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

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

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
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
.

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?