Paul,
Don't be alarmed. The so-called hum problem affects only the GK1-R and is a question of proper earthing, with a star earth set up at the volume control for all the wires to and from the input and analog pcbs. It does not affect the GK1-M.
On the R, insert a 1K resistor from the positive of the digital +5v supply to ground on the analog pcb, at one of the grounds on either left or right channel, preferably near the cathode resistors of the tube. This references the digital supply to ground, albeit at the positive output, and scotches any hum intruding at the relay coil buss.
Then separate the input earth and the output earth on the input/relay pcb but cutting the track, and then run sheilded cable (quality microphone coax is fine, no particular kind I've found as this run is very short) from the two pcbs rather than unshielded wire. This kills hum completely when earthed to the ground connection on the motorized pot. When all earths to and from are attached to this earth, a single wire back to each earth of the analog pcb, at the bottom of the cathode resistor, is best.
I will endeavour to put up a graphic on this thread a little later, but I'll need a little time to get it together as I'm pretty full on at present with the end of the FY.
I apologize for these hum issues; the GK1-R is a highly complex piece of gear, and while the wiring as explained in the instructions works well, all implementations are a little different and a tiny deviation in intent can often cause hum. This can bamboozle even me, so no one here is Robinson Crusoe!! This is not a problem on the EL and M versions as they don't have the complication of relay switching and logic circuitry.
Cheers,
Hugh
