Scoping solid state amp output -- head check needed

[xtian]

I want to measure the power output of two SS amps, a Crate Power Block, and a 7-watt Rolls amp, into 4- and 16-ohm resistive loads.

I've done this a million times on tube amps, but I'm having trouble with the SS amps. As I turn up the volume with 1KHz sine applied to input, I get nothing on the output until some wild oscillation happens. Same on both SS amps. Swapped in my 10-watt tube amp, and all looks OK again.

I decided to use my $30 DSO Shell scope instead of my Hantek, and I see the signal now! Next, I hook both probes to the output, and it works...sometimes. There's something causing the SS amps to misbehave unless the DSO Shell or an actual speaker is attached.

What's going on here?

[xtian]

Ah ha. When I clip the probe's ground lead to speaker output -ve, it causes this fault condition. Obviously, my probe's ground lead is directly connected to earth. Why is this a fault state? Should I have known this already?

[glebert]

Just my guess. Transformer coupled outputs are not necessarily ground referenced, just a differential voltage between the secondary wires. SS amps will usually have one signal lead and one ground lead to the speaker. If your probe has one side tied to ground and you hook that to the signal lead you are basically shorting the ouput.

[Enzo]

Look at your schematic, neither speaker wire is ground. At least on the Power Block. I don't have a Rolls schematic, you might post one.

So you are shorting out one side of the amp when you ground it with your scope probe ground clip.

[xtian]

Understood. So how do I correctly measure output RMS?

[glebert]

Understood. So how do I correctly measure output RMS?

How about a DMM?

[g1]

Yes, use floating DMM. Scope at output with scope ground connected to chassis so you can see the onset of clipping, but use the RMS number from the DMM for your calculations.

[J M Fahey]

That.
No problem at all in scoping one and then the other as two standard Amps (which they are, after all, just "used" different).
Standard troubleshooting applies.

[Dave H]

If your scope has two channels it's usually possible to invert one channel and use it in A+B mode (differential) to measure between the outputs without grounding either of them.