Parasite cancelation on a LM386N amp

Question

I have put together:

1. an Raspberry Pi Pico that manages a keypad and
2. a simple audio amp based on LM386N.

The audio amp is connected via GP22 which provides PWM and plays the role of sound-audio feedback on a successful key-in pressed. The system works correctly and audio beeps are played from the speaker attached. However there is unwanted noise that is coming out of the speaker, sporadically.

Specifically the parasite or noise is very precise and is repeated every time the wifi/mqtt publishes data to the network plus some other times, but definitely it doesn't seem to be random noise.

How do I tackle this? Of course I would prefer this to be as silent as possible.

Here is the schematic:

This is the pcb layout:

Answer 1

The noise burst may get coupled via audio input or via supply.

Your circuit shows huge electrolytic caps, but no small bypass caps such as 100nF ceramic that are more effective at high frequencies.

Also, your LM386 circuit is set to operate at maximum gain possible, which means that the usable volume range will be quite low and audio needs to be attennuated by very large amount to have suitable level at LM386 input. It also means any noise coupled to audio will get amplified by huge gain.

And the audio is not AC coupled.

First solution is to set the gain to minimum by removing the capacitor C1 and then AC couple the PWM audio input between IO pin and potentiometer.

You may need to add high frequency supply bypass and audio lowpass filter to remove any leftover noise.