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Hum on Active Speakers (not ground loop?!)

Discussion in 'Audio Visual' started by Seb101, 5 Oct 2020.

  1. Seb101

    Seb101

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    Hi All,

    I'm baffled by a problem I'm having with some audio equipment. I have two active speakers (with built in amplifiers) each takes a two-pin mains connection and an unbalanced RCA input. I have these connected to a laptop, through a 3.5mm jack to RCA breakout tail. There is no earthing terminal on either the laptop or the speakers.

    When the laptop is switched on, everything is fine and when nothing is playing, the whole system is silent, however as soon as the laptop switches off (or goes into Sleep mode) there is instantly a VERY LOUD 50hz humm from the speakers. I have tried different devices, and the same occurs, if the unit is powered, no hum from the speakers, if the unit is unpowered, very loud hum from the speakers.

    I'm used to dealing with ground loop issues, and normally this sort of thing would be corrected by ensuring all the equipment was on the same earthing point, but none of the kit has earth terminals and all the equipment is plugged into the same multi-socket in any case.

    If I disconnect the audio cable from the laptop end - the buzzing remains but is quieter, if I disconnect the audio cable from the speaker end, the hum stops. So from this I am guessing a current is getting induced in the audio cable from the power cable? But if so - why does the humming stop when the source equipment is powered on?

    How could I fix this permanently? Another forum suggested buying RCA cables with an earth wire... but there is no-where to attach this on either the speaker or the laptop.

    Help!
     
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  3. Alec_t

    Alec_t

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    With the laptop switched off the impedance at the laptop end of the cable is possibly higher than when the laptop is on and actively driving the cable (albeit with muted audio). The high impedance would mean the cable (plus the attached laptop) is more prone to pick up 50Hz interference. You could try a shunt resistance (say ~10k?) on the cable input end.
     
  4. Tigercubrider

    Tigercubrider

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    Ok so it's not a ground loop, but have you actually tried a ground loop isolator?
    £10 on amazon and has sorted many an issue for me
     
  5. DIYnot Local

    DIYnot Local

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