Our PA System is Messed


The sound system in our gym SUCKS!

We have several problems with the sound system in our gym (the gym is used for thing like assemblies and our annual arts showcase). First of all, whenever the sound system is on, a few of the speakers play the radio. Nothing loud, but still enough to be noticable and annoying. When we were working on the aformentioned arts showcase, we tried to fix the problem. Our school TD thought it was a grounding problem, so we put little grounding plugs on the speakers that were playing radio. But whenever we couldn't hear it on one, it started up on another speaker. We didn't have enough time to fix it before the show, so what we ended up doing is turning off the system when no one was using the microphones so you wouldn't hear the radio during musican pieces.

Another problem that we noticed with the same system is caused by the lighting dimmers. When we were setting up for the show, we noticced that whenever the lights were at anything but full and off the sound system would get a terrible hum in it. For some reason the dimmer system was screwing up our sound system. The guy who set it up (guy who runs the radio department at the school) said he was going to fix it, but has made no effort to (probably because he was the one who screwed it up in the first place). Our TD plans to tear down the whole system over the summer and set it back up again. *sigh*
hmm you might want to try and isolation transformer, if they make them that big, any decent power conditioner should have some sort of system like that in it. should filter out the EMF noise form your lightrs and dimmers, or alternativlay use your sound on a diffrent circut mabe

We once had cablew wired wrong in my old school and the music from the computer was going into the sound board, out throught the power wires, into the power bar, and into the amp and out the speakers.....explain that one.....
the radio problem seems like one i had when i was doing sound for a live TV broadcast, the length of the audio cable acted like a big antenna, and allowed a standing wave to form, we clearly heard the local radio station. i never checked the physics to determine this for sure, but changing the length of the cables helped. Also properly groundng them should help.
It would be helpful if you could provide more information on the equipment in the sound system and how it is hooked up. If you have wireless, how many do you have, what brand/model are they, what frequency(ies) do they run at, etc.

As far as the dimmer/sound system hum, this is best diagnosed by a licensed electrician. Chances are you won't have much choice as to where it is plugged in, and if that is wired incorrectly, you can't do anything about it without an electrician. DO NOT USE A GROUND ISOLATOR TO FIX THE PROBLEM! This is an electrical code violation and it's just asking for someone to get hurt if there is a potential between earth and the sound system's ground.
Interesting problems....

In regards to the radio interference..typically this can happen when your buidling is close in proximity for your location to a broadcast tower. Usually within a couple of miles for the strong towers. If you can find the station, you can call and ask them where their towers are. Unfortunately if you are in close range--not much can be done except to check all your cables and use properly shielded cables for all your runs--and do not run cables near power or over metal trussings, and see about some RF dampening in your building to reduce this problem. Not cheap...

But that said--there are occasions when because of either old/bad electrical power / ground or cable installations in all-metal truss structures such as in gyms and some grids...that a building can in effect become a large "reciever" and pick up such RF interference. Running unshielded mic or cheap unshielded snake cable (or damaged cable--one nick in the shield can screw ya) thru metal trussing or along side of power cables or high up in metal structures can make this part of the "antenna". Contrary to popular belief--speaker cables do not usually act as the antenna. Even if they did, the signal that is picked up and heard requires some sort of amplification...typically the snake, audio send or mic wires that go to a amp are what become the best antenna and are responsible for most of this. Old amps as well are not as well shielded and grounded as current and new amps of today as well...and may be suceptible to bad grounding and RF interference causing problems. Additionaly poor grounding issues in high voltage electrical power can also induce some ground/RF interference. Given that you have a HUM problem in your speakers with the dimmers on tells me you have power and electrical issues.

Your power for your audio needs to be NOT on the same feed and most importantly not on the same neutral/ground buss as your dimmer power. Lots of older buildings use chassis grounds or pipe grounds in breakout boxes and outlets--not up to NEC code or safe IMO, but most are grandfathered and not corrected. Additionally--some buildings main power comes in to a large box and is split improperly without the best returns or proper cable run. This could be your problem....or the power or outlets you are using is grounded to your building boxes or pipes in your building--not providing a true and adequate ground.

Would suggest you first have your building checked out by a qualified electrician for your main power panels to check for proper grounding and check for separation from your Dimmer power and other Building and audio power, and that theoutlets you are using have no voltage trickles or leakage between neutral and ground. Chances are, if you investigate and fix the power issues you may have, it may fix or reduce greatly your other problems... Lots of times there can be more then one factor playing out or contributing to a situation..so don't get discouraged if the first few things you do do not fix the problem....

For example...a few years ago I spent two weeks tracing out a similar RF problem a TV studio was having... They ran snake cable on two sides of the studio thru the open grid cieling to two home-made cheap snake boxes on either side of the studio..this ran into a rack in the back and out to the console etc. It would have been easy if the radio station occured on a single snake cable..but the RF occured on only a couple of channels in each snake line. After a long time tracing out each run and possible causes and checking the power feed to the building, I found several problems that needed to be fixed. The snakes were run in and out thru metal conduit that was attached to the wall and the trussing, and the snake itself when I pulled it out had some nicks in the jacket down to the copper (common when pulling cable thru conduit)...and the second problem was the snake ran to a patchbay with gear in it, and some of the wires were stripped back and exposed about 12" bare to the patchbay in the rack and hung around some of the AC cables to the equipment. Additionally some of the grounds were cut or missing on some of the channels (TV guys--when they hear a hum the first thing they do is cut a ground). So I grounded the rack and got rid of some of the cheap crap wall-wart stuff they had in the rack and moved other items to a new rack, I re-soldered cables and trimmed them to length with proper shielding, I pulled the snake out of the conduit and lost the conduit in the trash...and replaced it with a thicker jecketed higher quality shielded snake and new snake boxes, and lost the home-made crap boxes attahed to the conduit and screwed into the metal wall stud. Pain in the butt and many factors IMO contributed a lot to the interference problems...but in the end it was problem solved....

good luck...
In regards to the radio interference..typically this can happen when your buidling is close in proximity for your location to a broadcast tower.

Heh, we have a problem here. Our school runs a radio station, and the tower is on top of the theater building, probably no more then 200 meters away.

I am not sure what system we have installed, I will try to go and take a look on monday. Thanks for your help everyone :).
Arbron said:
In regards to the radio interference..typically this can happen when your buidling is close in proximity for your location to a broadcast tower.

Heh, we have a problem here. Our school runs a radio station, and the tower is on top of the theater building, probably no more then 200 meters away.

I am not sure what system we have installed, I will try to go and take a look on monday. Thanks for your help everyone :).

Is this the radio station you hear through the PA?
I would start by disconnecting the input leads into the amplifiers. Then I would turn on the amplifier and see if you can still hear the radio. If you can you could get the amplifiers output checked by a technician who can put in RF blocking. If the radio is not comming through the speakers in this test reconnect the input leads but leave them disconnected from the mixer. If the radio is now comming through try putting RF blockers around the cables at the end near the amp. You should be able to buy some at Radio Shack. Keep repeating this process until you have an idea where the radio interference is entering your signal path and use appropriate RF blocking methods.

Also, to check if it the school's radio station, quite probably, ask them for a schedule of when the station is not transmitting then turn on the sound system at one of these times. If there is no interference then it probaly is the school radio.
avkid said:
are the speakers shielded? keep the dimmers off the same circuits as the sound system!

I think you are confusing speaker leads with signal leads. Signal leads must be shielded but you should not shield speaker leads.

Also - as wolf has pointed out, noise is almost always introduced into the signal path and then amplified.

Arbron - an important question is in regards to the use of radio mics. Especially if your school has some older VHF mics.
I want to block all radio transmissions in part of our building, what is the best way to go about doing this?
My dad (who works for a constuction company) had to do this when they were installing some sensative hospital equipment, and they ended up putting thick metal sheets arround all the walls and over the roof and under the floor.... not really a pratical solution though!

Without blocking them out with lots of metal, I think you would have to overpower them... and that would require having something that would powerfully broadcast across a wide frequency, overpowering any other channels. The problem with this is it wouild probably overpower whatever devices you were trying to use too....

Hopefully someone else has some better ideas!
If you're attempting to stop cell phones from working and receiving signal in the room, that is illegal in the United States I believe. You can illegally purchase a scrambler which you attach to some sort of metal in the walls or roof to do this, but like I said, I pretty sure that it is illegal except for cases like hospitals where it is absolutely necessary.
The military and many gov't agencies use cellular phone blocking technology. I believe that one needs a license from the FCC(very hard to get) I am trying to block UHF wireless mic transmissions in the dressing rooms and bathrooms of our theatre. The dressing room and bathrooms are completely made of cement.(if that helps)
You either need a scrambler, which makes it hard to limit the boundaries of the blocking, so probably not a good option. You also need a license for that and have demonstrate adequate need for the blockage. The other way is good 'ole lead walls, also probably not an adequate solution. If you're trying to stop transmission from mics in the dressing rooms and bathrooms, shouldn't you be turning them off at the board anyway, or is this just another measure to be sure?
Although probably illegal, it can come in handy when your bored at intermission, or even useful when you need to know whether everyone is ready to start or not. If they're still talking, they're obviously not ready. You also occasionally catch people talking to themselves backstage, which gives a good laugh.
Foxinabox10 said:
If you're trying to stop transmission from mics in the dressing rooms and bathrooms, shouldn't you be turning them off at the board anyway, or is this just another measure to be sure?

Just another measure to ensure we don't hear someone taking a leak again(um..)
Had the same exact problem in a shity old theatre i worked in the fix was this for some reason the amps were in the tech booth by moving them next to the speakers on deck and using short 1/4" runs to the speakers and long xlr ones to the board in the booth this completly eliminated the problem. dont ask me why. I even used the same cables just reversed the set up. and it worke.

second lighting you have two options.

1- have an master electrician come in and get you your own power drop for sound (the only way to have truly clean power)

2- get an power conditioner and place it near the devices you will use. this type of hum from the lights has absolutely nothing to do with grounding. its to do with the change in the draw of power when the dimmers change percentages from 10% to 50% will have an audible hum on your sound system and even a power conditioer may not help you


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