If I power from a Pi USB PSU my DAC is quiet and also so is the boot.
I am trying to power from a 24v PSU with a 5v step down on GPIO and tried all sorts of EMI filters, Wide Input SHIM from Pimoroni. Its EMI in fact you can almost hear the SoC at work with a background noise that often sounds like an old analogue modem.
Is it because on the same 24v DC that powers a 100 watt amp it creates a ground loop as at whatever stage I add EMI filters to the DC supply I can not seem to get rid of noise that seems to be generated by the Pi unless I use a seperate PSU for amp and Pi. Which sort of negates my idea of a neat single 24v DC supply to a small enclosure for the Amp and a step down to power the Pi. Tried linear regs, a couple of different bucks and the Pimoroni psu shim with emi filters from input to the dac to pi power input and still noise.
Take out the buck and with a standard Pi PSU powering the PI with the same 24vdc on the amp and all is lovely.
I am scratching my head really but thinking maybe the GND in the audio out of the DAC is creating a possible ground loop that maybe with the 2x PSU isolation doesn't exist. Anyone got any ideas?