aieta@ils.nwu.edu (Michael Aieta) (08/25/90)
Is there any effective way to eliminate interference on my home Musical PA System (Mixing Board, Tape Machine, and Power Amps) The interference I get is strictly from (what I assume to be highly illegal) boosted CB signals from what sounds to me like Pakistani cab drivers in the city. It can be VERY VERY loud, much louder than any signal coming out of my mixer, and it occurs even with the volume from the mixing pre-amp is all the way down. I have balanced microphones running into the mixer (along with line-level signals). This sum is in turn fed to both a tape machine and the power amplifiers through normal 'guitar' cable (1/4" 2 -wire cable). All gear fed into the system is connected to the same ground running to a cold water pipe that runs to the bottom of the apartment building (I am on the third floor) and I assume it goes underground somewhat from there. I reduced the lengths of the cables running from the mixer to the amplifiers to attempt to keep them from acting as antennae. It seemed to make a small bit of difference, i.e. the cabbies still come through but somewhat lower and less clear (at least last night they were quieter,although they might have just been farther away. So, down to my questions to you RF,audio,radio,CB,op-amp buffs! How do I shield the cables running from the mixer to the amps? Do you run a twisted pair with a true shield that you attach to a ground at the mixer/pre-amp side only? Both sides? Neither? How would I go about putting an RF filter (I have some basic schematics from the FCC for TV's, stereos etc.) on my power amps? On my mixer? The mixer is an Alesis 1622 for you musicians, for you techies it is a monolithic integrated circuit so I don`t believe I can isolate any pre-amp components per se. How exactly do I get these and how do they get broadcast so f%*#ing loud? How much power are these guys putting out? FCC limits are 4W, but it sounds to me like these guys are cranking out hundreds. I have even jotted down plate numbers of cabs that are sporting 20 ft. antennae with fat bases and big coils wrapped around them, and I don't mean to sound prejudiced in any way, but it sure is some coincidence that these airwave pirates all sound (and look when I see the bigass antennae) like either Indians or Arabs - i.e. Paki's. Please no flames from Indians, Paki's, or Arabs, as I am sure there are plenty of plain ol' honky's, Black, and Hispanic, etc. cabbies that are equally offensive to the airwaves, it is just that in my area that is all I hear. Any suggestions, solutions to similar problems?
dg9g@maxwell.acc.Virginia.EDU (David Guercio) (08/25/90)
>Is there any effective way to eliminate interference on my >home Musical PA System (Mixing Board, Tape Machine, and Power Amps) Put .001uF bypass capacators across your input cables at both ends. You can solder them inside the jacks on the cables, if you buy the smallest capacitors that you can find. Radio Shack components are fine for this. If this doesn't work, send me some e-mail, and I'll suggest some more stuff. >I have even jotted down plate numbers of cabs that are sporting 20 ft. >antennae with fat bases and big coils wrapped around them, and I don't You probably keep your neighbors up with your power amps, so you ought to leave these guys alone. I used to be one of these 'Pakis', myself. David Guercio
depolo@eniac.seas.upenn.edu (Jeff DePolo) (08/27/90)
In article <1491@anaxagoras.ils.nwu.edu> aieta@ils.nwu.edu (Michael Aieta) writes: > > >The interference I get is strictly from (what I assume to be highly illegal) >boosted CB signals from what sounds to me like Pakistani cab drivers in >the city. It can be VERY VERY loud, much louder than any signal coming out >of my mixer, and it occurs even with the volume from the >mixing pre-amp is all the way down. I have balanced microphones running into >the mixer (along with line-level signals). This sum is in turn fed to both >a tape machine and the power amplifiers through normal 'guitar' cable (1/4" >2 -wire cable). A while back on rec.ham-radio, as well in a number of the ham rags, there was a discussion about cab drivers/dispatchers using the 10/11m CB and ham bands for their communications. It seems that when the popular 10m mobile radios (e.g. Uniden and Clear Channel) came out, the cab companies found that spending $250 on one of these 25 watt sideband/AM rigs was easier than paying Motorola or GE thousands of dollars and getting real radios. >How would I go about putting an RF filter (I have some basic schematics from >the FCC for TV's, stereos etc.) on my power amps? On my mixer? The mixer >is an Alesis 1622 for you musicians, for you techies it is a monolithic >integrated circuit so I don`t believe I can isolate any pre-amp >components per se. You can start by putting bypass caps across all speaker leads. You can experiment with ferrite chokes on some of the other equipment. I've had good luck installing chokes inside equipment such as preamplifiers and amplifiers in RF equipment. The same may work in your situation. On the shielded cables, try some of the popular wrap-around chokes available at Radio Shack and from most other electronics shops. You will probably find only one or two critical paths that the RF is getting in from. Try coiling all of your cables into tight loops as close as possible to where they connect to any equipment. In general, try everything until you can narrow it down. The problem here is that they aren't always transmitting nearby, so it will be hard to tell what is or isn't working. >How exactly do I get these and how do they get broadcast so f%*#ing loud? >How much power are these guys putting out? FCC limits are 4W, but it sounds >to me like these guys are cranking out hundreds. CB transmitters are 4 watts max AM, 12 watts PEP SSB. 4 watts doesn't sound like much, but if it's nearby, just about anything with a diode in it becomes an instant receiver. Complain to your local FCC field office. If you have cab numbers and companies written down, all the better. Give them all the info. Chances are that you won't see any immediate action, but who knows, they may get around to checking it out. Good luck. --- Jeff -- -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- Jeff DePolo N3HBZ Twisted Pair: (215) 386-7199 depolo@eniac.seas.upenn.edu RF: 146.685- 442.70+ 144.455s (Philadelphia) University of Pennsylvania Carrier Pigeon: 420 S. 42nd St. Phila PA 19104