Electrician's advice wanted: safely lifting ground

Dan ed, your solutions is good only for components in which the input and output jacks are isolated from the chassis. I have seen few where there is such isolation.

Lifting the chassis ground is dangerous if there is a hot wire in contact with the chassis and you are grounded and can complete the circuit, such as being barefooted or touching the defective chassis and another grounded one.

My line stage only operates at it max if all other components have no ac ground but it does. Would Underwriters Labs take exception to this and deny certification to a manufacturer doing this? Yes. Would you city electric department? No. Do I feel unsafe? No.

Tbg,

IT is not the chassis ground! Chassis ground is intact, still there, grounding away, at the AC input. The inputs to both amps are grounded back to the preamp.

The lift is to the input connections ONLY, and they are tied to the power supply ground. Everything is grounded! Why doesn't it hum anymore?

One amp had an ADDITIONAL ground from the inputs to the chassis, one did not, simply due to different type of connectors.

Take a look at Rogue, Bryston, any other manufacturer that provides a ground lift. This is what they do also.

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.