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From: rjr@mgweed.UUCP
Newsgroups: net.audio
Subject: Speaker wire/ rf interference/ etc.
Message-ID: <4303@mgweed.UUCP>
Date: Mon, 24-Oct-83 13:53:01 EDT
Article-I.D.: mgweed.4303
Posted: Mon Oct 24 13:53:01 1983
Date-Received: Tue, 25-Oct-83 23:37:53 EDT
Organization: Western Electric - Montgomery Illinois
Lines: 41


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