Serge Auckland wrote:
johnelstad wrote:
He shoots, he scores! My amp has a three-prong plug, so I added a
two-prong adapter and the hum disappeared completely. I'm so relieved.
Thanks so much, James, for your quick and helpful response!

John

James Lehman wrote:
Does you power amp have a two prong plug? If so try adding a wire from the
chassis to a real Earth ground. If your power amp has a three prong plug,
try it with one of those two prog adapters with no ground at all.

James. )


Be careful that you may have removed the safety ground connection, and
your power amp is grounded only through the audio cable screen to
another piece of grounded equipment. What you have proved is that the
hum is most likely coming from a ground loop. What you now need to do is
to find and eliminate the ground loop whilst still maintaining the
safety ground.

S.

Try plugging both power amp and preamp into a power Y adaptor (using
all 3 prongs). This will insure that both chassis are at the same
ground reference, and you have safety ground. If you still have hum
then break the shield on your interconnect at only one end (I'd break
it at the power amp end but shouldn't matter), this should eliminate
the hum loop. The interconnect will still be shielded mind you, but
that shield will only be connected at the preamp side making spurious
current flow in the shield impossible. All signal flow will then be
referenced to the safety ground. You can make a shield breaking
adaptor if you dont want to cut up your interconnect cables.

I install home intercoms as a side job and the first rule is to never
ground a shield at both ends. Because ground references throughout a
typical home are always a few microvolts off. So if you ground a
signal shield at both ends, AC will flow through the shield and make
the system hum every time.