On 27/07/2010 11:30 a.m., Peter Chant wrote:
Jim Lesurf wrote:

In , 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.


Thanks. This is all useful, can be helpful to discuss things when trying to
sort out problems.

Pre-amp powered - mains earth
2x power amps - mains earth.

cd player - no earth (double insulated)


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?

Kind of. When the left hand amp is connected both amps hum. The right amp
on its own does not hum. Swapping left and right on the preamp does not
cause this condition to swap - therefore it is not a fault on one of the
preamp channels.


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.


It sounded fine until recently - a slight hum, but you had to approach the
speakers to hear it with nothing being played. At present it is
unacceptable.

Incidentally, left hand amp (the problematic one) - phono sheild to earth
pin reads 0 ohms, right hand amp which is fine reads 20 ohms. Something odd
going on there.

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.


Something to do with the flyback transformer? A charge build up? All TV's
I've owned have not been earthed.

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.

I'm suspicious of the input circuitry. I suspect something has failed short
circuit.

Pete

Check the faulty amp to see if it has a ground lift resistor or capacitor
that has gone short circuit. This would explain the sudden onset of hum
from that amp. It shouldn't be that hard to find as you have another amp to
compare with.