Electrician's advice wanted: safely lifting ground

It is not safe. Thanks anyway, I got the answer and a better solution. :-) There is a ground wire that runs between the grd at the input connectors and over to a chassis ground near the ac inlet. I removed that connection and things are every bit as quiet as with the cheater plugs. I'll get a switch to mount on the back and use that as a grd lift. The power supply is still grounded and all is well. Happy feet!

Folks. I did contact the person who built/mod'd my amps and it was he who provided the solution based on my experiments to track it down. Once I opened both amps, the problem was clear. One amp was using wbt input connectors and had a connection between the ground rings and chassis ground. The other amp did not have such a ground connection. The are not mono-block amps, just two stereo amps bought from different people who chose different options as far as the input connectors are concerned. Things are still grounded through the power cords.

The cause is that the center taps on the transformers are not identical and you get a ground loop current between the amps when their inputs are connected together. Simple once I saw that one amp did one thing with input ground and the other didn't.

Not completely sure where I got the idea I originally posted about. Probably from reading forum posts. ;-) :-)

is why are you using equipment that is so poorly designed that you have to lift the ground to avoid hum?

Thanks for your opinion, but did you read my last two posts?

The only ground lifted is the chassis ground to the INPUT connectors. The chassis is still grounded at the power input to the amps. There is no safety issue with my solution.

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.

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. :-)

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.