On Mon, 26 Jul 2010 00:28:30 +0100, Peter Chant
wrote:

Don Pearce wrote:



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.

Two power amps and a pre-amp. As far as I'm aware the power amps and pre-
amps were designed to work together ie Cambridge A70 power amps and C70 pre-
amp - I've not mixed and matched. All have mains leads.


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.

Neither the pre nor power amps appear to be double insulated, all have IEC
leads and working earth connections in their leads.


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.

I'm using one power amp for the left speaker, one for the right.

With pre-amp connected to the mains, active powered with one of the power
amps, the one I use on the right, I have no hum. As soon as the left power
amp is connected - even if not switched on - I get hum. This hum is audible
in both amps and the headphones in the pre-amp.

Disconnecting the earth to the left hand power amp stops the hum. Swapping
the power amps over, left and right, and the hum still occurs when the same
amp is connected. It therefore appears that the fault is not in the preamp.
I am rather suspicious of the input circuity of the left hand amp. I am
rather confused though why hum is the only issue if there is a fault on the
input.

I'm loath to chuck transformers at the problem when it appears that there is
a genunine fault.

Pete

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.

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

d