How bad is using speaker wire for subwoofer signal?

Discussion in 'Home Theater Projects' started by AndyF, Aug 6, 2004.

  1. AndyF

    AndyF Agent

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    I'm trying to troubleshoot my brother-in-laws subwoofer hum problem.

    The sub is a Kenwood 10" with auto power on. When the sub is left on and the receiver is turned off, you get a loud hum coming from the subwoofer. When both are on you don't get a hum at all.

    The receiver and sub both use a two prong electrical plug (no ground connecion).

    The signal cable from the receiver to the sub is regular speaker wire RCA connectors soldered on the ends instead of coaxial cable.

    Could the speaker cable be picking up electrical noise from somewhere after the reciver is turned off and causing the sub to hum?

    Since there's no ground connections, I don't think it's a ground loop problem.

    The way the cable is run, it would not be easy to install a coax cable even using the existing cable as a pull line.

    Any ideas? Or would it better to switch to a passive sub?

    thanks,
    andy
     
  2. Bob McElfresh

    Bob McElfresh Producer

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    Try disconnecting the CATV coax. It is one of the major sources of 60hz noise. It basically acts like a antenna and then feeds 60 hz hum into what should be 0.00 volts in your electronics. The subwoofer could be seeing this as a signal.

    Second thing to try: does the sub have RCA inputs? If so, turn the receiver off (so the hum starts) and touch the RCA input. Does this affect the sound? It could be that the un-attached RCA jack is picking up noise. If so do this: go to Radio Shack and pick up 2 parts:

    - A "F-to-RCA Male adaptor plug
    - A "Resistor Cap" - this is a little cap that goes on the un-used ends of a splitter. They usually sell for about $1 a pair.

    Screw the cap onto the "F" end of the adaptor and plug the RCA part into the RCA jack on the sub. This will 'clamp' the input so noise does not act like a signal.

    Hope this helps.
     
  3. AndyF

    AndyF Agent

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    When I disconnect the subwoofer cable at the receiver, the hum starts. When I let my finger touch the center pin and the shield, it changes the hum a little.

    I also noticed that when the receiver is on but the volume is turned all of the way down, I hear a hum but it's very faint. Only when I turn off the receiver or disconnect the RCA cable from the receiver does the hum become very loud.

    The amp has left/right RCA inputs. Would you explain how to connect the "resister cap" again? Do I just connect the resister cap to an F-to-RCA male connector and connect that assembly to the unconnected RCA input?

    thanks,
    andy
     
  4. Bob McElfresh

    Bob McElfresh Producer

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    Oh Crap I totally missed this:


    Thats your problem.

    Look, nearly any signal that uses RCA connectors are considerd line level. These are very weak signals and WILL pick up noise if you use un-shielded wires which you have done by using ordinary speaker wire.

    Interconnects (with RCA plugs) are made with coax cable. Just like your CATV coax. The coax encases the signal wire in a 'pipe' that protects it from interference.

    I'm sorry, but you really need to replace the speaker wire wire with a coax wire.

    Go to Radio Shack and buy a hunk of pre-terminated CATV coax and 2 "f-to-RCA-male" adaptors. Get the stuff in white so it looks nicer. Now make a long RCA cable that goes along the floor to the sub. Just run it across the room to start and my guess is your noise problem will be solved. When it works, you can then route the wire along the base-board or however it takes to make a perminent install.

    This wont look as nice as in-wall, but it will work.
     
  5. AndyF

    AndyF Agent

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    Thanks.

    I think I solved the problem by convicing him to move the sub closer to the receiver (in a small alcove behind the entertainment center. There is an open air gap between the alcove and the room; so, sound will get out) so it's easier to run cable.

    It'll affect the bass response I know, but it's better than no sub and a visible cable will not pass any amount of WAF.

    thanks again.
     
  6. NickSo

    NickSo Producer

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    Nick So
    Yeah, its a REALLY REALLY bad idea to use unshielded speaker wire for line level signals likea sub signal.
     

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