SoulFetish 10/9/2017 9:29 PM
grounding load box w/BNC jack for 'scope?
I just finished building a new bench and I'm building a new load box rated for 200W(min) at 4Ω, 8Ω, & 16Ω. I'm going to install a BNC jack to the enclosure for easy access to the signal into the load. I want to double check the best way to handle the ground (in ref to earth).
Some builders/manufacturers ground the sleeve of the output jack to a ground reference somewhere in the amplifier. Others prefer to isolate the output signal to eliminate any ground loops.
So, my question in regards to the outer conductor in a 'scope probe is this: is there any risk in attaching the probe ground to the sleeve in an isolated signal? How would you handle grounding the enclosure of the load box so that it can safely take both isolated and non-isolated input signals?
 
The Dude 10/9/2017 9:48 PM
Do you always plug the gear your testing into an isolation transformer or isolated variac? (You should, but many don't.)
 
SoulFetish 10/9/2017 10:15 PM
Here is how I was thinking of doing it:
[ATTACH=CONFIG]45253[/ATTACH]
 
SoulFetish 10/9/2017 10:20 PM
Quote Originally Posted by The Dude View Post
Do you always plug the gear your testing into an isolation transformer or isolated variac? (You should, but many don't.)
Here is an article which makes a good case for not using an isolated transformer in many instances:
Isolation Transformers
 
Enzo 10/9/2017 11:38 PM
Problems may arise on solid state amps wher the speaker return is not to ground. If your amp is earthed, and your scope as well, then grounding the BNC tot
 
Mick Bailey 10/10/2017 1:15 AM
Think about how the box needs to be set up with a bridged amp. You don't want either the load or the scope grounding the amp's output.
 
J M Fahey 10/10/2017 8:16 AM
You know I love lateral thinking , if if I canīt solve a problem (happens all the time), I try to solve a simpler one instead
You know, 90% of results with 20% the problem/investment/effort.

In my load box, resistors go to an isolated jack ... so far same as you, so I can load standard/bridged/ungrounded amps , which is the main point.

The BNC "hot" pin can go, through a switch, to input jack Hot or "ground" (sleeve actually).

I add 2 posts for multimeter, one to jack tip, one to sleeve, I use spring speaker type red/black ones, so they hold multimeter probes there to measure VAC across load no matter what.

BNC ground goes to a crocodile clip so I can clip it straight to amp chassis so no problem even if amp is bridged or speaker is grounded through a small (but important) resistor.

But ... but ... thatīs not what "they" suggest !!!! "they" talk about expensive scope differential preamps, floating ground, the works !!!!!

Really????? letīs see what problems does my setup bring.

1) plain grounded output (think Twin Reverb or tube Marshall): no problem at all .

2) mixed feedback guitar amp: you are measuring (with scope) slightly higher voltage (typically around 1V) than actually going to speaker, because you are including the drop across current sensing resistor ... so 20VAC is probably 19VAC .
No big deal, scope is used *mainly* to check for distortion, clipping, oscillation, etc.
You want exact value for precise power measurement? ... thatīs what the multimeter terminals are for .
No multimeter?
You want to be precise?
Flip the switch, read voltage across grounding resistor, substract from main voltage measurement.

3) bridged amplifier.
Again no big deal, itīs made of two out of phase **exact same** amplifiers.
You check one for anything you want (yes, from hot to ground) , flip the switch back and forth as many times as you want to check the other.
You will miss NO problems at all (clipping/distortion/oscillation/hum/buzz/spikes/noise/etc.)

But .... but .... I see only half the actual voltage sent to the load!!! How can I calculate, say, full power?

Think again
Maybe itīs twice what you see on screen?

Besides, meter is floating and across the full load.

Whenever I get a computer back Iīll post a schematic.
 
Mick Bailey 10/10/2017 8:31 AM
I have my load so I can split it for working on stereo or dual-mono power amps where I want to simultaneously load both channels. I bring out my 4 ohm load resistors to isolated combo binding/banana posts. I can get patchable 1x16, 1x8, 1x4, 1x2 or 2x8, 2x4, 2x2 ohm loads.
 
xtian 10/10/2017 8:56 AM
Perfect time for this thread. Just ordered some thick film, 100w resistors from Newark.com for cheap, in order to build a new load box. Pulling up a chair...
 
SoulFetish 10/10/2017 9:25 AM
Quote Originally Posted by Mick Bailey View Post
I have my load so I can split it for working on stereo or dual-mono power amps where I want to simultaneously load both channels. I bring out my 4 ohm load resistors to isolated combo binding/banana posts. I can get patchable 1x16, 1x8, 1x4, 1x2 or 2x8, 2x4, 2x2 ohm loads.
This is the setup we use at work.
 
SoulFetish 10/10/2017 10:37 AM
Quote Originally Posted by Mick Bailey View Post
Think about how the box needs to be set up with a bridged amp. You don't want either the load or the scope grounding the amp's output.
Mick, could you explain what you mean by the box being set up with a bridged amp in this sense? do you mean using two probes in differential mode? If so, how would you configure tapping off signal points in reference to the isolated resistive load?
 
SoulFetish 10/10/2017 10:57 AM
Quote Originally Posted by J M Fahey View Post
You know I love lateral thinking , if if I canīt solve a problem (happens all the time), I try to solve a simpler one instead
You know, 90% of results with 20% the problem/investment/effort.

In my load box, resistors go to an isolated jack ... so far same as you, so I can load standard/bridged/ungrounded amps , which is the main point.

The BNC "hot" pin can go, through a switch, to input jack Hot or "ground" (sleeve actually).

I add 2 posts for multimeter, one to jack tip, one to sleeve, I use spring speaker type red/black ones, so they hold multimeter probes there to measure VAC across load no matter what.

BNC ground goes to a crocodile clip so I can clip it straight to amp chassis so no problem even if amp is bridged or speaker is grounded through a small (but important) resistor.

But ... but ... thatīs not what "they" suggest !!!! "they" talk about expensive scope differential preamps, floating ground, the works !!!!!

Really????? letīs see what problems does my setup bring.

1) plain grounded output (think Twin Reverb or tube Marshall): no problem at all .

2) mixed feedback guitar amp: you are measuring (with scope) slightly higher voltage (typically around 1V) than actually going to speaker, because you are including the drop across current sensing resistor ... so 20VAC is probably 19VAC .
No big deal, scope is used *mainly* to check for distortion, clipping, oscillation, etc.
You want exact value for precise power measurement? ... thatīs what the multimeter terminals are for .
No multimeter?
You want to be precise?
Flip the switch, read voltage across grounding resistor, substract from main voltage measurement.

3) bridged amplifier.
Again no big deal, itīs made of two out of phase **exact same** amplifiers.
You check one for anything you want (yes, from hot to ground) , flip the switch back and forth as many times as you want to check the other.
You will miss NO problems at all (clipping/distortion/oscillation/hum/buzz/spikes/noise/etc.)

But .... but .... I see only half the actual voltage sent to the load!!! How can I calculate, say, full power?

Think again
Maybe itīs twice what you see on screen?

Besides, meter is floating and across the full load.

Whenever I get a computer back Iīll post a schematic.
I see what you're doing here Fahey, and I LIKE it! It's a similar setup to what I had in mind, albeit, you thought to accommodate most commonly found audio amplifier outputs (home stereo, plus solid state and tube instrument amps). That was good thinking, and would be improvements to my initial idea. Wouldn't take but a few modifications to my original schematic. Also, I like the idea of running the sleeve of the BNC to an alligator lead. It's more consistent, and could minimize careless mistakes.
 
Mick Bailey 10/10/2017 11:29 AM
Quote Originally Posted by SoulFetish View Post
Mick, could you explain what you mean by the box being set up with a bridged amp in this sense? do you mean using two probes in differential mode? If so, how would you configure tapping off signal points in reference to the isolated resistive load?
Just that you need to ensure that in bridged mode one side of the load isn't connected to earth if you connect across the full bridged output. This includes the ground side of your BNC scope connection. Equally, your load resistors have to be isolated from earth. I've had plenty of success grounding the scope to the chassis. Having the BNC ground connected to a croc clip (as Juan suggests) rather than being hard-wired forces you to make a decision where to clip it. As you mention differential measurement I'm assuming you already have the techniques nailed.
 
g1 10/10/2017 11:51 AM
You probably have a reason you want to use BNC.
I'm with Mick in preferring dual bananas. The stackability and quick reverse function makes them very versatile. And they are isolated by default.
 
SoulFetish 10/10/2017 11:55 AM
Quote Originally Posted by Mick Bailey View Post
Just that you need to ensure that in bridged mode one side of the load isn't connected to earth if you connect across the full bridged output. This includes the ground side of your BNC scope connection. Equally, your load resistors have to be isolated from earth.
Oh. Yeah, I'm with you. I don't know why I thought you were talking about Isolating the probe through an active devise or something.

Quote Originally Posted by Mick Bailey View Post
As you mention differential measurement I'm assuming you already have the techniques nailed.
Is that actually a thing?? I was just throwing out tech jargon so I could sound like one of the guys
 
SoulFetish 10/10/2017 12:03 PM
Quote Originally Posted by g1 View Post
You probably have a reason you want to use BNC.
I'm with Mick in preferring dual bananas. The stackability and quick reverse function makes them very versatile. And they are isolated by default.
I was planning to put in some bananas for the multimeter. I'm not in love with using a BNC other than a convenient supply of terminated RG58. But you make a solid case for the ease of switching back and forth.
 
g1 10/10/2017 12:21 PM
You can put dual banana at the other end of the RG58 and BNC on the scope end.
 
Enzo 10/10/2017 12:33 PM
Aside from regular scope probes, I had a few BNC to BNC cables for my scope, and I had BNC female to dual banana male adaptors. For that matter I have BNC male to dual banana female binding posts.

[ATTACH=CONFIG]45263[/ATTACH]

[ATTACH=CONFIG]45264[/ATTACH]
 
SoulFetish 10/10/2017 12:57 PM
Quote Originally Posted by Enzo View Post
Aside from regular scope probes, I had a few BNC to BNC cables for my scope, and I had BNC female to dual banana male adaptors. For that matter I have BNC male to dual banana female binding posts.
Wouldn't ya' know, in a moment of "I have too much shit around here", I tossed (or gave away) a couple of pamona BNC to banana jack adapters during a shop purge. I haven't been burned for the most part doing that in the past, but, perhaps I should have made some room for those and held on to a couple.
 
Enzo 10/10/2017 1:11 PM
You know, I would toss those project of the future amp carcasses or whatever to make space, but tools and adaptors... never.

I probably should clean house now, I still have adaptors made for pinball machine CPU boards and dollar bill changers.
 
The Dude 10/10/2017 2:50 PM
Quote Originally Posted by SoulFetish View Post
Here is an article which makes a good case for not using an isolated transformer in many instances:
Isolation Transformers
FWIW: I find nothing in that article that convinces me to change my opinion. Quite the opposite. Of course you should check for AC-chassis leakage before you send something out the door, but that's not a reason for abandoning the isolation transformer. You can check for leakage with a meter, or find out with your fingers. I prefer to be safe. Of course, it still requires some common sense. Just because you're isolated doesn't mean you can't get a jolt.