grounding load box w/BNC jack for 'scope?

[SoulFetish]

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]

Do you always plug the gear your testing into an isolation transformer or isolated variac? (You should, but many don't.)

[SoulFetish]

Here is how I was thinking of doing it:

[SoulFetish]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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.

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