giuliomoro Thanks for that (but i need stereo and it turns out, that the ADC isn't the real culprit).
Here's details. The real noise is
• produced by the DAC (not audibly coming into Bela from the ADC).
• Is is prominently a (very annoying) high pitched tone at around 2750 Hz (plus harmonics/multiples). Connecting the tip of an output cable to the ground (ring) of either main outputs or a rack screw holding Pepper makes that high note even more audible.
• However it is also present with a cable just normally stuck into the DAC.
• Blinking LEDs, especially the first two, increase the volume of the high tone, when on.
Further: The volume of this also depends on the type of eurorack case/power supply Pepper is connected to.
• I had major trouble here on a powered 4ms POD, but
• much less with the massive transformer/power supply of a Dopefer LC6 with a PSU2 (guessing that the newer PSU3 are similar or better in that regard). Still there, though.
Also: It is important to use a direct USB connection without going through a (cheap) hub.
• Otherwise a lot of other noise and crackle leaks from the USB to the DAC.
• Direct connection to my Macbook works much better.
Last but not least: Using the IDE leaves hearable traces, mouse movements, file transfers and all.
• Disconnecting from IDE improves that a lot.
I'll investigate further, as i noticed, that volume and quality of the noise depends a lot as well on the equipment connected to the Bela/Pepper output. On my Lavry converters - sampling at 88.2/24bit - i get the most terrible (as in clear) picture, much less on cheaper converters at same rate/bitdepth.
I am writing all this just so you have a detailed overview.
Hope it helps for future Bela versions!