We just can't get rid of this hum.

[Lambda]

- Is the run between the mixer in the booth and the amp rack on the stage a proper, balanced XLR? Have you verified that this cable is wired correctly?
The cable seems to be wired fine, although the two ends are much too far apart for me to use a tester on it. Suggestions?
- Have you lifted pin 1 of that XLR, AT the stage?
Yes. I've lifted it at the booth, too. It actually makes the hum slightly louder.
- Is anything bonded to the conduit that the XLR runs in? Measure the impedance of pin 1 to the conduit with it disconnected at both ends.
I'll try this.
- Does all equipment in the stage rack share a single common ground? E.g., is all equipment plugged into a rack rider or some similar, which is plugged into a single outlet?
Stage rack is powered by a single outlet. Booth equipment was on two different ones, but I moved it all to 1. No effe
- Same question for the Booth.
- Is there any additional equipment in the facility connected to the sound system which is plugged in to power somewhere else?

 

[howlingwolf487]

Is there any additional equipment in the facility connected to the sound system which is plugged in to power somewhere else?

and

A test you can try: Does the hum go away when an audio source AT the amp rack is connected? Use a CD player or small mixer and microphone and make sure everything is plugged into the same power source and that you have a single ground point for that rack.

I think that answering these two questions will explain a lot. It would be good for Lambda to see if there are up-to-date electrical plans available so they can see what circuit paths are shared between outlets and various systems (audio, video, lighting, stage power, etc.).

 

[howlingwolf487]

When I disconnect the main send from the crossover in the amp rack, the hum disappears...(bypassing the crossover, going direct to the amps, does not remove the hum).

Okay, next series of question/advice:
Does the level of hum get louder when you move the main fader up?
Is the mixer connected directly to wall power or is it going through some other device (power strip, power "conditioner", etc.)?

Honestly, here's what I would do at this point:
1. Disconnect any cables going to the amp rack from FOH as well as the mixer's power plug into the wall.
2. Next, use the line out from a battery-powered, line level device to feed the amp rack some music or signal. Connect it using the appropriate snake channels/returns.
3. Next, do the test again, but plug the device into FOH wall power.
4a. If the hum reappears, you know the issue with with the AC power and not a particular piece of gear.
4b. If the hum does not come back, the issue is with a particular piece of gear.
5. Repatch cables/wires into the mixer in this order and check for hum after each step: mixer-->amp rack; AC wall power-->mixer; other returns (from stage, monitor world, etc.)--> mixer; inputs (one-at-a-time)-->mixer; inserts-->mixer.
6. Let us know the outcome.

You say, "wires pulled backwards". What exactly do you mean by this?

Heh, I should have phrased that better. I wasn't saying that the wires themselves were pulled backwards, just that if the hum went away, them problem would most likely be isolated to anything before the amp rack (amp connections<--snake fanout<--conduit<--snake fanout<--mixer connections<--mixer, etc.).

 

[mbenonis]

The cable seems to be wired fine, although the two ends are much too far apart for me to use a tester on it. Suggestions?

Rat Sniffer!

 

[Lambda]

Rat Sniffer!

Well, we have pretty much no funding, but it doesn't look too hard to build something like that.
In the meantime, I'll just take a real close examination of the connectors on the ends.

 

[Ric]

I think all the testing we have done shows pretty clearly that the dimmers have a significant effect on the hum. No lights on, no hum. All lights at 20%, loud hum. All lights at full, as far as I remember, no hum again. Weird thing is, as I believe has been said before, is that the sound stuff and the light stuff run on completely different lines.

This is the most common issue with any venue. The power is not separated or run as you would expect/hope. You'll likely find that Lighting Power & Audio power; if actually separated will still use a common neutral which is likely the hum cause. Ideally, the active & neutral for your dimmers is totally separate to your active & neutral for your audio. If they're not separate, when your dimmers kick in & chop the waveform creating all sorts of distortion & harmonics, this also appears on the neutral, and as the neutral is a common line to audio, appears there.
A lot of equipment also ties neutral to ground or uses the neutral as a psuedo ground exacerbating the situation.

 

[Lambda]

Well, I've figured it out. In checking every connector, I checked one that I had neglected to check before. From the EQ output, there's actually a splice in the cable consisting of a 1/4" TRS plug and an inline jack. I never checked either of those before.
Turned out that on that plug, the ring connection wasn't making contact. That resulted in essentially an unbalanced line running to the amps, free to pick up all that 60hz noise that floats around when the the dimmers are up. The reason a ground lift made it louder but audio still got through, is that it severed the return line. So, instead of having a direct return up through the snake, the ground connection between the EQ and amps was made through the ground bus in the distro panel.
So, now I have no hum at all, plus an extra 6db. Thanks for all the advice. Just goes to show you, always check EVERY part of the signal path.

 

[andyw3]

Well, I've figured it out. In checking every connector, I checked one that I had neglected to check before. From the EQ output, there's actually a splice in the cable consisting of a 1/4" TRS plug and an inline jack. I never checked either of those before.
Turned out that on that plug, the ring connection wasn't making contact. That resulted in essentially an unbalanced line running to the amps, free to pick up all that 60hz noise that floats around when the the dimmers are up. The reason a ground lift made it louder but audio still got through, is that it severed the return line. So, instead of having a direct return up through the snake, the ground connection between the EQ and amps was made through the ground bus in the distro panel.
So, now I have no hum at all, plus an extra 6db. Thanks for all the advice. Just goes to show you, always check EVERY part of the signal path.

I guess it's good you found it eventually, although it seems like something that we could have found easily before hand.

 

[Lambda]

I guess it's good you found it eventually, although it seems like something that we could have found easily before hand.

You know how it happened? The strain relief on the plug gave out, and for who-knows-how-many years, it's been hanging right on the solder connections. This might not be the only plug that's like that, we should check them all.

 

[tjrobb]

I like that you want to check all of them. Too many people assume there is only one problem, problems can easily be hidden by other problems. Best of luck!

 

[FMEng]

Good work finding the culprit. I recommend replacing the 1/4" TRS plug and jack with a pair of Neutrik XLR connectors. They make for a more robust connection in all respects.

 

[mbenonis]

[Whoops, replied to the wrong thread!]

 

[TimmyP1955]

The mention of power strips brought something to mind. It could be one of the strips (or rack units). O had a group bring their wireless mics in to the theater for a show. Buzzzzzzz as soon as I plugged in their rack, which had a Furman rack strip. I plugged all their gear into one of my strips instead of the Furman: Buzzzzzz. I unplugged the Furman (which was now doing nothing): Silence. My guess is that one or more of the Furman's surge suppression components was leaking current to ground.