Ground loop with a amp and a preamp in a GCX

[JohnClark]

Does the power supply you are using provide 9V AC? What is the current capability of this power supply?

Is there anything else that is connected to the amps or GCX when testing the simplified setup from above?

[Steven Latch]

Well no. All other jacks are disconnected.

Some units are plugged into the furman but not turned on. So only the GCX, preamp and marshall are hooked up, powered by the furman.

[JohnClark]

Try pulling the power cables to the unused devices one at a time to find if you are getting the ground loop through the rack rails via the earth ground connection somewhere.

[Steven Latch]

I will try.

However, I do have humfrees to isloate the racks and isolating tape on the top of the racks

Is there any chance not using the right power supply could be part of the problem?

[JohnClark]

Is there any chance not using the right power supply could be part of the problem?

Does the power supply you are using provide 9V AC? What is the current capability of this power supply?

[Steven Latch]

Sorry John!

The US robotics transformer just says 9V - 1000MA

[JohnClark]

Id guess that the power supply isn't the problem, particularly since the problem only occurs when the loops are switched a specific way.

[Steven Latch]

Well thanks for your replies John. I willl keep experimenting then

[Steven Latch]

Hi John,

Many more attemps, and still no luck. I have tried blaming either the amp or the preamp with a simple test: swapping either of them with another preamp, powered by the furman but sitting on top of the rack without any contact with the rails.

In any configuration, the result is the same: as soon as preamp 1 is playing and I plug the input jack of preamp 2, a significant hum appears.

Starting to run out of ideas...

[JohnClark]

As a test, try using a ground lifted cable from the out of the second preamp to the Return of the GCX Loop being used to A/B the preamp sections.