I am using a Raspberry Pi 3B to output music to an audio amplifier board. Without the Pi connected, there is no noise from the amplifier. Once the Pi is connected (but not outputing any music yet), there is loud crackling and humming which is associated with the activity on the Pi.
BTW I am a (retired) digital engineer and my analogue experience goes back to university.
I am using a "brick" power supply (J1) which outputs 20V. This feeds the amplifier directly. There is a separate buck/boost DC-DC converter (J2) which generates 5V from the 20V for the Pi. Here is a diagram of the wiring:
The amplifier board uses a TPA3116D2 audio amplifier IC.
The (partial) schematic for the Pi can be seen here. The audio output is generated on the Pi by a PWM which is buffered using a separate 3V3 supply, then filtered before output to the 3.5mm jack socket. Unfortunately the Pi schematic doesn't show how the 3V3 supply is derived from the 5V input.
If I power the Pi directly from a standard USB charger (with the amplifier still supplied by the power supply), there is no such noise and it sounds pretty good.
I can think of three potential problem areas with my design:
- The noise is being generated by the power supply. The amplifier has a good PSRR so can cope with this noise but the DC-DC converter is essentially passing it through. The Pi then passes the noise onto the 3V3 supply.
- The noise is being generated by the DC-DC converter which the Pi is passing through.
- All the wiring is generating ground noise.
Is there a more likely source of my issue? Which is the most likely area for me to investigate further? Any suggestions on what I can do to remove this noise?
All help gratefully received.