I wonder if anyone can offer me any advice.
I'm experiencing a loud hum through the speakers from my 541 processor. I'm using this with a 555 power amp, but the buzz is definitely coming from the processor because there's no noise using an Arcam pre amp instead, and it even occurs with nothing else connected to the 541 other than the 555.

If instead of using the front left and right pre outs, I use a tape output to connect to the 555, there's no hum at all, but the volume is deafening because the volume level is fixed. There's also no hum if the power amp is connected to the rear or centre pre outs instead of the fronts.

I've tried different interconnects and mains cables / supplies but this makes no difference.

I'm suspecting a fault on the 541, but any suggestions would be welcome.

Thanks, Jeremy