so i put in a 100nF cap between +12v and GND. i also put two ferrite beads in, one in the +12v line (trace cut, worked just fine) and one right after the 5v regulator (on the jumper.)
- it has improved somewhat, signal to noise-wise. still a bit whiny though, peaking at 2750hz.
- it doesn't change with different PSU's (same PSU as rest of rack, separate PSU).
- disabling LED's doesn't make any difference.
- it still has a tiny bit of creep into neighbouring modules, but that is negligable, so great!
- on it's own, still using the tape delay patch, the whine is louder on the dry output than on the wet side.
now in a real-world live situation, i guess this is totally workable.
i do feel, however, that every track i'd record with it will have to get eq'd quite steeply at 2750Hz.
just pushing by +6dB puts the whistle prominently in the mix. which is not unreasonable when compressing a track, for instance. the problem with those kinds of frequencies is that, even at lower levels, they're still very present..
i guess this is about as far as it goes, so i'll learn to live with it.
i really want to start using it, and maybe focus more on all the wonderful stuff it CAN do..