I tried many different audio sources etc. and the problem persists. So, the issue is tied with the DAC. Typically such EQs are implemented with IIRs. My quess is that they are using something like CM6533 . Samplerate matching control loop could cause this. (I don't think there is FT anywhere in this part of chain.)
The timescale is on top in seconds. So the pitch sharply jumps every couple seconds. But also this seems to vary and can be more rapid. This was very audible for me and made the device useless. I also wonder why others haven't figured this problem right away. When you once hear it with some static tone then you cannot unheard it even with more complex music.
There has been talk of weird audible effects, group delay measurements and uneven frequency responses with some methods. These are all reflections of this same underlying problem. Maybe some buggy frequency locked loop generating the DAC reference clock.
Acceptable-Exit-9212 OP t1_jd7slio wrote
Reply to comment by Bal_u in Moondrops DSPs defective ? : Quarks DSP by Acceptable-Exit-9212
I still think they are the best idea ever when even half-properly implemented. Sadly this was not the case here.