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From: kenw@col.hp.com (Ken Wyatt)
Newsgroups: sci.electronics
Subject: Re: RF interfereence
Message-ID: <16120013@col.hp.com>
Date: 30 Nov 90 16:18:30 GMT
References: <1990Nov29.053422.17859@ux1.cso.uiuc.edu>
Organization: HP Colorado Springs Division
Lines: 29

It is possible for audio amplifiers to "pick up" strong radio stations
via their speaker leads (long leads make great antennas).  The rf energy
then becomes coupled into the input stages of the preamp circuitry.  Try
connecting a 0.01 uF ceramic capacitor (50 volt rating, or higher) across
each speaker wire as shown.  These may be purchased at Radio Shack.


      _______                                 _______
      |     |                                 |     |
      |     |---------------------------------|     |
      | AMP |   |                             |SPKR |
      |     |   O  0.01 uF                    |     |
      |     |   |                             |     |
      |     |---------------------------------|     |
      |     |                                 |     |
      -------                                 -------

Also, you might be try winding two or three turns of the speaker
cable around a ferrite torroid core or rod material.  This will tend
to "choke off" the rf from the cable.

If neither of these ideas work, the rf may be coming in via some other
piece of equipment.  Try unplugging the audio patch cords of each piece
of equipment one at a time, while listening on the headphones.  For  
example, occasionally rf energy may enter a system via the pickup wiring
of your turntable.  These are the wires that connect the pickup cartridge
and travel through the arm into the base of the turntable.  A couple of
0.01 uF ceramic capacitors across each channel pair (install these in the
turntable base, not the tone arm) may solve the problem.


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