Testing bridged output amplifiers?

[Axtman]

I have recently come across two amplifiers that have bridged outputs. They both say to NOT ground the amp. What is the best way to hook up a load and oscilloscope to these amps?

[g1]

Make sure your load is fully floating. No grounds of any sort connected (unless you have isolation transformer for device under test).
You can measure the AC volts at the load with a hand held multi-meter.
You can ground your scope to the amp ground and look at each side of load, you will see half power at each end (the 2 ends will be out of phase with eachother).

[Helmholtz]

Each side of the load is connected to the hot output of a separate power amp.
Using a dual channel scope, you can simultaneously scope both sides and see the out-of-phase signals.
If your scope provides channel adding, this would allow to see the full signal across the load (both scope channels must have identical input sensitivity settings).

[Enzo]

They both say to NOT ground the amp.

It does NOT say that. It says do not ground the OUTPUT.

Make sure each channel works on its own. If they do, then there are limited ways for bridging to fail. Even with a load connected in bridge, you can ground your scope to chassis and scope either hot. They need to be out of phase, which is hard to see on a scope, but they waveform is the same as in stereo.

In service, I could scope it up across the bridged, but I never need to. Just fixing the channels will get me where I want to be.

[Axtman]

Should I use a ground lift plug on the amp, oscilliscope, signal generator or all three? (The dummy load is not grounded.)

[Helmholtz]

They need to be out of phase, which is hard to see on a scope.

Easy with a dual channel scope.

[glebert]

It does NOT say that. It says do not ground the OUTPUT.

Make sure each channel works on its own. If they do, then there are limited ways for bridging to fail. Even with a load connected in bridge, you can ground your scope to chassis and scope either hot. They need to be out of phase, which is hard to see on a scope, but they waveform is the same as in stereo.

In service, I could scope it up across the bridged, but I never need to. Just fixing the channels will get me where I want to be.

I think the amp shown above is single channel, bridged-mono sort of deal.

edit: thinking about it, you can treat each speaker conductor as a channel and test each relative to earth ground, which is presumably what you meant.

[Helmholtz]

Should I use a ground lift plug on the amp, oscilliscope, signal generator or all three? (The dummy load is not grounded.)

Certainly not!
Everything can be grounded except neither end of the load.
Just proceed as G1 recommends.

[Enzo]

Yes, you can hook up dual channel scope to it all, But by using one channel you can't look and see which phase it might be. Point was I rarely need to do that. I service the two channels individually.

Ax - the only thing to not ground is the load.

[Helmholtz]

But by using one channel you can't look and see which phase it might be. Point was I rarely need to do that. I service the two channels individually.
.

Getting your point.

Of course you can't read a phase relation from a single trace.

But in bridged mode the 2 power amps are not completely independent as they share the output current (meaning that the output current delivered by either amp must flow through the other one with the load connected between the 2 outputs).
Nevertheless if either amp works fine (wrt ground) and there is a 180° phase shift between outputs, there shouldn't be a problem.

[g1]

Even if you only have a single channel scope, the hand held multimeter reading AC volts across the load will tell you if the outputs are properly out of phase.

[Helmholtz]

Even if you only have a single channel scope, the hand held multimeter reading AC volts across the load will tell you if the outputs are properly out of phase.

Only if you're getting full undistorted output.

Don't get me wrong, changes to find a phase difference other than 0° or +/-180° are close to zero.