Okay, so a couple of months ago I got gifted a GrubDAC kit. Built it up nice and clean, added a few mods here and there, and I must say that I really, really like the sound of this little thing. Balanced sound, no coupling caps, nice output level. BUT what was annoying me to no end were the HF interferences manifesting themselves as clicks, pops, and especially drop-outs of the DAC whenever any power switch of some sorts was actuated. Anything from the fridge, oscilloscope, soldering station, light switch in any room of my apartment, maybe even one or two power sources that my neighbour is running, it all disturbed the DAC in some varying degree.
For reference, here's the setup that I was dealing with: Netbook to (bus powered) GrubDAC via super short USB cable, from there shielded RG316/U cable with BNC plugs to the input of my preamp. At the input was a 25k Alps pot, connected via Cat5e twisted pairs, going into a DC coupled unity gain JFET buffer (a so called DCB1 Mezmerize), which then connects to an active crossover that feeds two stereo amps. Here's a pic:
The crappy boar shaving brush (too floppy when wet, no backbone) is my dedicated electronics dust-off brush, I highly recommend it.
On with the story. Trying to get rid of these interference issues, I implemented many a thing. My source is a netbook, so I ran it on battery. Didn't change a thing, it seemingly wasn't entering the DAC from this side. Added ferrite cores, chokes, line filters to preamps and amps (100% DIY), bypass and decoupling caps, bigger resistors and bigger caps (not that I expected anything from those last two), nothing changed. Then I googled and saw that other people are having that problem too, but no solution was to be found. The GrubDAC looks like this now:
Unrelated to the DAC issues I added something to my system yesterday: Another JFET buffer with relay based source selection, for convenience really. Here's another pic:
The GrubDAC connects to the two BNC connectors (to the left of the two unoccupied RCA jacks). As you can see the whole thing is not exactly inside a chassis. The whole pre doesn't even have a line filter at the mains input. Noticed how the first paragraph is in past tense? That's right, the issues are gone.
What changed? I left the volume control where is was before, at the input of the Mezmerize. In front of the volume pot I added the additional JFET buffer, and at this buffer's input I simply put two 22k1 Dale RN60D resistors to ground (instead of a pot), which in parallel to the buffer's own input impedance form a fixed(!) input impedance of pretty much 20k. The rest is virtually the same. The same cables, the same plugs, the same circuitry. The relays are Nais TQ2-5V and I doubt that they have something to do with the resolving of the issue. All the buffers use the same JFETs (2SK170BL) and resistors (RN55D), all unity gain, no coupling caps.
So the new setup is: Netbook, GrubDAC, fixed 22k1 RN60D resistor, buffer, volume pot, buffer, crossover, amps.
I'm still not sure what is actually going on, but the simple resistor at the input must have a flatter impedance than the pot for the HF portions of the power switch spark e/m crap. Or something like that.
TL;DR: Buffer your volume pots.
I hope this helps. Comments, criticism and alternative explanations welcome.