Probing power amp output with resistive dummy load : isolation ??

[ricothetroll]

Hi,

I've come accross some issue I've never had (noticed ?) before  : while testing the output of an Eden 500W bass amp (transistor) with my resistive dummy load, I've noticed some groth distotrion on the waveform. Seemed strange as the amp sounded pretty OK when loaded with a speaker.

I've plugged headphones on it (headphone out is an attenuated version of the power output) I've noticed that it was when I put my 1/10 probe on the dummy load that the distortion appears. I've then configured my scope (Tek 466)  for differential measurement (CH1 minus CH2) and then could see a perfectly nice waveform.

Have you guys ever experienced the same thing ? I'd like to keep use of the 2nd channel of the scope while doing those measurements to be able to monitor the input signal along with the power output, so I though of building a diff input. I don't need 100MHz of BW, but 1MHz could be nice to be able to check for oscillations. Would a THAT1200 do the job ? Could the isolation be sufficient ? Of course I'd use it with a 1/10 probe to prevent overloads.

Best regards.

Eric

 

[CJ]

distorted wave forms, oscillation, circuit breakers tripping, earthquakes, global warming, not fun to blow up a new set of output transistors or tubes, so we grab a power transformer, 120:24 volt out of the junk drawer, and hook it to the dummy load,

now we have no worries of 120 pwr getting on the output,\ due to a non isolated scope, or ground loops, any any other gremlins you can think of,  we also step down the level to be more compatible to the scope input, and, we add a bit of inductance to the load via the 120 primary, who could ask for anything more?

is not the transformer the best thing that man ever invented?  ;D

 

[Samuel Groner]

Could it be that the amp has a differential output, and you shorted the negative output to ground with the probe ground clip?

Samuel

 

[moamps]

Could it be that the amp has a differential output, and you shorted the negative output to ground with the probe ground clip?

Yes, this is most likely reason for massive distortion.  If the amp in question is WTX 500 there is warning at rear side about grounding. Those are obviously D class diff. type amps.

 

[ricothetroll]

Hi,
Thanx for your answers!
Well I must admit that this experience has showed a huge flaw in my diy dummy load: the banana/bnc measurement connectors were reversed! I've been using it for like a year now, but just changed my scope from an early 60s Phillips PM5600 to a "new" Tektronix 466. The Philips wasn't earthed so the problem never showed up, as it was actually floating.
Happily enough I monitored the signal with scope while testing,  so I never ran signal to the amp's shorted output more than a few seconds and with the master very low...
However I did some tests with a 220v/18v trafo (35H measured at the primary) and a signal generatorand a scope at the secondary, it showed a surprisingly flat frequency response up to about 50KHz. Past that frequency I've noticed a few resonances, like twice the voltage that I get at the other frequencies. I also pit some square wave through. It shows some serious overshoot.
Couldn't this show some undesirable oscillations with some amps? As the goal is to make sure the amp is working correctly,  I'm afraid getting wrong results with such a setup.  What do you think?
BTW the amp's a WT550, that's 500W with a regular push pull output (a complicated schematic though). Working fine finally ;-)
Best regards
Eric