Hello there. I've been, over time, re-wiring/re-patching our sound rack. One thing I have run into, is there is a ground signal all the time coming out of our loudspeaker arrays. The ground loop starts in the rack. There isn't any problem with inputs into the mixer, or anything going out of the mixer. All of the outputs on the mixer go over CAT5 to an expansion card on the rack. In a sense, the rack has no outside ground influence. However, there is a ground loop starting somewhere in the rack. I'm trying to identify the source of the problem, or find some way to eliminate the issue. We have wireless mic receivers, antenna distribution units, a bi amp distribution system, and 7 power amps on our rack. All of which, are connected to the same ground circuit. Hoping to get some help on this. I'll attach pictures, although good luck deciphering the back. Haha.
 

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jkowtko

Well-Known Member
Looks like fun! Start unplugging everything, one at a time, beginning at the speakers and moving backwards through the amp and continue until you find the culprit.

My ground loop story -- after days of searching I finally realized that, even though my sound cue PC was plugged into the same outlet as the board and rack, the monitor for the PC was plugged into a different outlet that happened to be on a different circuit ... so the differential went through the monitor cable and made it's way into the sound board ... ugh!
 

MNicolai

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Premium Member
Fight Leukemia
This is less likely to be caused by the AC power connection and more likely to be a low-voltage signal wiring issue.

General recommended practice is to lift the ground wire at the inputs of the amplifier. (LOW VOLTAGE AUDIO SIGNAL CABLING -- NOT LIFTING THE AC POWER GROUND) Just loosen those screw terminals and heatshrink or zip tie the ground wires back.

There are other troubleshooting steps you can take if that doesn't do it, but looking at the photos it is obvious that the installer didn't lift the signal grounds when they installed.
 
This is less likely to be caused by the AC power connection and more likely to be a low-voltage signal wiring issue.

General recommended practice is to lift the ground wire at the inputs of the amplifier. (LOW VOLTAGE AUDIO SIGNAL CABLING -- NOT LIFTING THE AC POWER GROUND) Just loosen those screw terminals and heatshrink or zip tie the ground wires back.

There are other troubleshooting steps you can take if that doesn't do it, but looking at the photos it is obvious that the installer didn't lift the signal grounds when they installed.
Thank you! Ill try this. Just to clarify, do you mean to just take the ground wire out of the input terminal on the power amps?
I assume that wouldn't cause any issues since it's a low voltage signal?
 

MNicolai

Well-Known Member
Premium Member
Fight Leukemia
Correct.

The shield along the length of the cable acts like a faraday cage, sucking up electromagnetic interference and dissipating it into the ground instead of letting it absorb into your signal wiring. For audio signals, it only needs to be connected at one end of the cable for it to be effective at that task. If you connect it at both ends, any/all difference in ground potential across your system will induce a small current across that rolls along your signal wiring and carries the ground hum all the way out to your speakers.

In the diagram below, the red line shows the current you are trying to disrupt. If you lift the shield/ground at the input to the amplifier, the other end of the cable will still be connected to something that allows it to dissipate that interference into the earth.

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DrewE

Well-Known Member
Correct.

The shield along the length of the cable acts like a faraday cage, sucking up electromagnetic interference and dissipating it into the ground instead of letting it absorb into your signal wiring. For audio signals, it only needs to be connected at one end of the cable for it to be effective at that task. If you connect it at both ends, any/all difference in ground potential across your system will induce a small current across that rolls along your signal wiring and carries the ground hum all the way out to your speakers.

This is accurate info, assuming that the connection to the amplifier is a balanced connection. If it's an unbalanced connection, lifting the ground will break the audio signal as well. (Also if they're unbalanced connections, it would be a very good idea indeed to make them balanced if at all possible, particularly as you seem to have some noise issues; the balanced connection is much more immune to picking up noise via induction, etc.)
 

FMEng

Well-Known Member
Fight Leukemia
I agree that's it's most likely a ground loop that can be cured from isolating shields at inputs, but I also see some interesting things in the photos.

I would move the Audia processor a couple of rack spaces away from the power amps. It's rare, but I have encountered pieces of equipment that pick up hum from adjacent units, and power amps can radiate significantly.

Near the top of IMG_0867, I see a gray cable feeding the input of a Crown amp that looks like it's unshielded with three conductors, red, black, green. That could be an issue. Audio signal cables must be shielded, twisted pair. The twisting, in particular, makes common mode noise cancel in balanced connections.

The other thing I note is 2 to 3 inches of un-jacketed wire at many of the Buchanan plugs. Those are little antennas.I don't like to see more than 3/4" of un-jacketed conductors, not counting the stripped portion in the terminal.

Coils of audio signal cables should be avoided. Cut them to length and re-terminate them. Bundle power cables apart from signal cables. Line level audio cables should be bundled apart. Mic level cables should also be bundled away from all others. Keep speaker cables away from audio signal cables.
 
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RonHebbard

Well-Known Member
Premium Member
This is accurate info, assuming that the connection to the amplifier is a balanced connection. If it's an unbalanced connection, lifting the ground will break the audio signal as well. (Also if they're unbalanced connections, it would be a very good idea indeed to make them balanced if at all possible, particularly as you seem to have some noise issues; the balanced connection is much more immune to picking up noise via induction, etc.)
@DrewE And worked for miles after miles for telephone companies across our nations and beyond for decades WITHOUT shielding.
Toodleoo!
Ron Hebbard
 

ACTSTech

Well-Known Member
To tag onto FMEng’s post, in img_0867, there’s a transformer plugged in near ckt 19. I’ve had these start to go bad or someone bangs them or drops them (occasionally me) which can give you your hum.

Any 120 generators around? We were having trouble at a show once, my hired hand was a music student and he kept complaining that something was running ”around B-flat.” Outside was a truck with a generator powering some carnival-type games. Combine that with poorly shielded wire and other crap, we had our culprit.
 

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