We just can't get rid of this hum.

[Lambda]

So, we've got a hum in our sound system. It's 60hz and its associated harmonics. Our system is structured as such: the amplifiers and crossover are backstage, and all other equipment is in the booth. It's a mono system and there's only one send to the backstage amp rack. In the booth, the mixer's mono out goes to a digital graphi-Q unit, then through an analog EQ, and down the snake into the crossover.
The hum is dependent on the dimmers, it's completely absent when they're at 0 or 100. This is odd because the dimmers get their power from a completely different place. They were added later than the facility was built, so they run off a separate three-phase feed.
Things I've tried: Unplugging the send from either the crossover or the booth EQ's output stops the hum, so it's in that line. Any setting of the EQ has no effect, nor does disconnecting its input. Bypassing the crossover has no effect. Lifting the ground has no effect. Isolation with a transformer has no effect. Increasing the gain of the crossover or amplifier will affect the volume of the hum as with all other signal.
Any suggestions as to where to proceed?

[Clarkwg3]

follow your snake from out of end to end. somewhere it's parallel within 24" of a dimmer circuit(s). R U able to narrow down which dimmer(range) creates the problem?

[chausman]

Start disconnecting things until it goes away. Somewhere in that signal path there is something causing it.

Is this something new? If it just started, what changed when it started?

[avkid]

Get a long known good cable and bypass the snake.

[FMEng]

It would be helpful to know the models of the EQ and crossover. It would also help to know if the both the EQ output and the crossover input are balanced. Just having XLR connectors does not mean the electronics behind them are balanced. It would also be good to ohm out the cable between the two locations to make sure it is OK.

[Lambda]

follow your snake from out of end to end. somewhere it's parallel within 24" of a dimmer circuit(s). R U able to narrow down which dimmer(range) creates the problem?

It is in a conduit that runs paralell to a condiut carrying dimmed circuits, but it's a bit more than 24" away. But circuits that aren't run through conduit will create hum as well. It gets worse as more circuits are turned on, wherever they may be.

Get a long known good cable and bypass the snake.

We don't have enough cable, unfortunately.

It would be helpful to know the models of the EQ and crossover. It would also help to know if the both the EQ output and the crossover input are balanced. Just having XLR connectors does not mean the electronics behind them are balanced. It would also be good to ohm out the cable between the two locations to make sure it is OK.

The EQ is a DOD 831, and the crossover is a Rane MX22. Bypassing either or both of them does not change anything.

I sort of forgot about this thread with tech week, sorry.

[Ric]

It's quite possible that it's not induced hum into the line, but a ground loop issue, regardless of whether it changes with dimmer levels.
This is pretty common when you have desk and amps/eq's etc. far apart and powered from different places.
Try powering all the devices from the same power outlet and see if the issue disappears.

[Lambda]

It's quite possible that it's not induced hum into the line, but a ground loop issue, regardless of whether it changes with dimmer levels.
This is pretty common when you have desk and amps/eq's etc. far apart and powered from different places.
Try powering all the devices from the same power outlet and see if the issue disappears.

The problem is, the amps and the other equipment are over 200' away from each other. We simply don't have enough extension cord. Plus, if it was a ground loop, wouldn't the ground lift have solved it?

[FMEng]

The DOD EQ is balanced on the XLR output and "pseudo balanced" on the 1/4" jack. Avoid the pseudo balancing by using ONLY the XLR output. If the line plugged into the EQ uses the 1/4" jack, it's time to get out the soldering iron and put on an XLR. The Rane crossover has balanced XLR inputs, so no problem there.

Now, let's take a good look at the cable running between the two. I would open the shells on the XLRs at both ends of the line to verify that the connector shells are not tied to pin 1 (that was a fad for awhile). Then, I would take an ohmeter and check all three conductors for continuity and proper connections. The wires of the twisted pair must be on pins 2 and 3, with the same color on the same pin # at both ends, and the shield drain wire on pin 1. The resistance of both wires should be the same (no more than 0.2 ohm difference).

If those steps don't turn up anything, then I would examine the power cord to ensure that the the AC plugs of both units have an intact ground pin and that they are plugged into a grounded outlet. We don't want anything like an AC ground lifter lurking around. Next, I would disconnect (desolder or cut) and isolate the shield drain wire from pin 1 inside the XLR connector at the crossover input. Dropping the shield connection at one end prevents ground loop currents from flowing in that shield and inducing noise into the signal conductors.

And, to answer your question, an AC ground lifter often will not solve a ground loop. That piece of equipment may still get grounded from the rack rails and from other equipment via audio shield connections, because audio grounds are almost always tied to AC ground inside the equipment. What ground lifters mostly accomplish is to create a shock hazard. There are many, better tricks to solving hums and buzzes. I will shamefully admit to grabbing an AC ground lifter in a pinch as a last resort, but the things are a really BAD idea and should not be used.

[TimmyP1955]

Did you put an isolation transformer on EVERY send from FOH to the stage? Even one without could cause the problem.

[andyw3]

Something seems very familiar about this.
I have a sneaking suspicion Lambda and I work in the same theater :P

The first I thought of when I heard the hum was the noise when a 1/4" TRS has a bad connection, like when a guitar loses connection with the amp.
Other than that, my minimal expertise can't add much to thread. I just wanted to say hi, sorry for disrupting.

[Lambda]

Did you put an isolation transformer on EVERY send from FOH to the stage? Even one without could cause the problem.

That's... a good point there. I didn't do anything to our greenroom monitor send, although there's hum there too, I didn't really care about it.

Something seems very familiar about this.
I have a sneaking suspicion Lambda and I work in the same theater :P

Suspect you're right... ANDY.

[Lambda]

Did you put an isolation transformer on EVERY send from FOH to the stage? Even one without could cause the problem.

Well, it was a good idea, but it didn't work.

[howlingwolf487]

Hey Lambda,

I know this thread has been soaking for a bit, but I wanted to ask a question:

What has changed in the system since the time you first noted the hum? Has any gear been added or taken out of the system recently?

If you can get into the amp racks, disconnect the main loudspeaker sends from the console (don't forget to label them!). If the hum goes away, you know it's from the snake/pulled wires backwards. If the hum remains, then there is a problem in the amp rack.

[Robert]

I had a similar issue when a technician plugged an input to the console into an insert jack. Had a slight hum that increased with the dimmers coming on. Just a thought.

My first thought was that the power supply to the board might have gone bad. Our equipment is over 15 years old and I thought there might be some age related deterioration of the electrical components. Check those wall worts that power you offboard gear. Try swapping those out as well. If you have any dimmer switches in the room turn those off and see if that clears up the noise. I've had trouble with those in the past inducing noice in my systems.

[andyw3]

I had a similar issue when a technician plugged an input to the console into an insert jack. Had a slight hum that increased with the dimmers coming on. Just a thought.

My first thought was that the power supply to the board might have gone bad. Our equipment is over 15 years old and I thought there might be some age related deterioration of the electrical components. Check those wall worts that power you offboard gear. Try swapping those out as well. If you have any dimmer switches in the room turn those off and see if that clears up the noise. I've had trouble with those in the past inducing noice in my systems.

I would hope the power to the board isn't bad, as we just purchased a brandy new board :P
Of course it may simply be the outlets, as we tend to overwork them a bit (powerstrip plugged into a powestrip plugged into a powerstrip, etc), although I don't know if that would have any affect on the amps. Granted I'm no electrician so I probably shouldn't be talking...
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.

[FMEng]

I would hope the power to the board isn't bad, as we just purchased a brandy new board :P
.

Ah ha, witholding information! Does the hum go away when the console output is disconnected from the EQ? What kind of console and which output buss and connector type are you using? Also, do you hear any of the noise in the headphone output when listening to input channels or busses?

[Lambda]

Ah ha, witholding information! Does the hum go away when the console output is disconnected from the EQ? What kind of console and which output buss and connector type are you using? Also, do you hear any of the noise in the headphone output when listening to input channels or busses?

I didn't mention the new mixer because when I unplug it from the system, the hum doesn't go away (or even change slightly). There's no hum in the headphones.
It's a Mackie Onyx, using the Mono send with a balanced XLR. Not that it makes a difference: for our last production we rented a Midas Verona and there was hum with that, too.

I'm convinced it's some sort of grounding issue. I don't know if I said, but when I unplug the EQ's input jack, there's no change. If I bypass the EQ, still no change.

[Lambda]

Hey Lambda,

I know this thread has been soaking for a bit, but I wanted to ask a question:

What has changed in the system since the time you first noted the hum? Has any gear been added or taken out of the system recently?

If you can get into the amp racks, disconnect the main loudspeaker sends from the console (don't forget to label them!). If the hum goes away, you know it's from the snake/pulled wires backwards. If the hum remains, then there is a problem in the amp rack.

When I disconnect the main send from the crossover in the amp rack, the hum disappears. Which would be an excellent solution, except then we're in permanent ultra-quiet mode.
(bypassing the crossover, going direct to the amps, does not remove the hum).
You say, "wires pulled backwards". What exactly do you mean by this?

Not much has really changed. The hum has been there the whole time, I've just recently had to increase the gain of our power amps, making it disturbingly noticeable. They were set quite low, presumably to make the hum less noticeable. Of course, that way, we've had trouble with the actors' mics being too quiet and getting drowned out by the orchestra, driving the console's and eq's output amplifiers nearly into clip and it still wasn't loud enough.

[mbenonis]

Ahh ha. Now we're on to something. Some questions:

- 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?
- Have you lifted pin 1 of that XLR, AT the stage?
- 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.
- 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?
- 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?

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.

OH: Is the amp rack metal or wood? Is that rack on wheels or mounted to the wall? If it's physically installed and metal, unplug EVERYTHING in it that goes out (signal and power) and measure the impedance of the metal part of the rack (scrape paint off if needed) to the ground pin of the outlet it normally plugs into. What is it?

[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.