I have made a DAC with Soekris dam 1021 which is also connected with rpi4 via i2s interface. I am using a digital switch to switch between rpi and amanero combo384. All the connecting cables with rpi4, switch, amanero combo384 and dam 1021 are shielded signal cables. The rpi4 is powered by ups and ups is powered by smps 3 amp mobile charger. Previously I used rpi 3b + and that time there was no noise even at the full volume. But recently I changed to rpi4 and I am getting noise even at low volume. The noise goes off when rpi4 is powered off. My amp and preamp is parasound halo 21 and p6 and it is perfectly alright as it playes other sources quietly. Need yor kind help and suggestion.
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RPi4 produces more RF noise than RPi3 did. That the price you pay for more speed. It means you may need to take more steps to control conducted and or radiated EMI/RFI between RPi4 and the DAC. Some experimentation might be helpful. For example, increasing physical separation between the boards may give a clue.