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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?

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This is a common problem when the step down converter is lower in operating frequency. The power supplies oscillator interferes with the DAC and ADC subsystems.

One concept is to use a soundcard which has a built in isolated supply with a high frequency oscillator controlling the power supply. This high frequency oscillator ensures that the original 5V usb supply is broken up and reconstructed on the other side of the isolation transformer. This deconstruction and reconstruction of the DC signals stops AC noise components on the power line from entering the DAC and ADC converters on chip.

A review of the available sound cards for the Raspberry Pi indicates that the Audio Injector Pro. and Isolated soundcards have isolated power supplies onboard. At this point in time no other soundcards use isolated power supplies.

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  • Thnx Matt. think I might just try a 2 rail ac to dc psu than the step downs then Oct 1, 2022 at 16:57

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