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AC (Power) Noise


What are the possible sources of AC (power) noise in a PA system, and how do I identify, isolate and eliminate them? How should a PA system be electrically wired, and what should/should not be powered by the same circuit(s) as my PA gear?
The setup in question is my church PA. Major equipment includes a Mackie SR32, comp/limiters, effects units, and eq. There is one amp for FOH, and one for monitors. Stage sound sources include multiple electric and/or acoustic guitars, bass, two keyboard rigs (all instruments DI'd), 5 handheld dynamic mics, 1 (phantom-powered) headset mic, 1 wireless headseat mic and 1 wireless lapel mic.
I'm no electrician, but I think the church's sodium-vapor lights are on the same circuit as some of the sound gear. And some of the incadescant spots are, too.
Also, do balanced/unbalanced audio signals play into this?
Help! The buzz is driving me nuts!
October 22, 2001 @04:58pm

Industrial lighting can really wreak havoc on an audio system. You may have to have a consultant (tech) come in and look at your situation more closely as there can be many subtle causes.
A few things that may help:
1) Try using all balanced connections between gear.
2) Find an AC circuit that has nothing else on it and run ALL of the audio equipment power directly to one outlet on that circuit. This may mean running an AC cord from the FOH mixing position up to the stage. Plug everything in at one point if at all possible.
3) Disconnect things one by one until the hum/noise goes away...or...start with the bare essentials connected and add to it until you get hum/noise.
By going through these steps you'll learn a lot about where your problem lies. You may still need a tech to come look at it, but at least this will rule out something simple.
October 25, 2001 @09:57pm

It seems only common sense to me that all audio equipment should be on one circuit, or if more than one, the only thing on the the group of circuits. I have contracted with an technician to come in and trace our electrical system and look for trouble spots.
In the meantime, I'll remove devices one-by-one from the audio chain and see if I can find offenders. I know for sure that our dual 15-band EQ is quite noisy, and might need replacing. (Hello, Sweetwater?)
Another area that concerns me is whether the industrial lighting and other mechanical equipment in the building might be generating RFI that's getting into the audio chain. I do have two 16x4 150-foot snakes that run between the stage and the sound booth, and they go through the attic near the sodium vapor light transformers, some HVAC blowers, etc.
I just want to eliminate hum and buzz from the system so I can concentrate on mixing the music (which as you know is enough trouble in itself!) The way I see it, when my gear's all powered up and there's no music being played/sung, I should hear dead silence - not hum, hiss and buzz. As it stands, I get these all the time, music or not...
Thanks for the reply.
October 26, 2001 @12:32pm

I've been there. First thing I did was grab a full set of the architectural drawings for the building. Found that all the phases had HVAC, lights, and other electric motors. We ended up putting in a new 200 amp service off the main disconnect. EVERYTHING was re-wired with new outlets and everyone was told to use the orange outlets for sound system purposes. We were thinking of putting in a new ground bar, but cutting a hole in the basement floor/wall, and installing sump pumps was not a viable option. The electricians said the whole works may be a bit of a gamble, ground problems can be quite an animal, but we won.
How noisy is your board? Unplug all inputs to your board, turn up the gains one channel at a time, listen to which is the noisiest, work the faders. We had an old Allen and Heath that needed some TLC every once in a blue moon.
Fluorescent light ballasts make noise by themselves, we had a high noise floor just on lights alone. Especially on a cold winters morning.
October 29, 2001 @05:22am

Of all the potential sources for noise you list, the incandescent spots (IF they are on dimmers) would be the first place I would look to. I've had bad experiences with dimmers before.
As others have mentioned, try balanced cords wherever possible, replace any cords with questionable shielding. I've even got desperate & wrapped bundles of cable with aluminum foil to add shielding. I've also installed lower output incandescent bulbs to avoid using the dimmers at all.
Run the dimmers down really low (lights barely on). If the dimmers are your trouble, that will make things lots worse.
Also, any guitars (especially with single coil pickups) are simple RF receivers. They really pick up noise from ballasts, etc. They also get noisy if the player stands too close to an amp, especially their own.
Hope this helps.
December 18, 2001 @11:56pm

Sounds like you may have a ground loop. Telltale indicators of this as a possibility (besides the noise, of course) are the use of multiple circuits (especially those terminating in different locations) for sound system components. In church systems, often, the amps are often located separately from the FOH gear, and often connected on different power, which can be trouble. Also, any DI'ed electronic instruments on-stage connected to AC power can cause a loop if the stage power is suspect. These can often be eliminated or reduced by lifting the grounds at the DI boxes (most have ground lift switches).
December 19, 2001 @03:10am

Finding the problem may take a while. Another approach might be to use a noise gate with the threshold set higher than the buzz level. The buzz will still be there, but it will be covered by the program, and there will be dead silence when the program (plus buzz) level falls below the noise gate threshold level. Good luck
December 24, 2001 @07:39am

I had that problem. I do local sound & lighting in LA. Depending on your power amps (I use crowns) they might have a ground lift on the SIGNAL wires coming from the console where the croosover wires go into the back of the amp. If you don't have a power amp that allows this, you might try disconnecting the dround on the signal wire at the power amp inputs. I found if i didn't do this i'd get hum. I'm assuming you're running balanced TRS or XLR connections. If not, it's now time to start. DO NOT lift the ground on the a/c connections on the console. (Which I've seen alot of bands do) It's dangerous to do so.
February 18, 2002 @03:57am