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Noise Injection

Discussion in 'General Electronics Discussion' started by kogi, Jan 22, 2013.

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  1. kogi


    Jan 22, 2013
    Hi all. First time poster here :D I used to be an EE major but that was a looonnng time ago and I've forgotten most of what I knew :(

    I've got a simple audio amplifier. Doesn't have a ton of power, and fidelity isn't great, but it suits my needs in this situation.

    Connected as a source to the amplifier is a little bluetooth receiver. When the items first arrived, I hooked them up and they worked exactly as I'd hoped, and it really sounded pretty good!


    Turns out the inside of the amplifier has just enough room to put the bluetooth receiver inside, so I cracked them both open and got to work.

    The amplifier runs on 12v, so I wired up a 7805 to power the bluetooth off of the amplifier's power supply. Then ran 3 wires to connect the bluetooth headphone out jack to the amplifier line in jack (both 1/8" audio jacks).

    Fired it up and nothing exploded, bluetooth powered on, amplifier powered on, and my phone connected and was playing music. But there was a high-pitched whine added in. Very noticeable even with music playing. :(

    First I disconnected the right audio, then the left. The whine got significantly quieter, but didn't go away -- it was present in both the left and right signals. Then I reattached the audio and disconnected the 5v supply (removed the 7805) and powered the bluetooth independently (isolated) from the amplifier. The whine was still there (still softer)

    Then I disconnected the bluetooth entirely, powered it separately, and connected the audio with a standard cable just like I did when they first arrived. The whine is still there (still softer). WTF??

    What am I doing wrong? How do I fix it?

    Thanks in advance!!
  2. davenn

    davenn Moderator

    Sep 5, 2009
    The bluetooth electronics is probably picking up the whine from the switchmode PSU
    (SMPS) inside the amplifier.
    when the BT unit was outside the amp and the amp was closed up in its metal case, this shielded the BT receiver.
    But now you have mounted the BT receiver inside the metal case and probably within a few inches of the SMPS.
    You will probably find you have no choice but to have the BT receiver mounted outside the amplifier case and powered externally as it was originally supplied

  3. kogi


    Jan 22, 2013
    Sorry, it took so long to get back to you.

    You're right on the money! As soon as I moved the bluetooth back outside (still wired up) and put the metal case back on the amp, the noise disappeared. I guess it's just gonna have to stay on the outside :(

    Thanks for your very helpful reply!
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