn 4/21/2018 8:45 AM
Quote Originally Posted by Helmholtz View Post
"Terman's method" is fine, at least as long as inductance does not change with frequency, one takes care of parasitic capacitances of the measuring setup and the system (PU) has only one resonance. Maybe a little time-consuming. The theory behind is simple. Always nice to have different options. But each method has its drawbacks. I am not going to revive an old, fruitless discussion, though. Prefer being pragmatical instead, as PU capacitance is not a primary influencer of sound.
Not really. The inductance does in fact change, and one can quantify the change using Terman's Method. It's exactly those parasitic capacitances (of the coil) that one is measuring. Parasitic capacitances of the leads are far smaller, and more or less represent wiring inside the guitar.

I published the method some time ago in a separate thread.
 
Sooner Boomer 4/22/2018 2:16 AM
I've been lurking for a while, and decided to post. I know quite a bit about electronics, but have only recently gotten into guitar-related stuff. It's pretty hard to separate lore from fact when discussing guitars, and especially pickups. Putting number to descriptions helps immensely.

Please keep the discussions public. Yeah, you might be going over old ground, but each time a small kernel of knowledge is unearthed.

Has anyone ever wound a bi-filar pickup?
 
Antigua 4/23/2018 12:55 PM
I've made a four conductor tapped single coil, and I stand corrected: there outer coil brings the resonance of the inner coil way down, regardless of whether it's connected at one end or not. I'm not sure exactly what is happening in terms of circuit analysis, but obviously some combination of inductive and capacitive coupling. I'll post more details a little later today.
 
Helmholtz 4/23/2018 3:04 PM
I've made a four conductor tapped single coil, and I stand corrected: there outer coil brings the resonance of the inner coil way down, regardless of whether it's connected at one end or not. I'm not sure exactly what is happening in terms of circuit analysis, but obviously some combination of inductive and capacitive coupling. I'll post more details a little later today.
The phenomenon is called reflected capacitance by magnetically coupled windings. Now you may also measure interwinding capacitance between the windings.
 
Antigua 4/23/2018 7:05 PM
I finally got around to creating a 4 lead tapped single coil of my own:

[ATTACH=CONFIG]48601[/ATTACH]

Here are measurements of the four lead tapped single coil:

Inner coil without outer coil in place, 4000 turns
DCR: 2.935k
L: 578mH
Peak f: 16.3kHz

Inner coil with 4000 turn outer coil in place, not connected
DCR: 2.900k
L: 575mH
Peak f: 10.4kHz

Outer coil, 4000 turns, inner coil not connected
DCR: 3.117k
L: 711.8mH
Peak f: 10.0kHz

Both coils in series, 8000 turns total with connections at 4000 turns:
DCR: 6.028k
L: 2.282H
Peak f: 9.48kHz

So, the presence of the outer coil does bring the resonant peak of the inner down by quite a lot, from 16.3kHz down to 10.4kHz, but interestingly the 100Hz inductance measurement with the DE-5000 didn't change.

Here are several plots:

[ATTACH=CONFIG]48600[/ATTACH]

The most important takeaway is that the outer coil limits the resonant peak of inner coil, regardless of whether it's connected at one end or not, so the current industry practice of using three leads doesn't represent a drawback. There is very slight difference, but that would surely disappear in a load context. When the inner coil was by itself on the bobbin, the resonant peak was 16.4kHz, but with the presence of the outer coil, it dropped to 10.4kHz, and having a single connection at the ground side only reduced the resonant peak by another ~200Hz. And as was seen with production tapped single coils, the resonant frequency of the tapped coil(s) is nearly the same as that of both coils connected in series.

The two overlapping lines that dip way down show that when the outer coil is shorted, it becomes a huge load upon the inner coil, similar to eddy currents caused by covers and base plates, but much more extreme. The fact that they overlap means that the result is the same even if the shorted outer coil is connected to ground at the short point.

With this four conductor tapped coil there are other things that can be tested for, but overall it looks like
Helmholtz is correct, that the practical value of a tapped single coil is very limited by an unavoidable interaction between the two coils.
 
bbsailor 4/23/2018 8:22 PM
Antigua,

You made a nice 4 wire strat test pickup. I have one more test for you to try on this pickup that will put other issues to bed. Using a dual trace scope connect one scope channel to the inner winding and the other channel to the outer winding.

Hand hold the pickup above guitar strings and note the peak output voltage from each coil. Swap scope channels and coils to see if you get the same result. This will eliminate any bias due to the scope channels not being amplitude balanced.

If you follow this to its logical conclusion you could make a pickup coil with an upper winding and a lower winding and see the interaction of the coils in this configuration.

What at you want to do if form a good mental model about what happens to the pickup when you make a flat and wide pickup versus a tall and narrow pickup.

Another good new thing to try is to wind the pickup with AWG 32 to AWG 34 to about 500 to 600 turns and feed the pickup into 2 conductor shielded cable with an XLR connector on the amp end of the cable. Feed the XLR into a mic mixer input and listen to a different sound without the resonant hump in the typical electric guitar sound location. As an alternative you can feed the XLR ditectly into a Shure A95U mic matching transformer directly plugged into the guitar amp input. At the low impedance mic input impedance of 150 to 250 Ohms, the cable capacitance and winding capacitance will have a minimal audio effect.

These are just some tips from tinkering with guitar pickups for over 50 years.

Thanks

Joseph J. Rogowski
 
Antigua 4/23/2018 11:06 PM
Quote Originally Posted by bbsailor View Post
Antigua,

You made a nice 4 wire strat test pickup. I have one more test for you to try on this pickup that will put other issues to bed. Using a dual trace scope connect one scope channel to the inner winding and the other channel to the outer winding.
I know some people aren't satisfied that an inducer coil accurately represents a moving guitar string, but the inducer coil I use come pretty close to resembling the magnetic orientation of a magnetized guitar string

...

What at you want to do if form a good mental model about what happens to the pickup when you make a flat and wide pickup versus a tall and narrow pickup.
[ATTACH=CONFIG]48606[/ATTACH]

This plot shows shows the inner versus the outer winding, not plotted simultaneously, but with the inducer coil not having been moved at all between plottings:

[ATTACH=CONFIG]48605[/ATTACH]

The outer coil shows a 2 dBV increase over the inner coil. It would be interesting to experiment with a Jazzmaster style coil, add winds, then test it, add more winds and test again, etc. For a given load, the voltage generated is proportional to the total flux change through the loops of wire, so it seems apparent that the loops of the outer coil experience more overall flux change than the loops of the inner coil. I'm guessing there's a coil area of peak efficiency, beyond which the magnetic field becomes increasing parallel with the loop, and eventually even containing portion of the return path of the magnetic field, which would mean less overall flux change, less overall voltage.

Also note the extremely similar resonances, apparently because the coils share capacitance, which helps ensure the peaks will be close, if not identical.

Quote Originally Posted by bbsailor View Post
If you follow this to its logical conclusion you could make a pickup coil with an upper winding and a lower winding and see the interaction of the coils in this configuration.
We kind of have a top/bottom coil in the form of stacked pickups. I've compared the two halves of those before, and as to be expected, the top coil produces a greater voltage. I'll have to look at it again though and see if there was a strong "reflected capacitance" in that case.

One thing I think is worth trying though, is designing a pickup that puts the greatest number of turns possible, as close to the strings as possible, so you'd use 44AWG, or 45AWG, the smaller the better, and created a short coil of 8,000 turns, and then compare the voltage production of that small, short coil, compared to the a fatter, taller 42 AWG coil. In theory, since the smaller coil places more loops of wire closer to the string, it should capture a greater flux change than a typical 42 AWG coil, where many of the winds end up being farther away from the guitar strings.

If you think about what a PAF humbucker is doing, it's similar in theory: you have a tall Strat pickup with 8000 turns, you take 4000 off the bottom, give them their own coil, put it beside the first coil close to the strings. Those 4000 turns that were unproductive on the bottom are now just as productive as the 4000 turns that were above them, so it's like you have two "tops", and a greater voltage in turn.

AlNiCo poles pieces are less permeable than steel pole pieces, so the issue of distance between string and wire loop would disproportionately effect AlNiCo pickups.
 
David Schwab 4/26/2018 1:28 PM
I would have expected the inner coil to be the larger of the two. What happens when it's done that way?

And what if the two coils are stacked instead of coaxial?

Note: I haven't read all the posts yet

Quote Originally Posted by Antigua View Post
I finally got around to creating a 4 lead tapped single coil of my own:

[ATTACH=CONFIG]48601[/ATTACH]

Here are measurements of the four lead tapped single coil:

Inner coil without outer coil in place, 4000 turns
DCR: 2.935k
L: 578mH
Peak f: 16.3kHz

Inner coil with 4000 turn outer coil in place, not connected
DCR: 2.900k
L: 575mH
Peak f: 10.4kHz

Outer coil, 4000 turns, inner coil not connected
DCR: 3.117k
L: 711.8mH
Peak f: 10.0kHz

Both coils in series, 8000 turns total with connections at 4000 turns:
DCR: 6.028k
L: 2.282H
Peak f: 9.48kHz

So, the presence of the outer coil does bring the resonant peak of the inner down by quite a lot, from 16.3kHz down to 10.4kHz, but interestingly the 100Hz inductance measurement with the DE-5000 didn't change.

Here are several plots:

[ATTACH=CONFIG]48600[/ATTACH]

The most important takeaway is that the outer coil limits the resonant peak of inner coil, regardless of whether it's connected at one end or not, so the current industry practice of using three leads doesn't represent a drawback. There is very slight difference, but that would surely disappear in a load context. When the inner coil was by itself on the bobbin, the resonant peak was 16.4kHz, but with the presence of the outer coil, it dropped to 10.4kHz, and having a single connection at the ground side only reduced the resonant peak by another ~200Hz. And as was seen with production tapped single coils, the resonant frequency of the tapped coil(s) is nearly the same as that of both coils connected in series.

The two overlapping lines that dip way down show that when the outer coil is shorted, it becomes a huge load upon the inner coil, similar to eddy currents caused by covers and base plates, but much more extreme. The fact that they overlap means that the result is the same even if the shorted outer coil is connected to ground at the short point.

With this four conductor tapped coil there are other things that can be tested for, but overall it looks like
Helmholtz is correct, that the practical value of a tapped single coil is very limited by an unavoidable interaction between the two coils.