Don Pearce wrote:



This is all very odd. A hum appearing when you plug something in, even
if not switched on, is classic ground loop. And as you describe the
system, each item individually grounded to the mains, is a guaranteed
generator of hum. You may not need to resort to a transformer though.
First try some coax cables with broken grounds for your interconnects.
I don't know how practical/technical you are, but this is certainly
something you can easily do for yourself. Break the ground connection
in just the left hand interconnect first. That should have the same
effect on the hum as disconnecting the mains ground. If it does, stick
with that. There are no safety implications and it is standard
practice.

I made the interconnects, so breaking the screen is not an issue. This may
well be an expedient fix before I get around to finding the real culprit.


But the system as you describe it should always have hummed unless you
have recently changed the interconnects.

I'm suspicious of the input board on the amp. With just the offending amp
switched on (all connected) I get hum, but it is much stronger in one
channel than the other. If it were a regular earth loop resulting from the
design I would have thought it would be equal in both channels. I have
previously checked the diodes on the input board of that amp, all seemed
well, as far as I could tell. I'd think that it is not one of the op-amps
or I'd have a dead channel.

Pete

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http://www.petezilla.co.uk