SVT 4 Pro gets low and distorted when loaded

[Andrew58]

I got a svt4 on the bench and noticed it was distorting way to early when running bi amped both channels at once to the speaker. and it was a bad sounding distortion. So i checked power amp a and it sounded fine then checked power amp b and it was very low volume and distorted. I checked the preamp outs and they looked fine and equal on both channels. Then i noticed when i unplugged the speaker from the b amp all of a sudden the signal cleaned up and got just as large as the A side when compared on the scope. How should i go about finding the problem of amp distortion when loaded down. Any one seen this? Thanks

[Enzo]

NO guarantees here, but when I see a power amp that puts out a consistent clean signal unloaded, but the signal collapses when the load is connected, it is USUALLY a problem with the output stage being unable to provide the full current the load demands. Unloaded, there is no demand for current. Current doesn't come leaking out and dripping on the floor when no speaker is connected. SO the output can run the full waveform.

I'd look at the drivers first, myself. What does the waveform do when you watch the gate busses for the outputs?

REmember that solid state amps are heavily fed back, so if part of the waveform disappears at the output, the input circuit will try to compensate. SO you MIGHT see something like the top half of hte wave is missing at the output, but at the op amp the top half of the wave becomes exaggeratedly large, because the amp is trying to drive the amp to make the output full. You might think of that as the equivalent of your guitar amp making very low output, so you crank the volume trying to hear it.