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Cable hum

  • 15-06-2014 3:49pm
    #1
    tony glenn
    Registered Users, Registered Users 2 Posts: 609 ✭✭✭


    Why do some of my guitar leads hum when plugged in (disappears if I touch the casing of the plug, and reduces a lot when I plug the other end into an instrument?
    Is it because they're the cheaper leads?
    Can I fix it? They seem ok on the multimeter...


Comments

  • #2
    TroutMask
    Registered Users, Registered Users 2 Posts: 743 ✭✭✭TroutMask


    tony glenn wrote: »
    Why do some of my guitar leads hum when plugged in (disappears if I touch the casing of the plug

    Your amp inputs are 'normaled' to ground when nothing is plugged in, this keeps the highly sensitive instrument inputs from picking up and amplifying stray electromagnetic and radio frequency interference (EMI & RFI). When you plug in a guitar cable, you interrupt the normalising switch and, with nothing connected at the other end, you are basically connecting an antenna to a high gain input - fluctuations in the ambient magnetic field are then amplified through your amp.
    tony glenn wrote: »
    disappears if I touch the casing of the plug
    your body acts as an addition path to ground and effectively shunts the minute variations in the magnetic flux to ground (via your own human flesh).
    tony glenn wrote: »
    reduces a lot when I plug the other end into an instrument?
    In this scenario you have completed the circuit and also introduced the shielding built in to your guitar into the equation = a reduction in hum.


    tony glenn wrote: »
    Is it because they're the cheaper leads?
    Not necessarily - cheaper cables are more likely to leak EMI into the signal path, but if the hum is not caused by the cable in the first place then switching to an expensive cable will not solve the problem.
    tony glenn wrote: »
    Can I fix it?
    Sure you can - the actual cause of the hum needs to be diagnosed though. It might be caused be something other than your cables.
    tony glenn wrote: »
    They seem ok on the multimeter...

    A multimeter will only test for continuity. It will not report screen/shield integrity or show up hum issues not related the the cable.


  • #3
    tony glenn
    Registered Users, Registered Users 2 Posts: 609 ✭✭✭tony glenn


    Now that's what useful replies! Many thanks both.


  • #4
    martinedwards
    Registered Users, Registered Users 2 Posts: 2,045 ✭✭✭martinedwards


    apparently Hendrix used the cheapest leads he could get because he LIKED the spurious RF collected by the strat single coils and poor cable.....

    fluorescent lights are a nightmare!


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