Did some further tests now:
I flipped the belts, so all Idlers are run on the smooth side of the belt (and are changed to smooth Idlers with no teeth) except the Pulleys of the Stepper motor. Therefore I had to flip the belt. This reduced the ringing a lot. It is still there but almost gone. Here a photo of the new belt-lineup (only for one motor otherwise my painting skills would be to confusing )
I also had to loose the belt tension a lot to reduce the ringing even more. So it is still visible but you can't feel it any more...
Also performed a second test:
changed to 0.9° stepper motors to increase the steps/mm. (and also tested 16 teeth Pulleys instead of 20. So I could compare the print quality from 200 steps/mm with 80 steps/mm… But there was no difference.
Next, I will try to replace the belt by a fishing line. So there won't be any toothed pulleys in the hole system any more. (the line just wrapped around the motor shaft 5 times to ensure enough friction). I know, that this isn't a solution, but it may help to find out whether the ringing is really caused by the belts...?
It is really strange!I think I just masked the problem with this conversion, but I did not fix the cause…
Almost everybody else uses the Hypercube evolution with a regular belt line-up...?