Audio Systems: Ground Loop from Hell!, ground loop isolator, 8 ohm speakers

Question
I'm installing a PA system in a church basement, and I think I'm experiencing a ground loop. But no matter what steps I take to remediate it, it persists. The ground loop hum is also accompanied by some hissing at higher volumes.

The amplifier is a QSC GX3 driving 2x 8 ohm speakers in a parallel configuration, per channel. The mixer is a Behringer RX1202FX. Both mixer and amplifier have balanced XLR connections, which is what I tried first. There was a nasty hum and hiss, so I have also tried:
- Lifting the ground (pin 1) of the XLR connector (the amp end)
- Going in to the unbalanced inputs on the amplifer (XLR to 1/4 cable that connects pin 1 and 3)
- Going in to the unbalanced inputs on the amplifer (XLR to 1/4 cable with a transformer in it)
- Inlining a Radio Shack "ground loop isolator" (model 270-054) between the amplifer and mixer
- Connecting the chassis of each component together
- Lifting the ground on the power cables (both for the power strip and then for each component individually)
- Using the ground loop isolator *with* the XLR transformer adaptor (made it worse!)
- Using a Power Conditioner instead of a power strip
- Verifying proper wiring of the outlet with one of those plug-in outlet testers

I have also tried connecting the 1/4 outputs (for the "control room") to the 1/4 inputs on the amp. Same buzzing and hissing.

Then, I tried connecting the RCA outputs (for the "tape") to the RCA inputs on the amp. Same buzzing and hissing.

I also tried the ground loop isolator with the RCA to RCA route.

Then, I tried connecting the headphone output on the front of the mixer to the 1/4 inputs on the amp (using a 1/4 stereo to 1/4 mono "Y" cable.) Same problem.

I know the amplifer is good because when I plug a microphone directly into it, there's no ground loop or hissing.


Any ideas? I'm getting desperate!

Thanks!

Answer
It would seem it is not a ground loop - but, rather, no ground where you need it.  Such as between the amp and the mixer.

Here is how I would try to isolate the problem.

ONE:  Disconnect the connections between the mixer and power amp.  Hum?  No Hum?
TWO:  Connect the output of the mixer to the input of the power amp.  Hum? No Hum?
THREE: Turn the volume down to zero of the power amp. Turn the volume down on the mixer masters.
FOUR:  Try to determine when/how the hum is coming in and where.

You may have to check a few items such as when you connect the lines to you get instant addition of the hum? Then find out why it is bringing up hum with that connection.

The QSC may have floating inputs that need to be grounded, for example.

So, keep investigating and let me know more.


Hope this helps.