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Author Topic: Picking up AM radio through snake... help!  (Read 2181 times)

Garrett Nelson

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Picking up AM radio through snake... help!
« on: February 17, 2013, 11:38:22 pm »

Hey everyone, first post here but I have been poking around on the site for a while now. Sorry if this post is kind of long...

My church has had problems with picking up an AM radio station through the sound system ever since the building was constructed 7-8 years ago and the sound system was installed. I was not around when the church was built, but I am now trying to figure out the problem because it is driving me nuts. I believe the station I am picking up is about 20-30 miles away.

The AM signal is almost always there, sometimes we seem to pick it up a lot lot louder than other times. We mainly notice it on a podium mic that we really crank the gain on in order to be able to pick up people who speak quietly. Several other channels pick it up as well but it is virtually inaudible under normal gain conditions.

The snake is run through walls and not in any sort of conduit at all. The individual channels in the snake have a ground wire and a foil shield. The snake is probably about 150 feet long roughly. There is no shield over the whole bundle, just a rubber jacket.

The snake runs to the middle of the stage and from there explodes into a spaghetti mess of wires... none of the individual channel wires were cut to length when installed, or routed in any sort of reasonable way. I spent a couple hours under the stage and routed a couple of the wires that go to the stage box near the podium in the straightest route I could, I cut off about 20 feet of extra wire and soldered on some new Neutrik female XLRs to replace the original Whirlwind ones because they were getting loose. I thought for sure this would help, but if anything the radio got even louder on that channel after my attempt at a fix.

Multiple different channels on the snake seem to pick up radio, some worse than others. Most channels get some degree of radio, we do have a couple channels that seem to be perfectly quiet for some reason though.

I have taken apart some of the XLR connectors and examined the wiring. Pin 1 is connected to the ground wire, and the shell of the XLR connectors are not connected to pin 1.

I have disconnected pin 1 from the XLR going to the mixer on a few of the problem channels, and the AM is instantly gone. However, some of our mics need phantom power so that also killed some mics.

I brought in a different mixer for testing, a brand new Behringer that I was able to borrow for a day. I connected a problem channel from the snake to the Behringer and the only other stuff connected to it was the power cord and a set of headphones. I was still picking up radio, but it was definitely at a much lower level than through the Peavey mixer.

Connecting different mics to a problem channel makes no difference. Different mic cords make no difference. You can unplug the mic cord from the stage box and still pick up radio with nothing connected to that channel.

I have been doing some reading and it looks like there are two different things I can try... ferrite cores or small capacitors wired between pins 2 and 3 to pin 1 on the XLRs. Which should I try first? I assume that I should be doing these things at the mixer end of the snake, right? Or is there something else I should be checking into first?

Any advice would be appreciated.

I don't think this is an equipment problem, but I am listing this stuff just in case. If you need actual model numbers on the stuff I don't have here I can get them.

40 channel snake, switchcraft male XLRs connected to mixer, Whirlwind female XLRs in stage boxes
Peavey RQ 4332C mixer
Some sort of analog EQ (dual 16 band I believe, not sure how it is connected)
DBX DriveRack 260 crossover/limiter/etc
3 amps for main speakers/subs in rack close to mixer
1 amp for stage monitor installed under stage
3 main speakers and large sub suspended from ceiling of church in vertical array
1 wedge monitor on stage
A bunch of mics... Senheiser and Shure corded, Shure and Audio Technica cordless.

There are 3 rackmount surge protector type boxes that the mixer, amps, EQ, etc are powered from... I believe they are on 2 different AC circuits. The amp for the stage monitor is on a 3rd circuit.
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John Roberts {JR}

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Re: Picking up AM radio through snake... help!
« Reply #1 on: February 18, 2013, 12:30:39 am »

Picking up radio stations is caused by "rectification" when the audio path electronics is not fast enough to follow the radio frequency signal. AM radio while relatively low frequency for RF at around 500kHz to 1MHz is still too fast for audio circuity.

Being 20-30 miles from the broadcast antenna should experience only modest local RF levels, so suggests either A) a really bad mixer/preamp design, or b) an unusual quirk of the building ground system or wiring that forms an antenna.

While the RQ4332 was after my time, I know the lads who designed it so I expect it is probably serviceable, and not the fault.

I would suggest experimenting with running separate mic cables from the problem mic to the mixer and see if the separate mic cables pick up as much RF.

Ultimately adding inductors and caps as extra RF filters on the mic input may be needed if you are in some unusual high RF signal area, but first confirm that there isn't something funny about your snake- input wiring. Wiring can sometimes act as antenna and make the RF louder instead of attenuating with the shielding connected to ground (hint there should be a good solid ground or the shield is just a big antenna). 

If standard mic cables cause the same radio interference, consider rolling some add on RF filters.

JR
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Tune it or don't play it... please

Mac Kerr

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Re: Picking up AM radio through snake... help!
« Reply #2 on: February 18, 2013, 07:24:41 am »

Wiring can sometimes act as antenna and make the RF louder instead of attenuating with the shielding connected to ground (hint there should be a good solid ground or the shield is just a big antenna).

Poor solder joints can also cause AM radio pickup. Make sure that all solder joints are smooth and fully melted, and that the wire does not move while the solder cools and solidifies.

Mac
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Frank DeWitt

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Re: Picking up AM radio through snake... help!
« Reply #3 on: February 18, 2013, 09:59:59 am »

This came up not to long ago,  There was a lot of discussion and some good suggestions.  The OP chose to go with ferrite beeds.  One on each channel installed as close to the mixer as possible.  Below is his summery.

"My research uncovered that as one would expect four factors are important in the performance of a ferrite. Material, geometry, installation, and placement with the first two being of preeminent importance. I chose toroids made of Laird Low Frequency material that are 1.5" long and only have a .5" ID which means I will need to remove every XLR connector to install them. It also means they achieve over 2600 ohms at my 1.2mHz target with only five wraps and as a bonus nobody else can take them off once installed. I will try to get some before and after photos of the meters on the board to show reduction in interference."

On my X32 one can just barely hear a difference in noise between a channel run with a ferrite on it and an open channel on the console. On the venues BK-2442 console the difference is inaudible as the noise floor in the console is higher. Lots of soldering hassle but the ferrites only cost $2.75 each so material cost for the fix was under $100.

http://forums.prosoundweb.com/index.php/topic,141118.0.html
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Bob L. Wilson

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Re: Picking up AM radio through snake... help!
« Reply #4 on: February 18, 2013, 10:22:02 am »

This came up not to long ago,  There was a lot of discussion and some good suggestions.  The OP chose to go with ferrite beeds.  One on each channel installed as close to the mixer as possible.  Below is his summery.

"My research uncovered that as one would expect four factors are important in the performance of a ferrite. Material, geometry, installation, and placement with the first two being of preeminent importance. I chose toroids made of Laird Low Frequency material that are 1.5" long and only have a .5" ID which means I will need to remove every XLR connector to install them. It also means they achieve over 2600 ohms at my 1.2mHz target with only five wraps and as a bonus nobody else can take them off once installed. I will try to get some before and after photos of the meters on the board to show reduction in interference."

On my X32 one can just barely hear a difference in noise between a channel run with a ferrite on it and an open channel on the console. On the venues BK-2442 console the difference is inaudible as the noise floor in the console is higher. Lots of soldering hassle but the ferrites only cost $2.75 each so material cost for the fix was under $100.

http://forums.prosoundweb.com/index.php/topic,141118.0.html

Frank is referencing a thread about a fix I implemented for a homeless shelter before Christmas.

If using ferrites one must be certain about the frequency of the station. Do whatever you have to do but figure out what station it is, in my case I took an AM radio along and listened to stations until I found the culprit.  Even if the rectified signal in the snake is too distorted to understand what is being you will know when you have a match. In the referenced case it was like listening to the dry signal on the radio and some crazy effects return of it on the console headphones. I probably wouldn't initially disassemble the end of a quality snake instead try making some ferrite equipped extension cables. These will tell you how well any specific ferrite choke solution will work for your particular situation without permanent alterations. IE if you have a station that falls between the peak attenuation frequencies of readily available chokes buy a couple of each make a few extension cables and compare them.
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Tim Perry

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Re: Picking up AM radio through snake... help!
« Reply #5 on: February 26, 2013, 05:45:00 pm »

Please tell me the callsign of the radio station in question.

It is rare, but occasionally stations use boosters to cover an area within their coverage contour that is otherwise blocked by terrain.

In other words, there may be a transmitter closer then you think.
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Garrett Nelson

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Re: Picking up AM radio through snake... help!
« Reply #6 on: March 02, 2013, 12:22:06 pm »

Well I was placing a Parts Express order anyways, so I ordered a small assortment of ferrites from them. These ones did the trick:

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=110-454

Running one of the wires through a single time helped cut down the radio by quite a bit. I looped a bad one through there 3 times and the radio was completely gone. No detrimental effects on sound quality that I was able to notice.

So, I got lucky, as I did not figure out the station and did not research which ferrites would be best... but the ones I linked above worked wonders and the AM station is now completely gone.
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