# PA system question



## Wireless (Jan 22, 2007)

Are the speaker wire runs long?


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## JohnR (Apr 12, 2010)

I had that problem once, and the answer was to ground a EMT conduit that the mic wires were running through in the ceiling. It was just a chase for the wires no box on either end. The conduit was acting as an antenna and inducing the audio from the station (AM) into the Microphone wires. 

That said , you could get some ferrite cores from RadioS and wrap the cables affected through these as many times as possible. That will take out most of the unwanted signal by itself.


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## 480sparky (Sep 20, 2007)

Wireless said:


> Are the speaker wire runs long?



Room is about 40x60. 10' ceiling.


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## JohnR (Apr 12, 2010)

That is almost exactly the size of the room I was dealing with.


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## 480sparky (Sep 20, 2007)

JohnR said:


> That is almost exactly the size of the room I was dealing with.


There won't be any EMT to deal with.


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## JohnR (Apr 12, 2010)

Well I re looked at the OP and saw that the signals are real faint, Mine were in some cases louder than if you were speaking at a mic.

I took the mic cables that were affected, and wrapped 3-6 wraps through the ferrite core and that took most if not all the signal away. Even on the loudest. 

The install there was much more complex however and The mic cables were balanced xlr style. 
If the installation has unbalanced mic cables, you could try balanced ones which are for anything over 25 ft or so. The balanced cables have 1 ground/sheild, and two signal wires inside 1 is + one is + side of the signal. 

If you find you need more answers, there is a book put out by Yamaha that is very good at explaining in detail but in somewhat laymans terms how sound works and what is involved with sound. I THINK it is called Sound Reinforcement Systems


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## Bob Badger (Apr 19, 2009)

480sparky said:


> I'm no audiophile, but I've been asked to look into problems in a small PA system.
> 
> System consists of just 3 mics and 4 speakers. On occasion, a radio station can be heard over the speakers. Not enough clarity to determine what station, and there are no nearby transmitters.
> 
> Anything I should look at first?


OK I am not an expert but I work on a fair number of commercial type PA systems. 

IMO it cannot be a problem on the output side of the amp, it has to be on the input side.

If it was somehow getting this radio signal from 'long speaker runs' than you could take the amp away and you would still have the same problem. 

I would look to the input wiring, each input should be a on a different volume control, turn them all down then crank one of them up full, then turn that down and crank up another. You should be able to pinpoint which channel has the issue. You also might find it effects all channels.

Either way I would look to make sure the input wiring is using shielded cables and that the shield is made up to the correct terminal at the amp. 

Good luck.


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## drsparky (Nov 13, 2008)

Also look at the mic inputs they may have options for balance/unbalanced.


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## LJSMITH1 (May 4, 2009)

I second the method Bob described. If the input cables are not shielded, you will get all kinds of wierd stuff through the speakers.


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## Wireless (Jan 22, 2007)

Bob Badger said:


> OK I am not an expert but I work on a fair number of commercial type PA systems.
> 
> IMO it cannot be a problem on the output side of the amp, it has to be on the input side.
> 
> ...


 
Who said taking away the amp solves the problem? The long wire runs on either side are acting as an antenna. I have picked up radio signals with my tone generator on long cat 5 runs when placing the probe near the wire you can hear the signal through the speaker.


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## 480sparky (Sep 20, 2007)

The RF audio is only heard when the system is on.


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## Bob Badger (Apr 19, 2009)

Wireless said:


> Who said taking away the amp solves the problem?


No one, but it will.




> The long wire runs on either side are acting as an antenna. I have picked up radio signals with my tone generator on long cat 5 runs when placing the probe near the wire you can hear the signal through the speaker.


Yes I am sure you have as the probe you are using is an amplifier.

But no signals picked up by those speaker wires will be strong enough to drive the speakers without amplification.


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## jw0445 (Oct 9, 2009)

480, Ditto on the Badger comments. You need shielded wire to bleed the noise to ground.


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## RIVETER (Sep 26, 2009)

If you can hear understandable audio it is not RF. A long wire will pull in radio stations and can push through the IF stages of some. Is it possible that you are near a cell tower? Is MUZAK involved when you are not talking on the PA?


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## LJSMITH1 (May 4, 2009)

RIVETER said:


> If you can hear understandable audio it is not RF. A long wire will pull in radio stations and can push through the IF stages of some. Is it possible that you are near a cell tower? Is MUZAK involved when you are not talking on the PA?


480 - One quick way to check if it is the AMP or the inputs (mic's or music) is to disconnect all 3 mics and line inputs from the AMP and see if you still hear the noise with the volume maxed out. If it goes away, focus your efforts on troubleshooting the mics and each line that goes to them as well as scrutinizing the cables connected to any line equipment. 

More than likely it IS a RF (3kHz to 300GHz) frequency being picked up by un-sheilded (or poorly sheilded) microphone or line inputs (i.e. BCI - Broadcast Interference). Sometimes a broken or loose sheild connection at a mic plug will cause the problem. Fixing that with higher quality cables usually solves that.

Based on similar stories I have heard with guitar amps or small PA systems, I would bet it is a strong AM station. AM frequencies are lower and travel further than FM or Cell frequencies - and can be easily picked up by long runs of cable. The only thing lower than AM is submarine communication or mine communications...and somehow I don't think its those...:blink: 

http://en.wikipedia.org/wiki/Radio_spectrum


One other thought I had is with regards to grounding. 480 - Check the amplifier grounding all the way to the outlet and through the building. Sometimes, inadequate grounding may also cause this issue. Also, many PA amps have a separate chassis ground that many installers forget to connect to the building ground. I am also assuming that the mics are wired and not RF. If they are RF, then try changing the frequencies that they transmit at.

If all shielding and grounding check out, try wrapping the line input connections through ferrite cores. You can get them at Radio Shack. A couple of turns through a ferrite at each corded connection is usually enough to attenuate the RF signal to where it may not be heard through the AMP.


Good Luck!


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## Wireless (Jan 22, 2007)

Bob Badger said:


> No one, but it will.
> 
> 
> 
> ...


Agreed. My initial point before I got carried away was a long wire will act as an antenna, I should have wrote long wire runs not speaker wire.


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## MDShunk (Jan 7, 2007)

Many of the better mixers will have a "ground lift" switch at each XLR jack. Switch it the other direction, and the problem normally goes away.


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## willadwardz (May 27, 2010)

Hello guys...,

I have query ...How can I hook up both Main House speakers & a floor monitor at a same time....?

Thanks...


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## jw0445 (Oct 9, 2009)

480, what did you find and how did you correct it?


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