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Shared power source ground/noise isolation

Discussion in 'Electronics & Electrics' started by dufflover, Jun 20, 2018.

  1. dufflover

    dufflover Member

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    I recently rewired up a wireless audio bluetooth transmitter to the battery terminals of an old portable player as I always use them together but battery on the speaker is much higher capacity (both 4.2V Li-on). But after doing so, there is a now a background static noise added to the transmitted audio.

    I did some poking around with the multimeter and the short test seemed to indicate the audio common ground was the same as the battery negative terminal for the transmitter. I'm not much of a signals guy so the only theory I have is that sharing the ground/common (by now being from the same battery) is now giving some kind of ground noise? I've already tried adding a small ceramic cap across power terminals too.

    I've read 1:1 transformers are a way to solve this as it breaks the wired ground connection, but they don't seem readily available, like a small part I can buy of AliExpress/eBay type of thing.
    I reckon I'm over-thinking it here; any tips? Anyone had similar issues?
     
  2. mjunek

    mjunek Member

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  3. RussellK

    RussellK Member

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    I don't know if they've improved, but way back I tried a ground loop isolating transformer for my car, and it did terrible things to the sound quality - I can't remember whether it was the high or low end that it rolled off, but it was one of them, and it was not good.
     
  4. aXis

    aXis Member

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    I had similar issues and ended up buying an isolated 5V-5V DC converter. This way your power/ground is isolated instead of introducing a coupling of questionable performance into your output.

    I haven't actually wired it up yet but could be worth considering.
     
  5. mjunek

    mjunek Member

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    Years ago, I used to use a similar iso transformer to the ones above between a Foxtel box at home and my old mixer, it worked a treat. No discernable issues with the quality. The double insualted Foxtel box was causing a loop with the earthed TV (video) and the earthed mixer (audio) connection.

    Note that they will only clean up ground loop issues, however. If the problem is related to dirty power or other induced noise it wont help.

    In the specific issue for the OP - seeing you're using the same power source, why don't you connect the signal at one end of your lead, and the signal and ground at the other end? The audio return path will be via the B- connection from the battery, eliminiating the loop.
    It's also possible that there is some interaction between the two power supplies - maybe a Li-Ion charger - which could be causing the noise.
     
  6. OP
    OP
    dufflover

    dufflover Member

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    One doubt I have is these transformers are about decoupling the audio right? But what I'm not sure of is whether these sorts of transformers would fix the issue in that it may be too late and the noise is already added in to the transmission made by the bluetooth transmitter (if that makes sense).
     

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