Electrician's advice wanted: safely lifting ground

I'll try to explain, which means I just may botch this up. :-)

The hum was only between the two amps. Everything upstream unplugged. Only the amps were on. Ground loop between the two amps. The cheater plugs proved this.

Now, amp A had a ground connection from both input (-) rings over to chassis ground as well as the (-) wire going back to the power supply. Amp B had no such chassis ground on the inputs. Both amps have chassis ground from the transformer.

The transformers in the power supplies of the two amps don't tap to identical potential. So you get a ground current between the transformer taps on both amps and it hums like crazy because there are two paths to ground through both amps due to the leakage. Remove the connector ground to chassis (which is really the transformer tap) and no more leakage.

Did I confuse things by not stating that there are no cheater plugs in use? :-) I do have a habit of glossing over fine details.

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.