Fantastic measurements! Thanks very much for your patience.
I do see a rise in noise in the 4-8MHz region (guessing at frequencies), but that appears to be the only problem with the „asymmetric“ ferrite configuration. This region is suppressed by the symmetric ferrite configuration so that it is better than the non-ferrite configuration. So there is a small problem with asymmetry here, but clearly swamped by the rest of the spectrum.
Overall the symmetric ferrite configuration looks great. I wonder how a cable covered by the cheap Topnisus ferrites compares (about 40 per metre of cable I think). I also wonder about a broader range of frequencies, upto 2GHz because sound quality problems have been reported with common-mode noise far beyond 100MHz. But in the end, the measurements already shown in this discussion demonstrate that sound quality benefits are easy to achieve with USB cable connections 🙂
I also wonder how much quieter the spectrum can be made. It still seems to me to be very noisy, all of this unwanted common-mode noise.
Changing subject slightly, this article:
https://audiowise-canada.myshopify.com/blogs/news/sound-quality-rf-energy-best-inputs
shows a Hugo 2 on battery power with various source and cabling configurations. Here we can see a low-powered PC with a USB connection can perform pretty well in terms of RF energy (as measured in the output of the DAC).
We can also see that there are differences between the two configurations that both deliver data to the DAC using a TOSLINK connection („direct from NUC“ and „from XMOS converter“). You can click the entries in the table to see a spectrum of noise from 10MHz to 4GHz.
I have been away for a while and then I didn’t turn on this computer, so that’s why it’s taken me a long time to reply.