Is it possible that what you are hearing is the result of 3V3 coming up after 5V which is a characteristic of the pmic on Pi4 ?
Hi tim236! Thanks for replying!
I'd really like to investigate this further but I dont have a multichannel oscilloscope to check the power up state of GPIO22 compared to the 5v and other power pins.
in my 2 setups, RP3B+ and RPi4B-4G, both with X400 v3.0 (even tried interchanging both X400 v3.0 between the two RPis), only the RPi4B produces the "thump sound" during power ON, whereas in the RPi3B+ there is none.
also in the RPi4B, I noticed that if I enable
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though I do have audio output from the speakers, the RPi4B does not "shutdown" properly, (using GUI, or CLI poweroff, or CLI shutdown now, HDMI monitor's power ON indicator remains ON), so I have to turn OFF the RPi on the AC power side.
Reboot commands function properly though
if I disable.... the Amp side of the DAC is disabled so I dont have audio output from the speakers (DAC functional, as confirmed by my VU meters connected to the DAC-Amp's RCA jacks)
and the RPi shuts down properly (HDMI monitor's power ON indicator turns OFF).
Reboot commands also function properly
the above 2 conditions for the shutdown process are not present on the RPi3B+
if there is a discussion on this matter on a another topic, appreciate the link
if this were to be forked as a separate topic, please do...
in the mean time... I'll continue enjoying it with the current state... until a solution (interim or permanent) is available...
thanks again for your time!