Jim Lesurf wrote:

In article , Mike Coatham
wrote:

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.


No ground lift resistors diodes etc. Why it was not humming before I don't
know as there appears, at least on the power amp side, no attempt to address
the issue. Suspect it was small enough to generally not be significant.

Cable box is the culprit, perhaps, via the audio and video connections with
the media PC and TV. Suspect somewhere there is a big fat earth-loop via
the cable feed that was not there before, and it has been dumping current
into the multiway adapter that the TV, cable box, media PC and left hand amp
are plugged into. Result, unhappy pre-power combination as the left hand
amp's earth jumps up and down relative to the rest. Removing the cable feed
from the cable box removes the hum The hint was that just earthing the
screen of the pre to left hand interconnect to the respective power amp's
chassis caused hum - yet doing the same thing on the right hand channel did
nothing significant.

Plugging in the left hand amp into the same 4 way adapter as the pre and
other power sorted it.

This explains why by rears were humming at the same time as the fronts when
effectively a separate setup - it was also connected to the same multiway.
BTW, multiway's earth checks out fine.

But it is odd. I can measure no significant voltages or currents anywhere.
0.3V, open circuit between cable feed and earth, but that is small.

That was also my reaction. However you may be unlucky. It may be that your
amp's transformer has an earthed 'electostatic shield' and it has become
shorted to the windings in a way that gives 0 Ohms. That can mean
significant earth currents that should be flowing in the Neutral. Not
good.

Fortunately not that. Toroid, have part number but there is no spec on it.
Looks fairly standard - could probably replace if I needed to - though I
cannot forsee, unless I want to reuse it in a home made welding set up, why
it would fail.

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