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[Ryan Lalum]

I getting a very random noise interference from a recent install that we did. The noise that is produced is a high pitch almost siren sound. At first I thought it might be the wireless components, but I did a radio frequency sweep/scan, switch the wireless channels and are still getting the sound once every two weeks. Each cable is about a 200’ run and we used Belden 9451 shielded cable. I am under the impression that it has to be radio frequency due to the unusual interference. If the interference was coming from the lights or HAVC system, we would hear the siren sound every time they kick on. The sound system is comprised of an 8ohm front of house system and 70v distributed system. The siren sound is only coming from the 70v system. We used shield Belden 6100FE wire for both systems. I am currently looking at purchasing (50) Fair-Rite EMI 1-300Mhz filters (part number 0431164951) to eliminate some RF interface, but it would be shooting in the dark. Does any one have a suggestion to this crazy noise occurrence.   

[Hal Bissinger/COMSYSTEC]

Since the sound only affects the 70 volt system it shouldn't take too much effort to determine how it is getting in. You don't say much about the system. I assume there is a separate 70v amp driven off the FOH?

-Hal

[Ryan Lalum]

The system is comprised of QSC CXD 4.3 and 4.5 amplifiers. These amps are 4 channel and have the ability to drive each channel either high or low impedance. Only the system microphones run through the 70v(high impedance) channels. The area that is producing the random siren sound has 12 microphone home runs that are about 200' in length. The low impedance amplifier channels is designated for music only, no speech. We are using a Biamp Tesria to handle the processing.

On a side note, I was doing some reading and found the UPS's can also be a cause of RF interference. The amplifiers do not reside on the UPS, but are mounted 1RU about it.

[Keith Broughton]

If possible, try to figure out a way to replicate the problem on demand.
Then you can go through the system input by input, and figure out where the problem is coming in.
Otherwise, it's a crap shoot to figure out.

[Robert Healey]

On a side note, I was doing some reading and found the UPS's can also be a cause of RF interference. The amplifiers do not reside on the UPS, but are mounted 1RU about it.

I would start here. If you are sending the same signal to both amplifiers, it has to be the amplifier or after. Try pulling out the UPS and hardwiring the power. Is the 70V amplifier the one next to the UPS?

[Mac Kerr]

The system is comprised of QSC CXD 4.3 and 4.5 amplifiers. These amps are 4 channel and have the ability to drive each channel either high or low impedance. Only the system microphones run through the 70v(high impedance) channels. The area that is producing the random siren sound has 12 microphone home runs that are about 200' in length. The low impedance amplifier channels is designated for music only, no speech. We are using a Biamp Tesria to handle the processing.

On a side note, I was doing some reading and found the UPS's can also be a cause of RF interference. The amplifiers do not reside on the UPS, but are mounted 1RU about it.

What you have described doesn't sound like interference to me. It sounds more like actual feedback, or oscillation. With a 70V system it is possible to get electronic feedback from the speaker output wires being too close to the microphone input wires. You should check that out. It may also not be a good idea to use shielded cable for the speaker cables, some amplifiers are sensitive to the capacitance caused by the shield. There is little or no reason to use shielded cable on amp outputs unless you are in a very high RF environment.

Mac

[Hal Bissinger/COMSYSTEC]

If possible, try to figure out a way to replicate the problem on demand.
Then you can go through the system input by input, and figure out where the problem is coming in.
Otherwise, it's a crap shoot to figure out.

I agree. Once every month doesn't give you anything to work with. How long does it last when it happens?

-Hal

[Jeff Robinson]

I getting a very random noise interference from a recent install that we did. The noise that is produced is a high pitch almost siren sound. At first I thought it might be the wireless components, but I did a radio frequency sweep/scan, switch the wireless channels and are still getting the sound once every two weeks. Each cable is about a 200’ run and we used Belden 9451 shielded cable. I am under the impression that it has to be radio frequency due to the unusual interference. If the interference was coming from the lights or HAVC system, we would hear the siren sound every time they kick on. The sound system is comprised of an 8ohm front of house system and 70v distributed system. The siren sound is only coming from the 70v system. We used shield Belden 6100FE wire for both systems. I am currently looking at purchasing (50) Fair-Rite EMI 1-300Mhz filters (part number 0431164951) to eliminate some RF interface, but it would be shooting in the dark. Does any one have a suggestion to this crazy noise occurrence.

I like this one:
http://www.mediacollege.com/forum/showthread.php?1388-Speaker-Wire-Shielded-Versus-Unshielded&p=22784&viewfull=1#post22784

Jeff Robinson

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