PA system - AM radio broadcast

jwl868

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Aug 31, 2004
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Pittsburgh, PA
The PA system at our church picks up the broadcast from a nearby high-power AM radio station. Sporadically, one can hear a barely audible broadcast of the station from the PA speakers while the PA system was on. The problem comes and goes. The problem appears to be with one particular microphone and then “jiggling” the cord where the jack plugs into the wall will make the broadcast go away. As far as I can tell, its XLR cord. The PA system, microphones, and cords are at least 15 to 20 years old.

Any ideas about what causes the AM signal to be picked up and any solutions? (I don’t think anyone has truly troubleshooted the problem. I don’t know if people have tried swapping cords, or microphones, or replacing cords etc.) I know I’m short on details here, but I can try to collect some more information.

Thanks

Joe
 

Grog12

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Sounds like an unsheilded cable or loosely sheilded...swap out the cable with a newer one and see if that doesn't do the trick.
 
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Van

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And check for loose strands of wire loose strands love AM, or vice versa. Keeping cable runs as short as possible, and cables in well maintained condition, will help a lot. Also make sure you have a decent ground on all your equipment. Over near where I live we have " The 50,000 watt Blowtorch of the Pacific Northwest, KEX" everyone in the area with old grounds, and old wiring starts picking up KEX on the phone, intercoms, plumbing..........
 
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mbenonis

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Van, I've always wanted my toilet to double as an AM radio!

Seriously though, replace the mic cable with a new one and see if that fixes the problem. If not, run good mic line directly from the mic to the console, skipping any in-wall wiring. Continue to simplify the system until the problem disappears - when this happens, you've found the culprit.

Any sound system located near a powerful source of EM radiation, such as an AM radio transmitter, should have properly shielded and balanced wiring at all points in the system, as well as properly designed equipment. Even the smallest hole in shielding, or unbalanced run, can act as a wonderful antenna for that AM signal, and the solid-state devices we all use today act as rectifiers for the signal, injecting it directly into the audio path. THe only way to avoid it is shielding, shielding, and more shielding.
 
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SHARYNF

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Sep 3, 2006
are you by any chance using a Mackie mixer? there is a know rf problem with them that can be fixed.
In most cases it is a shielding issue or pin one problem . Here is a link for some suggestions
audiosystemsgroup.com/SAC0305Ferrites.pdf

Sharyn
 
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jwl868

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Pittsburgh, PA
Thanks for all the input. it will probably take a few weeks for me to check things out. I don't know if there are spare cables around. (That leads other questions: is all audio XLR cable shielded? Do some manufacturer's make better cables than others?)

The particular cable needs to be longer than what would appear necessary. The mic is mounted on a podium for the lector, but occasionally the priest takes this mic in hand if he steps down off the altar to talk to the congregation. The excess cable is coiled by the podium.

Regarding the mixer (or any of the sound equipment for that matter) - I don't know the make or model. Its back in the sacresty, which is of somewhat limited access. I'll see about checking that out.

Joe
 

Grog12

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Considering you say he has to walk around the congregation, it seems to me that given you're purchasing such a long run (for some reason I have an image in my mind of 500+ feet), which will be traipsing through the house, it might be worth looking into a wireless mic. I mean, I don't know sound, maybe you'll run into more interference problems, but in the end it seems a lot easier to set up, a lot cleaner looking, not have 500 feet of XLR through the house, and you avoid the possibility of someone tripping on the cable, the cable snagging something while he walks through the house, etc.
True..but then you run into battery expense.

And honestly I doubt its 500+ft
 

SHARYNF

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Here is another suggestion, you can make sure that you connect the shield on your mic cable to the chassis of the mixer. Here is additional information on what is called the pin one problem

http://www.tvtechnology.com/pages/s.0071/t.1589.html

If you have access you need to also look at the entire cable path from the mic to the mixer, I am guessing there are various plug in boxes along the way, and these can be problematic. Get your self a good quality mic xlr cable, and see if plugging in the mic to the cable and the cable directly to the mixer fixes things. Look at the power plug on the end of the mixer and see if it is three pin and if all the pins are intact


Sharyn
 

jwl868

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Pittsburgh, PA
Sorry - I wasn't clear about the length of the cable. Its not much more than 20 to 25 feet. The priest just occasionally speaks from in front of the altar, at the floor level of the congregation.

Joe
 

avkid

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SHARYNF

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Obviously there can be a variety of different types of problem causing the problem but most of the time when I have had problems on a site that was close to an am transmitter, the problem has been more with the mixer, or the mixers implimentation of the shielding connection vs the cable itself. Again not a universal statement on the problem/solution, but at least something to look at

Sharyn
 

renegadeblack

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Way back in middle school, ahh, yes, those were the days...

I had been helping to set up and in school tv station that every morning would show the morning announcements televised. Well, it was the very first time that we were doing the show and somewhere in the line was too much exposed wire and we got what the TD deemed as what sounded like a cordless phone conversation. Very interesting. We had a professional TV studio that got converted into a classroom :evil: but most of the infrastructure was still in place. Things got semi-jerry rigged (what is the origin of that phrase) and we had a working TV station again (complete with a relay that changed the video input depending on whether or not the light switch was on).
My $.02
 

zuixro

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Way back in middle school, ahh, yes, those were the days...

I had been helping to set up and in school tv station that every morning would show the morning announcements televised. Well, it was the very first time that we were doing the show and somewhere in the line was too much exposed wire and we got what the TD deemed as what sounded like a cordless phone conversation. Very interesting. We had a professional TV studio that got converted into a classroom :evil: but most of the infrastructure was still in place. Things got semi-jerry rigged (what is the origin of that phrase) and we had a working TV station again (complete with a relay that changed the video input depending on whether or not the light switch was on).
My $.02
The youth room at my church used to do that. Funilly enough it picked up a local Christian talk AM station. I think it was more of a grounding/cheap cabling issue than a bare wire thing.
 

FMEng

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Normally, I'd agree with the previous suggestions, except for one detail. Since jiggling the connector makes the problem disappear, that points to the solution.

What we have here is a case of rectification in the XLR connector contacts. Dirty and oxidized metal to metal contacts can act like a diode detector in the presence of strong RF signals.

Go to Radio Shack and buy their Deoxit/Progold twin pack. Spray all the pins of both connectors, first with Deoxit, followed by Progold. The problem will go away.
 

mbenonis

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Normally, I'd agree with the previous suggestions, except for one detail. Since jiggling the connector makes the problem disappear, that points to the solution.

What we have here is a case of rectification in the XLR connector contacts. Dirty and oxidized metal to metal contacts can act like a diode detector in the presence of strong RF signals.

Go to Radio Shack and buy their Deoxit/Progold twin pack. Spray all the pins of both connectors, first with Deoxit, followed by Progold. The problem will go away.
Huh, I never thought of that - but after thinking about it, it makes perfect sense. I always first thought about poor input stages on consoles, etc...

Mike
 

TimmyP1955

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Indianapolis
Although it usually causes more problems than it solves, in this case I would make sure that in all of the cables from the problem mic to the mixer, pin 1 is connected to the shell of the XLR.

If they already are, then break those connections one by one, starting with the XLR that plugs into the mixer, and working your way back to the mic.

However: If there are any junction boxes along the way, the problem may be that pin 1 is connected both the the XLR shell, and to earth via the box. This would likely be a bad thing.

It could also be that there's a short in that mic or mic line that is unbalancing the line.
 

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