Electrician's advice wanted: safely lifting ground

Are the jacks on both amps insulated from the chassis? You mention that Amp A has a ring suggesting that the inputs are in fact isolated. Have you tried only having Amp B on to see if you have hum? Or only Amp A on? I suspect that Amp A is causing your problem as it has two paths to ground. I have never known two components causing the ground loop, but rather two paths to ground causing the problem.

Yes, both sets of jacks are isolated from the chassis. However, one amp had it's inputs tied to chassis ground so in effect one set of inputs was connected to chassis and one was not.

Either amp, used by itself, is completely quiet.

Yes, it is two paths to ground when one amp has a connection from input to chassis ground and one does not. Because chassis ground is relative to the transformer ground on each amp, and the transformers don't tap at the exact same voltage, so there is a potential created between the two amps. The slight ground current is running from one amp to the other via the input cables. Two paths to ground.

I agree, amp A was creating the potential through that input chassis ground. Lifting the ground on the inputs on amp A did break the loop.

As I said, this does fix the problem safely. I don't mind continuing the discussion in the hope that someone else will find this helpful. Ground loops can be very difficult to solve and I know how much time I spent on this.

This stuff is only fun after you find the solution. :-)

Yes, it is two paths to ground when one amp has a connection from input to chassis ground and one does not. Because chassis ground is relative to the transformer ground on each amp, and the transformers don't tap at the exact same voltage, so there is a potential created between the two amps. The slight ground current is running from one amp to the other via the input cables. Two paths to ground.

Dan_ed,

Then what you are saying amp A had the signal ground connected to the chassis ground, amp B did not. Correct?

Because chassis ground is relative to the transformer ground on each amp, and the transformers don't tap at the exact same voltage, so there is a potential created between the two amps.

the transformers don't tap at the exact same voltage, so there is a potential created between the two amps.

Primary or secondary windings? Can you be more specific....

Over the years I have read threads where guys with mono amps tried everything to rid them selves of a ground loop hum. Only in the end forced to use ground cheaters.

Jea48, our forefathers did not have this fun as their ac was ungrounded. With their stuff in wood boxes, I guess they were safe.

sorry, Jea48. I am not familiar with the specifics of the transformer. But I believe the answer to your question is yes. One amp had signal ground connected to the chassis and one did not.

Coincidentally, I believe that it was work on a pair of new monoblocks that my electronics guy has been doing which gave such a quick solution. He had been struggling with just such ground loop before finding the answer. ;-)

I think it was probably comments from some of those guys using cheater plugs that prompted my starting this thread.