B&K Power Amplifier - Fluctuating DC Offset

Guess is that one of the parallel MOSFETS is open circuit or you have a few dud caps.

You are setting the offset with a load on ?

for example, HH amps hunt up/down if the quiescent current is set without a load on.

Hi, I checked the voltage drop on the emitter resitor at each mosfet and all seems to ok, 12-16mV on 0.47ohm resistors. There are still a few old small green caps around. I'm trying to set the offset without any source or speakers on it. There is a separate pot for the quiescent current.

Thanks!

Hi, I checked the voltage drop on the emitter resitor at each mosfet and all seems to ok, 12-16mV on 0.47ohm resistors. There are still a few old small green caps around. I'm trying to set the offset without any source or speakers on it. There is a separate pot for the quiescent current.

Thanks!

Good morning,

While I don't know this amp specifically, I would suspect the input differential pair. Since this amp uses FET output devices, I'd guess that it uses an FET differential pair, though bi-polar is possible. The differential pair could be discrete transistors or some form of multi-leaded device. If replacing individual transistors, matched ones are a good idea.

If you find this device, try blowing on it, and you'll likely see the offset drift around. Lastly, I'd suggest monitoring the offset with the covers on the unit, so that everything gets up to temperature the way it normally would. Some drift in offset is normal on some amps.

Regards, Tim Schwartz Bristol Electronics