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I've made a bluetooth audio receiver using XS-3868 module (using this diagram) and connected a TP4056 charger module and a lithium-ion battery too. Everything works fine.

When the charger module is powered (using its micro-usb port), there is no noise as long as something is being played. But when the music stops, the speakers become silent, then after exactly 5 seconds a merely loud noise starts. If another song is started the noise stops. Even if I change the volume on my phone, to which the module responds by a small beep sound, the noise stops again for another 5 seconds.

Of course there is no noise if the charger is not powered and the module is on battery.

I can somehow understand the reason for noise when charger is powered (although I don't know how to fix it, and any help would be great), but my main question here is:

If the XS-3868 can stop the noise, how can I tell it to keep doing the good job, and not give in to the noise? I could not find anything about this feature in XS-3868 properties, or how to prolong that 5 seconds.

Edit 1: I just found the same question here and tried adding the capacitor mentioned there. The noise got much louder! Somebody explained it this way: "The OVc3860 audio output is differential. So If You connect it to amplifier You should do this as differential input. In fact on AGND of OVC3860 is avarange 0.6 V." I have no clue what that means.

Edit 2: I tried adding the resistor according to this too. No difference.

Edit 3: I just found a curious bridge between pin 17 and 18 of the OVC3860. I don't know if it is intended or I should try removing it! Here is a photo:

Bridge photo.

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    \$\begingroup\$ Bridge is intentional. Pins 4 and 5 are both a voltage input. \$\endgroup\$
    – Passerby
    Feb 7 '20 at 1:51
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Sounds like a ground loops and or a missing output capacitor. When the bluetooth module stops playing for x seconds, it turns off its output, so it stops actively driving the output jack. So the line starts floating and picks up the noise from the charging module. You can try a 10 uF ceramic capacitor in line/series on each of the left and right output, to block any DC voltage component on the line.

Secondly try a different charger. Some chargers are more noisy than others and you are picking up the AC or switching frequency noise from the AC to USB/5v adapter.

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  • \$\begingroup\$ You should be right about the floating output. I tried with two electrolytic (I didn't have ceramic ones of that value), but didn't help. Checked with another charger too, the sound of noise changed, but not its volume. \$\endgroup\$
    – Sohail
    Feb 6 '20 at 21:21
  • \$\begingroup\$ @sohail the capacitor on the ic pin 20 needs to be an electrolytic. But the DC blocking ones inline with the audio should be non-polarized. Try a smaller ceramic if you have one. A 1uf or 0.1uf. \$\endgroup\$
    – Passerby
    Feb 6 '20 at 23:59
  • \$\begingroup\$ I tried with two 100nf ceramic capacitors, and that didn't help. I can get some bigger value ceramics next week, if you think that should fix it. \$\endgroup\$
    – Sohail
    Feb 7 '20 at 1:22

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