try a 1:1 transformer from AB-2 box to mixer 1

try removing sheild on cable for AB-2 box to mixer 1 (that probably won't work)

try a a cap on the sheild(ground return) on cable for AB-2 box to mixer 1

Plug all the equipment into a Furman Pro II power strip.... with the ground lift engaged.

Bad Advice

GoshenSound wrote: Plug all the equipment into a Furman Pro II power strip.... with the ground lift engaged.

No! That safety ground is there for a reason. That reason is safety! Never lift the ground on a device that uses a 3 prong electricity cable.

Now that we have that point behind us, lets focus on a safe and correct resolution of the problem. Kev makes some good suggestions, and they are certainly worth trying. I am going to suggest that you reverse which outputs from AB Box 2 are feeding which mixers. You said that the AB Box 2 --> Mixer 1 output was causing the problem (since when you disconnect that cable the hum goes away). What happens if the AB Box 2 output currently going to mixer 1 goes to mixer 2 instead (and vice-versa)? Does the problem now follow the AB Box 2 output (problem in Mixer 2) or does it follow the mixer input (problem still in Mixer 1)? Knowing this will help you determine if Mixer 1 or the output of AB Box 2 that is going to Mixer 1 (in your diagram) is the source of the problem.

These problems are never fun to track down. Good luck with it!

for some reason the hum goes into both the mixers regardless of the AB setting. When I fade the volume on either of them up it increases the humming sound.

Camel42 wrote: Can they damage equipment?

No, only if it gets so bad it starts to induce some sort of "harmonic feedback" and the speakers start to amplify the signal, then you could, at the worst, blow you monitor's speaker cones.

Camel42 wrote: How do you get rid of them?

Use a ground lift switch. As far as the comment about "never use a ground lift". That's why they make those things. To lift a ground loop, and get rid of noise.

Camel42 wrote: When the audio is disconnected from AB Box 2 to Mixer 1 the ground loop is broken.

It sounds like you have it narrowed down to what piece it is. That's the most important thing, now just try lifting the ground that goes to that piece of equipment.

Camel42 wrote: All of the cabels except the audio from Mixer 1 to Mixer 2 have a shielded end.

since you have created a natural loop with this setup, is most likey why you need to "un-loop" the system. You have so much stuff that is somewhere else touching the same piece at a different location. This is normal, but it does create a way for the ground to share the noise, rather than get rid of it. (down the grounding path or wire)
But if you use a ground lift on the piece that is inducing the noise, then you should be able to keep it away from all the other equipment that is not having the same issues.
Ground lifts are ALWAYS safe and fine in ANY circumstance. The (white wire) or neutral "is" the ground. And if you don't believe me, open your electrical panel in your house and tell us where all the grounds and neutral's are tried to. (the same terminal strip).
The noise you are getting is in many cases normal, you just have to narrow it down to one piece of gear and rid that piece of interference.
Good Luck

go ahead and fry yourself

If you lift the safety ground, and you have a short-circuit or other condition within the equipment with the lifted ground, where is the electricity going to go? NOT to the ground, which is where it should be going!

Look, if you want to lift the ground at a device to see if it resolves your problem, fine. If it resolves your problem though, all that it does is tell you where the problem is. There are safer, and more correct, methods for fixing a ground loop than lifting the electrical safety ground.

If you want to agree-to-disagree about this, that's fine. I have nothing further to say.

Re: go ahead and fry yourself

hociman wrote: If you lift the safety ground, and you have a short-circuit where is the electricity going to go? NOT to the ground, which is where it should be going!

Wrong, the grounding electrode should NEVER have current on it! period.
That ground is per The National Electrical Code article 250. Ground's are for fault currents, NOT for short circuits. The breaker or fuse is for short circuits. The breaker and fuse are the only thing protecting you from harm.

hociman wrote: Look, if you want to lift the ground at a device to see if it resolves your problem, fine.

So now your saying to do it? :? What gives?
Are you trying to help the man or what? First it's "Never do that"...now it's "Fine if it resolves your problem"

hociman wrote: If it resolves your problem though, all that it does is tell you where the problem is. (read 3 X)

:roll: Read this 3 times, it gets funnier every time.

hociman wrote: There are safer, and more correct, methods for fixing a ground loop than lifting the electrical safety ground.

I guess your gonna keep those "safer, and more correct methods" secret for yourself, rather than share information. Unless your are really just full of it and do not know what you are talking about. So what are these secret methods?

hociman wrote: If you want to agree-to-disagree about this, that's fine. I have nothing further to say.

Oh yes you do, you'll be back.
Tell us all this...
Why do companies put ground lifts on equipment?
And what are the "safer and more correct, methods for fixing a ground loop".

Camel42 wrote: for some reason the hum goes into both the mixers regardless of the AB setting. When I fade the volume on either of them up it increases the humming sound.

Some equipment works better with other equipment. And sometimes you get a piece of equipment that is a bigger challenge to keep quiet, for whatever reason. In addition, once you connect several pieces of equipment together, and everything is connected "through" another device to another device. This is what we refer to as a "loop". Because a stray signal or small noise can travel around and around the setup until it's traveling faster and faster, and the signal starts to ampify itself.
When I say "stray signal" I'm talking about fault currents. Almost all electronic and electical equipment will have some amount of a fault current. Some having very little, other gear having more.
My guess is that you have one piece of equipment that is creating most of your problem(s). Hopefully seperating it (Yes, using a ground lift) will help keep the noise from being induced into your setup.