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RFI with PC speakers

Discussion in 'Electronic Basics' started by Paul, Nov 25, 2004.

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  1. Paul

    Paul Guest

    My 2.1 speaker system "PC Works" by Cambridge Soundworks is picking up quite
    a bit of RFI. It's manifested in audible whispering radio station
    broadcasts. When I moved in here, RFI was very audible. So I was suggested
    to put ferrite chokes around audio and power cable. I did that, and it
    worked for quite a while. Recently, I've been getting loud RFIs yet again.
    Now I don't know whether those ferrite chokes did anything, and not mere
    rearrangement of the cables. I took them off and there seems to be no
    change. RFIs come in at varying volume rates lately to the point that I
    turn the amplifier off over the night as in complete silence it's quite
    audible. By process of elimination I ruled out several possible causes. At
    first I thought it was the computer's sound card (had that happen before
    too!). I plugged headphones directly into the Lineout, silence, no RFIs.
    I rearranged the audio cable but that didnt do much. I pulled the audio
    cable out the amplifier. RFI is still there. So i've come to conclusion
    that the amplifier is not well shielded and causes that. However, why does
    intensity of RFI fluctuate? Is there anything I can do besides getting
    another speaker/amp system? Ironically, I have a set of cheap speakers in
    the same room on another coputer that dont have this problem! I looked
    inside them, and there seems to be no shielding on them at all.
     
  2. Art

    Art Guest

    You indicate that it seems to be via the amp, or maybe vis the speaker
    leads?/ Have you tried relocating the amp and speaker leads, say from one
    side of the room to another, just as a trouble shooting procedure? RFI can
    be a real problem to diagnose and solve since even a small piece of wire
    within an enclosure may react as a resonant antennea receiving the signals.
    Diligence will be rewarded in finding out specifically what is the root
    cause however eliminating with appropriate shielding may be difficult.
    Resulting in trial and error methods to actually find a means that will
    suffice, inclusive of ferrite cores, decoupling caps, etc.
     
  3. Bob Masta

    Bob Masta Guest

    Err, is it possible that the cheap speakers also don't have
    built-in amplifiers? The RFI you are getting is most likely
    coming from the amp input stages. There are several
    things to look for here. Are the cable shileds grounded
    to the PC case, and is the case properly grounded to the
    power line ground? Even if you have a 3-prong plug,
    the outlet may not be properly grounded. Or the PC
    may be grounded, but the jacks may not be. Ungrounded
    cables may be acting like antennas to bring the RF
    into the amps. Good amp designs shouldn't be
    susceptible to RF, but not all designs are good ones.
    If you don't mind a little surgery on the amps, you may
    be able to put RF filters the input stages, but from
    looking at the innards of a few of these I'd guess that's not
    gonna be a fun job.

    Best regards,




    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
     
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