rjr@mgweed.UUCP (10/24/83)
David at Tektronix mailed a question to me about my statement concerning rf interference in audio amplifiers. I believe I said something in response to someone's query about using coax cable for speaker wire. The situation I had involved a Fisher 250 receiver. The speaker wires were each 6 foot long lamp-cord. The interference source was a C.B. transmitter located in the house behind me, about 150 feet away. I previously had the same problem at my other house where a C.B. transmitter was about 800 feet away. Yes, the two C.B. operators were running legal 5 watt equipment. The speaker wire runs directly to the output transistors in the amp and therefore acts as a great antenna feeding a semiconductor. I don't know just what stage in the amplifier is acting as the detector, but it is after the preamp/tone control/level control section since none of the amplifier controls have any effect on the amount of interference. The Radio Amateur's Handbook, and other ARRL publications, have chapters dealing with rf interference and what to do about it, and this type of situation is covered there. The cure may be ferrite beads on the leads, grounding the equipment, caps on the output leads, caps or beads on the input leads, or any combination therof. Naturally, my own amateur transmitter drove the amp wild, but then I never listen to stereo while I am on the air. Another case of rf interference occured in our church. This installation used Altec tube-type mic mixers with balanced transformer-coupled mic inputs. The system worked quite well until one Sunday when a prayer was ended by a loud "10-4" from the speakers. There, the problem was cured by adding 100pf caps between the grid and cathode of each 12AX7 preamp stage. Here was a case where interference occured even with balanced lines using shielded cable. Preamp stages used for tape heads, magnetic phono cartridges, and mics are particularly vulnerable to rf. I am curious how many audio buffs have been plagued by rf interference. Bob mgweed!rjr
dswankii@uok.UUCP (10/28/83)
#R:mgweed:-430300:uok:400004:000:478 uok!dswankii Oct 26 15:56:00 1983 Oh boy! Interference pproblems! Well, my stereo amp whistles when I use my (dare I admit to owning) C.B. but only on A.M. I guess it can't handle SSB signals. It also makes noise when the TV is on when listening to A.M radio (football days only). I think I can blame this on the high voltage section of the TV. Only once have the hams or clowns caused problems and that was when a signal cable from the equalizer went bad. David Swank II !duke!uok!dswankii (I think)