I want to build a simple, portable, MP3 decoder + amplifier. I use a PAM8610 amplifier and a generic MP3/bluetooth decoder.
When I power each of them from separate batteries (7.4V) it works nicely. When I power them from the same battery (7.4V) I have A LOT of noise, especially when I turn on the bluetooth.

I put capacitors (3300uF and/or 100nF) on the battery. Won't help.

How can I make these two circuits work together without using separate batteries?

enter image description here
Measured between the speaker "+" and the battery "-" (ground). The input probe is set to 10x attenuation.

Current consumption (MP3 decoder): 50mA in AUX mode, 70mA in BlueTooth mode, 45 in standby mode.
Current consumption (amplifier): 18mA
(without input signal)

Placing a capacitor on the battery won't help. However, placing a 1000uF capacitor directly onto the power connector on the MP3 decoder helps a lot. The noise is at half now. Unfortunately, I don't know which pins are the plus/minus on the IC so I can connect the capacitor directly/closer to the IC. I tried also a 3300uF but the noise decreased with only 5-10%. Considering the size of the capacitor (100uF vs 3300uF), it might not worth it (for a portable application).

Even more, now (after installing the capacitor in the new position) the coil also helps a little. I will try with a bigger coil.

enter image description here enter image description here

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    \$\begingroup\$ Probably ground loop. \$\endgroup\$ – Unknown123 Apr 2 '19 at 17:17
  • \$\begingroup\$ Can you show us your noise and measure ground shift noise separately from supply noise using Vbat- as 0V ref. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 2 '19 at 17:33
  • \$\begingroup\$ @SunnyskyguyEE75 - I this what you want? (see new screenshot). Measured between the speaker "+" and the battery "-" (ground). The input probe is set to 10x attenuation. \$\endgroup\$ – Ultralisk Apr 2 '19 at 18:56
  • \$\begingroup\$ I wanted to see Gnd noise , Vbat noise and ensure you get a flat line probing Vbat- to ensure no induced current loop noise and location of measurements, while speaker noise is a different issue from cap charging without mute during power on \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 2 '19 at 19:04
  • \$\begingroup\$ @SunnyskyguyEE75 -Sorry, as you see, I am only an amateur. So, you mean, that I should hook my oscilloscope Channel 1 probe to the ground (-) of the amplifier and to the minus pole of the battery? \$\endgroup\$ – Ultralisk Apr 3 '19 at 7:51

I see several issues that need resolving:

  • The capacitor has a rating of 6.3v which is lower than your supply voltage of 7.4v
  • The added inductance of the wiring to your capacitor might cause it to resonate
  • The connection between the grounds does not seem solid
  • For a class D amplifier there seems to be a lack of big decoupling capacitors on the amplifier pcb

Try the following:

  • Use a capacitor with a higher voltage rating; preferably 12v or higher. Your current capacitor could be damaged and show unwanted behaviour.
  • Ensure that the capacitor is placed as close as possible to the power supply pins of the amplifier (least amount of wire in between). This will reduce the area that the current travels and with it the voltage peaks.
  • I would remove the inductor; should the mp3 decoder suddenly peak or drop in its current demand than the inductor could create a voltage drop or spike.
  • Insert the ground wires into the holes opposing the copper pads and solder them. Take a clipped leg from a through hole component or some bare wire and lay it over the copper pads of all the ground wires and solder it. You now should have a decent connection between all the grounds. This prevents high resistance in your ground paths.
  • Make sure when soldering that you try to get as many wire strands as possible in contact with the to-be-soldered-surface and solder to prevent bad connections.
  • Try to keep your signal wires away from your 'power' wires as crosstalk can occur between the two.
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  • \$\begingroup\$ Hi. The coil does nothing. I added the coil AFTER I have observed the noise. It does not add or remove any noise. I know that the capacitor V is too small. As longs as it does not smoke I think it is ok for testing. Like the coil, it was added after I observed the noise. So we know it does not reduce OR induce noise. So I will simply remove it (the coil also). I will try the other stuff and let you know. Thanks. \$\endgroup\$ – Ultralisk Apr 3 '19 at 7:55
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    \$\begingroup\$ +1 because this helped: "capacitor is placed as close as possible to the power supply pins" \$\endgroup\$ – Ultralisk Apr 3 '19 at 19:31
  • \$\begingroup\$ the coil actually helps (a little) AFTER I moved the capacitor to the right location \$\endgroup\$ – Ultralisk Apr 3 '19 at 19:32

Placing a 3300uF capacitor on the batterry: no noise eliminated.
Placing a 3300uF capacitor on the power connector: 50% of noise eliminated.
Placing a 1000uF capacitor directly on MP3's circuit board: 80% of noise eliminated (In the end, I had to dismantle the case. There was already a 220uF capacitor there (hidden under the case) but obviously was not enough)

Placing a coil (*103.) on the Bat+ wire eliminated the rest of the noise.
Now the circuit is totally free of noise!

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  • \$\begingroup\$ The 1000uF on MP3 is placed on the MP3 power supply? \$\endgroup\$ – bodtx Apr 26 at 10:34
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    \$\begingroup\$ @bodtx - you put it as close as possible to the IC on the circuit board. Increasing the value won't help much. Probably a 100nF cap also needed? \$\endgroup\$ – Ultralisk Apr 26 at 11:18

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