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I've set up an audio amplifier using TDA7297 and RPi's 3.5 mm audio jack. And I'm using omxd, a daemon for omxplayer, and playing mp4 video files. The audio output has been set to jack.

But the problem is that there's a strange noise in the background of audio. (The recorded audio is http://vocaroo.com/i/s0c0RC3uJE2o and http://vocaroo.com/i/s0Ek3z3ltDSO. One is main audio+noise, the other is just noise with the main audio paused.) This problem occurs when the grounds of both systems (RPi and external audio amplifier) are connected. When disconnecting the grounds, this noise disappears.

But I need the common ground to use logic levels, control audio via MCU, use SPI, etc.

I tried simple RC low-pass, high-pass, band-pass, band-stop, notch, ... filters, but no help!

So, any suggestions !? What causes this noise !? And how can I eliminate it !?

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    Sounds like you have a ground loop ? Apr 4 '17 at 17:46
  • Ditto. I believe, however, de-soldering the ground from the audio jack might help.
    – goldilocks
    Apr 4 '17 at 18:06
  • @goldilocks , I disconnected the jack's ground, but no help.
    – Omid1989
    Apr 5 '17 at 4:36
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Powering everything with a single power supply and connecting all grounds to its ground terminal will eliminate the noise source in most cases but isn't always convenient. Replacing poor cables with better ones (with smaller resistance to ground) is not as drastic but often reduces the noise significantly.

If eliminating the noise is not possible, you'll have to break the ground loop.

Connecting problematic grounds via a small resistance (10 Ohm or so) might reduce the noise to a significant degree while still keeping ground potentials close enough for SPI to work.

Otherwise you can go for full isolation: either put a 1:1 audio transformer in the audio path, or use opto-isolators for digital signals and don't connect grounds. If you don't have a transformer, there's a bunch of audiophile isolation gadgets on the market (just make sure you don't get ripped off paying $100 for things you don't need or things which turn out to be just cables).

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Take the common ground directly from audio jack or RPI USB

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