I've set up an audio amplifier using TDA7297 and Raspberry Pi'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 !?

  • \$\begingroup\$ Show us a wiring diagram. \$\endgroup\$ – Trevor_G Apr 4 '17 at 15:57
  • \$\begingroup\$ @Trevor This circuits.datasheetdir.com/271/TDA7297-circuits.jpg is the schematic for TDA7297 section. The audio from RPi is connected to this. \$\endgroup\$ – Omid1989 Apr 4 '17 at 16:06
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    \$\begingroup\$ I'm more interested in how you wired up the grounds between those and your power supply. \$\endgroup\$ – Trevor_G Apr 4 '17 at 16:07
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    \$\begingroup\$ A daemon is causing strange noises in your audio background, sounds like you need an exorcist \$\endgroup\$ – Voltage Spike Apr 4 '17 at 17:06
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    \$\begingroup\$ Well, the raspi's stereo output is a simple PWM which uses the cpu's power supply as voltage reference, so really, any noise on the 3V3 supply will be fed straight into the output! \$\endgroup\$ – bobflux Apr 4 '17 at 17:12

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