In article , Peter Chant
wrote:


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.

Not commented previously as I have been trying to get clear which items
have genuine mains-earth wires to their sockets and which do not.

Also, am I correct in thinking that when you swapped over the power amps
the presence/absence of hum followed the physical amp, not the channel?

In general I'd advise only having *one* item in the system with a mains
earth connection and using the connector braids back to it for the ground
definition of all other items. The problem with using multiple mains
grounds is that the results depend on the inter-unit ground wiring and
other items that use it. So what seems OK can change if the mains wiring
alters (e.g. a connection becomes poor) or some other item starts injecting
return into the wires.

It may well be that in a given setup multiple mains earth connections will
sound OK. If so, fine to use them.

For AV systems I tend to regard conventional TVs as worth being mains
earthed as they can tend to float large potentials. This is true with CRT
TVs in my experience, but I don't if flat panels are better in this
respect.

FWIW I've been wondering if the problem is due to a poor connection in thr
ground wiring of the relevant power amp. Either in the amp itself or in the
cable or plug/sockets.

Slainte,

Jim

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