Ground Loop Issues

If you have two separate AC circuits and they both hum, it sounds like you have two separate ground loops, not a single one. This might also explain why you have two float both grounds.

Jdt332, you need to start from scratch. You will need to weed out the piece of equipment, cable, or what ever, that is causing the ground loop hum.

I assume from your post you have eliminated the possibility of a bad ic. Also from your post you have at one point disconnected all source inputs from the preamp, (removed all ics), and still the hum.

(A) Get back to basics first.
*Disconnect all ics from preamp inputs.
*Plug the preamp into the same power outlet circuit as the two Power Amps. Use a multi outlet power strip for the test if needed. (Do not use power conditioner.)
*Do not use any ground lifters. All you should have is the two Power Amps connected to the preamp by ics.

Check for ground loop hum.
>>>>>>>>>>>>>>

(B) If none, plug the preamp into the other dedicated power branch circuit. (With out the power conditioner connected.)
Check for ground loop hum.
>>>>>>>>>>>>>

(C) If none, add the power conditioner to the mix. (Connect the preamp to the power conditioner.)
Check for ground loop hum.
>>>>>>>>>>>>

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(A) If you experienced ground loop hum with only the two Power Amps connected to the preamp, disconnect one Power Amp from the preamp, (remove ic).
Check for hum.

If hum is still present from the single Power Amp connected, disconnect that Amp and reconnect the other Amp.
Check for hum.

If the hum is gone....this. could be the problem. One of the power transformers primary windings could wired with reversed polarity.

Or it could be an aftermarket power cord with the plug and IEC connector wiring reversed on one end. This is not uncommon......

(B) If you did not have the hum with preamp and Power Amps plugged into the same dedicated circuit but do with the preamp plugged into the other dedicated circuit, check for reversed AC polarity at one of the dedicated duplex receptacles. Easiest way to check is with a polarity checker. Pick one up at HD or any Hardware Store.

If the AC polarity is correct for both dedicated circuit receptacles the problem then could be the length of the two dedicated branch circuits.

May not be your solution, but something I found interesting in a similar circumstance. We have a three way light (dim, medium, regular/bright) in the same room as my system, different outlet/different circuit. My wife put a standard light bulb in this lamp (vs. a 3-way light bulb). Whenever the lamp was turned on to anything less than full/regular/bright, I got a buzz through my amp.

Once we put one of those small flouresent lights into a overhead light that had a dimmer connected, whenever it was set to dim (even very slightly) it results in audible issues.

Not sure if either may be an issue in your case, but thought I would share this experience FWIW.

If you have a cable box connected to the system, try a Jensen VRD-1FF Cable TV Ground Loop Isolator. See www.jensentransformers.com. After going around and around, that solved my problem -- the problem originated with the TV cable. The Jensen item costs around $70. Hope this helps.

Jdt332, Jbaxley, I had the same problem as you descibed. However, I also have an outside antenna.
Finally after unplugging everything and re-inserting one item at a time , I found the problem to be the cable line and the outdoor antenna line.
So, I needed rwo of the Jensen Isolators, which cured the problem.