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I have a set of speakers that have to work nearly the high end of their volume range (maximum volume). When the jack is connected to a powered-on laptop then everything is fine, no buzzing sound regardless if the laptop is playing music or not. However when I shut down the laptop, the speakers start to emit a buzzing sound. This doesn't happen if I lower the volume. I can also hear the buzzing if I remove the jack from the laptop and turn up the volume.

What is the cause of this and how can I stop the buzzing sound?

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  • It's mains hum. Powering down the laptop disconnects the laptop output and leaves the audio lead unterminated. I haven't tried it, but it may be possible to add a resistor tip to sleeve and ring to sleeve to reduce the input impedance. – Transistor Dec 30 '15 at 12:18
  • The unpowered laptop or disconnected cable picks up stray AC fields. Just turn the volume down... As @transistor says, if you lower the impedance across the input, that will reduce the hum. If the laptop output can drive headphones then it can drive 100 ohms easily, so use 100 ohms. If not, then 5 kilohms may help some. – user_1818839 Dec 30 '15 at 12:18
  • Could you please describe this process in more details please? I'm not an electronics guy and I'd like to do it. – Paul Dec 30 '15 at 13:02
  • This is off-topic here, it has nothing to do with electronic design. – tcrosley Dec 30 '15 at 19:00
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When your laptop is powered up the audio circuits inside the laptop are activated and, if the laptop isn't playing music or sounds the line output (to your speakers) is effectively held at 0V i.e. no audio signals are present.

Now when you unpower your laptop, the audio circuits inside are just inactive and high impedance - exactly the same high impedance as seen by your speakers when they are unplugged so, if you get buzzing when the lead is unplugged you are also likely to get buzzing when plugged into something that is "inactive".

To stop this sound you could put an inline jack socket and plug that contains two resistors to shunt the left and right inputs to ground (the screen in your cable). The resistors would nominally be 1kohm but you can try this experimentally on one channel by taking a 1kohm resistor and twisting the leads around the jack to you speakers on left or right. You should here left or right becoming noise-free (almost) compared to the other channel.

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