electrical interferance?

[cyrax037]

Hi, I'm new to the place, I came in search for some answers to a few audio questions I have.

I am the audio technician for my small church, I've only been doing this for about a year, before that, I had no experience with a sound board at all, never even seen one in person.(I've also had no formal training or anything, so some of the terms and some equipment stuff I don't know so bare with me LOL)

I am also the audio technician at the local theater. One issue that we have, is that there is often a buzz that comes through the speaker system, and occasionally the monitors aswell. It is my conclusion that this must be some sort of electrical interference, as I tested it with a condenser mic that doesn't work very well, and the same buzz only MUCH louder comes through the speakers when it gets close to any electrical boxes or anything.

The theater is very old, and is in pretty bad shape. I'm certain there is grounding problems all over the place, especially the stage.(There is 3 or 4 electrical outlets in the stage itself) also, the stage light control box is setting right next to the sound board. I suppose the reason for this is so the technician con operate both, since its been hard to get volunteers that stick to it.


It only buzzes when mics are plugged in tho, I noticed the other day, when the hand held mics are up really high, especially the gain, it buzzes alot, and the condensers will buzz pretty bad most of the time. It is also present in the audio recordings I do of the shows, I have to run everyone's songs through a cleaning filter to pull it out. I thought at one point maybe it was the old speakers, but like I said, its only when mics are in, and its not the cables(atleast not in that they are bad cables) cuz I've tested them at church, and they are fine.

Most of the time its not really bad, hardly noticeable with the music going, but sometimes it gets pretty bad. I noticed the other day, that the position of the stage light levels effect it. At full light power, the buzz seemed to stop, but when I dim them to half, its when it was buzzing more. Could the high voltage from the box be effecting the board, or the snake? I am thinking perhaps the snake, cuz the same buzz from the mics occur from the ones on stage(about 50-75 feet from the board) and the mic I have upstairs, which is no more then 2 feet from the board.

I guess what I'm wanting to know, is if there would be anyway to stop this, cheaply. Could I shield stuff somehow, (could aluminum foil do any good?) Budgets are not great, and there is a TON of other stuff that needs to be fixed/repaired in the place before the sound system can get upgraded.(Like the leaky flat top roof that should have been replaced 20 years ago)

If it would help to know more about the system and equipment let me know.
Our next show is tomorrow, and I'll make sure to write down exactly what we have, tot he best of my knowledge.(I don't remember the name of the board, but I do know its probably atleast 10 years old, maybe older)

Thanks so much!!!

[dtpuga]

I think you may have about 75 different sources of your problems. However, one main thing is lighting. It is not uncommon for lights to cause all sorts of noise in A/V systems and the more they are dimming the worse it gets. In an auditorium we set up in frequently, we found our only solution was to bring in a power tap and conditioner and run all of our A/V gear through that. When we isolate our lighting system from the rest, we had great results. Even one piece of gear still tied into the same power as the lights will let the ground loop back into the system.

In our setup, we had everything isolated except for one video projector that was up on a truss next to the lights and sharing power with a dimmer pack. That one projector caused the hum throughout our system.

[cyrax037]

LOL, I think your projection of problem sources is probably on the generous side HAHA.


What could we do about it? I dunno exactly how the light box is setup, not being an electrician. Its not a plug into an outlet thing obviously. I dunno, we're trying to get grants and funding to remodel the theater.(its a historic building) so perhaps they only way to fix it is re-wiring everything. I had hoped there would be a temp fix for it or something tho.

[dtpuga]

Process of elimination can help. Start unplugging things and see if one or two things are your source of the main noise. Maybe you will be fortunate and find a culprit.

Also, if you only get the buzz when mics are plugged in then MAYBE its a bad snake from the stage to the console.

[cyrax037]

THAT could very easily be it right there! Lord only knows how old it is HAHA.(Like I said, I believe the board itself to be around 10 years or older, quite possible the snake is as old.) Also, we've had trouble with the monitors, ever so often, they'll cut out. The culprit is the main monitor, its loosing feed from the board, I believe the 1/4th plug has gone bad or something. Its only occasionally tho, one time I would wiggle the plug and it'll cut in and out, today, I wiggled it like mad and it stayed on consistent! It could be the same issue that causing the buzz, maybe there is a sort in the mic snake, or heck, it could very well be the stage plugs themselves. (I mentioned that the roof leaks really really bad right?) Under the stage is very musty smelling, from the water that's collected. Could cause some problems electronically wise right? HAHA


LOL, electronic problems are like potato chips, you can never have just one!

[travisvwright]

With a system that old, I'd suggest getting some electrical contact cleaner. Then walking every inch of the system and spraying every connection. Your out $10-15 and may fix some problems but will also prevent a lot as well.

[mikeymo1741]

Dimmers can cause a couple of issues. The first is RF interference. Dimmers (triacs) basically work by turning on the current partway into the load cycle. That means that sixy times a second, the source is being turned on and off rapidy. So instead of smoothly bringing the current up, it is spiking many times a second in a vertical waveform. This creates a magnetic field around all of the sources - the bulbs. (sometimes you can even hear the bulbs buzz.) The high-current dimmers on commercial stage pots are even worse than normal consumer lighing.

They also cause power harmonics, which can elevate the current and cause noise.

Isolating the power is one step, and a necessary one, which will help with the power harmonics.

For the RF, check that all your line-level cables are shielded or balanced. If possible, move the board away from the dimmers. See if the cables from the board to the amps are balanced (TRS) cables - they should be. That's probably where a lot of your interference is.

[travisvwright]

Originally Posted by mikeymo1741

See if the cables from the board to the amps are balanced (TRS) cables - they should be. That's probably where a lot of your interference is.

To second that, you can think of the board as being in the middle of your system. If you go straight out from the board say to a recording device or headphones, do you have buzzing there? If yes then the problem is before the board if no then it is after. It's nice to be able to cut your search in half but more than likely it will be both, but here's to hope.

[louca]

Originally Posted by mikeymo1741

Dimmers can cause a couple of issues. The first is RF interference. ... sixy times a second, the source is being turned on and off rapidy.

Great advice MikeyMo. I would correct you on the frequency since most TRIACs will switch on at the zero-crossing of the cycle and turn off somewhere midway into the half-cycle. And then repeat that for the next half-cycle, etc. So 120 Hz noise is generated. My point in mentioning this is 120 Hz is much worse sounding than 60 Hz. Also, the 120 Hz is not "clean" and contains several other frequency components, especially at low (dimmed) levels. In high quality audio systems, those overtones are faithfully preserved (or even EQ'd higher) through the system right to the speakers.

Lou

[cyrax037]

Thanks for the advice, I will look into the cleaner stuff!

For the RF, check that all your line-level cables are shielded or balanced. If possible, move the board away from the dimmers. See if the cables from the board to the amps are balanced (TRS) cables - they should be. That's probably where a lot of your interference is.

LOL, there is no separate amps, I'm guessing the board has an internal amp, and its all from that, that powers the two main speakers, and both the stage monitors. Pretty sad I know.

If you go straight out from the board say to a recording device or headphones, do you have buzzing there? If yes then the problem is before the board if no then it is after. It's nice to be able to cut your search in half but more than likely it will be both, but here's to hope.

What do you mean by straight out to recording device? Like bypassing the speakers and stuff? I haven't tried that, but if there is a buzz through the speakers, its there in the recordings aswell.

How would I tell if they are shielded and/or balanced?

This is currently the best picture of the setup. This is from my very first show, so things are in disarray and a mess HAHA.(Not that is TOO much better now lol)


maybe it'll help. If you think it might help, I can try to get better pics next time I'm in there. Thank you guys SOO much for your help!