Audible buzzing from tablet computer driving speakers

Discussion in 'General Electronics Chat' started by John P, Nov 14, 2015.

  1. John P

    Thread Starter AAC Fanatic!

    Oct 14, 2008
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    For the last couple of years we've been using a cheap tablet computer to store our music, with an external amplifier plugged into the headphone jack, driving a pair of speakers. The computer and the amp run off separate wall-wart power supplies. This setup has worked well, but a few weeks ago the tablet decided it just didn't want to work any more, so I dumped it and got another one. Now any time the tablet is connected to its power supply, there's a buzzing coming from the speakers. My ear isn't good enough to detect it when there's music playing, but it's certainly audible when there's no other sound. It doesn't happen when the tablet is running off its battery.

    If I saw this item posted, I might suggest "Try the new tablet with the power supply for the old one". I've done that, and the buzzing sound is worse on the other supply, but the old computer never made any obnoxious sounds. So now I'm thinking that I should cut the cord from the power supply and add filter components, then splice it back together. I'm skeptical about whether this will work, though. Has anyone got an idea of what might be happening?
     
  2. Dodgydave

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    Jun 22, 2012
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    If its not buzzing on battery , then its the psu, you can try a filter, but i think its the DC side thats at fault.
     
    Last edited: Nov 14, 2015
  3. GopherT

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    Nov 23, 2012
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    The ground of each device may be at different levels (ground loop). More specifically, the output port may have common at 1/2 a Vcc and input common may be at power ground (same as the output common of your old computer).
     
  4. John P

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    I don't see how that could happen, as the two power supplies must be floating relative to power-line ground, and they'd only be tied together by the ground that's in the audio cable (stereo sound plus ground, coming out through a 3mm stereo plug). And wouldn't the input to the amp be AC coupled through a capacitor?

    Since the old computer didn't buzz but the new one does, and it's worse with one power supply than the other, I'd have thought that the issue must be that it doesn't filter its input power properly, and I might deal with that externally.
     
  5. John P

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    Oct 14, 2008
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    I attempted to solve the problem by cutting the power cable and splicing in a capacitor. I didn't expect it to work, and it didn't.

    But here's another fact I just observed. I tried playing music from the tablet with the volume (using the tablet's control) turned all the way down. In this mode, there was no buzz, but if I actually stopped the music, the buzz started up in a couple of seconds. Likewise, it buzzes if the music player app isn't running at all. So it seems to me that what must be happening is that when the audio output isn't in use, the tablet allows the power cord to function as an antenna. My guess is that this is made to happen by the tablet disconnecting the audio, or somehow connecting it to something that picks up noise. But that's not a complete explanation, because just yanking out the audio plug feeding the amplifier doesn't make it buzz, unless you actually put your finger on the electrodes in the plug, which is reasonable because the audio cable is shielded. Somehow the tablet computer is doing something worse than just leaving the cable disconnected. Therefore I'm continuing to be puzzled.

    Edited to add--the power cord doesn't just function as an antenna. It has to be plugged in to have any effect. But having said that, I do notice that if I grip the power pins of the wall-wart while it's unplugged and the music app is stopped, there is a very slight buzz, but I had to put my ear up to the speaker to hear it.
     
    Last edited: Nov 16, 2015
  6. Dodgydave

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    Jun 22, 2012
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    Gremlins mate....
     
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