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I'm working on a project to convert an old Bluetooth earbud into a wireless speaker by connecting it to a PAM8403 Class‑D amplifier. The earbud’s capacitive touch sensor (a metal pad which normally lets you pause/play or trigger Google Assistant) is causing problems—it activates unexpectedly when I connect the PAM8403. I don’t need the touch functionality for this project.

What I've tried so far:

  • I attempted to load the sensor by placing 4 Ω resistors in parallel, but that approach didn’t prevent the false triggering.
  • I suspect the issue arises because the amplifier’s switching or grounding effects are altering the sensor’s electrical environment.

My questions are:

  1. Is there a recommended hardware modification (such as adding a pull‑down resistor, shielding, or cutting the sensor’s PCB trace) to effectively disable the capacitive touch sensor without affecting the rest of the audio circuitry?
  2. Are there any best practices for isolating or filtering the sensor inputs?

Any insights, schematics, or similar project experiences would be greatly appreciated!

The capacitive touch pad is on the bottom left corner of the first picture The capacitive touch pad is on the bottom left corner

other side

The capacitive pad: enter image description here

Update: circuit diagram enter image description here

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  • \$\begingroup\$ Are you able to edit the question to include pictures of the earbud showing the layout and possibly component identities? That might help to get an answer. \$\endgroup\$ Commented Mar 1 at 20:35
  • \$\begingroup\$ We don't know if the problem is what you say - the connections of the earbud to PAM module - as you don't show how you connected it. The earbud likely already has a D-class H-bride output stage and you need to be compatiblr with it. \$\endgroup\$ Commented Mar 1 at 20:45
  • \$\begingroup\$ Where's the metal pad you mentioned? \$\endgroup\$ Commented Mar 1 at 20:51
  • \$\begingroup\$ The earbud has two wires for the audio output, when connected to a small speaker directly it works fine, when connected to nothing it works fine, the problem is just the connection with the pam module which likely cause interference. I've also tested my build with a simple jack connector directly and it works too \$\endgroup\$ Commented Mar 1 at 20:52
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    \$\begingroup\$ @Alex Yes but I already stated you need to know if original bluetooh speaker output is a H bridge output, which it likely is, so you can't short either of original speaker outputs to ground on PAM module. Figure out if the original output has one ground terminal or if neither terminal is ground. \$\endgroup\$ Commented Mar 1 at 22:08

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As you said yourself, the SP- output is not ground, and if so it can't be connected to ground, as doing so will short cirvuit the H-bridge output of the negative speaker side to ground via PAM and battery ground.

It's the short circuit that's causing the problems.

If you find a way to wire it properly then as you saw it works properly.

Simplest solution would be to leave out the speaker negative and PAM ground connection. Best solution is to use audio isolation transformer for a passive solution. You can also design an active op-amp circuit that converts the differential audio output to be single-ended and referenced to ground.

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  • \$\begingroup\$ Is that achievable with basic components ? Or do I need as dedicated IC ? \$\endgroup\$ Commented Mar 2 at 6:59
  • \$\begingroup\$ I guess the answer to that depends on if you consider any suitable audio transformer or op-amp basic or dedicated, not all op-amps or transformers are suitable. \$\endgroup\$ Commented Mar 2 at 9:11
  • \$\begingroup\$ I don't have such ICs on hand, is it possible with just resistors / transistors/capacitors ? When connecting only one wire, it seams to work but the audio is quieter, can I use 2 PAM in parallel or series ? \$\endgroup\$ Commented Mar 2 at 11:53
  • \$\begingroup\$ @Alex the original output is similar to PAM output, which is why you can't short SP- wire to ground, or use two in series because you can only use one wire and the voltage gain is what it is. Either you need to replace the bluetooth earbud to a bluetooth module, find/make an adapter circuit, or change the PAM module to something with differential input. \$\endgroup\$ Commented Mar 2 at 14:18

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