Uhh, I am experiencing a very disconcerting humming issue in my setup consisting of N6ii, C9 and Beyerdynamic Xelento. For now, let me briefly say just this:
1. How it sounds: Like a mains hum, plus sizzling noise when touching any metal cases or the N6ii screen(!). Important to know: No connection whatsoever of any device to any mains wall wart, all on battery. No computer involved. Sounds a bit like what musicians on stage create by touching cable plugs or sockets.
2. I tested a whole bunch of combinations. Single-ended and balanced from N6ii to C9 with the C9 stock cables. Xelento on single-ended and balanced. Switched N6ii modules. Used some old Sony MP3 Walkman earbuds instead of the Xelento. Tested with my Sennheiser HD 800S instead of the Xelento. Brought my Hugo 2 in, too.
I will report on the findings from all that later. As tempting as it might be to describe things now, and as tempted as you might be to ask for specifics, for the time being I‘d like to just ask one question only:
I checked my cables with a multimeter. There is one puzzling finding for the
4.4 mm stock interconnect cable that came with the C9: Out of the five TRRRS connections from one plug to the other one, the S (sleeve) connection has infinite resistance. @Andykong, please, is this correct? There is no sleeve (=ground) wire in that cable? Everything else is 0 Ohms for identical ones TRRR to TRRR, and any two non-identical connections like e.g. T to R correctly show infinite resistance.
Thanks for helping me to clarify this one thing up front. In case that particular cable of mine is good, I will for sure return with more specifics and questions.