On Sat, 24 Jul 2010 16:34:28 +0100, Peter Chant
wrote:

Don Pearce wrote:


Have you tried unplugging things to see what is causing the hum? Try
and arrive at the minimal setup that results in hum - even if that
means disconnecting one of the power amps - then describe the circuit.

Pre amp - plugged in
2XPower amps

I'll have to try them separately.

Are the power amps separate items, each with its own mains lead? This
is a very minimal setup to still be causing hum, and I suspect maybe
you have a power supply fault in the pre-amp.

There is always a bit of a battle between minimal hum and safety.
Minimum hum demands a single earth connection for the entire system,
while safety says each item should have its own. If the preamp or
power amps are double insulated, then they won't need a safety ground
and the signal grounds in the coax connections will do.

Anyway, temporarily disconnect the grounds from the mains plugs in
each item and see if you get a cure. If you do, then some more
isolation transformers will be a good idea. Maplin sell them for car
stereos. If there is no cure, then it isn't a ground loop but a fault
in the preamp.

d