The discussion thread at the headphone forum is here
, but I wanted to throw out the problem to the general blog-reading community here at diyaudio to see if anyone can nail this.
The earthed chassis (light blue) must connect to the circuit common i.e. "ground" (pale green).
I do not know where the best place on the circuit ground is to tie that connection.
(COM and GND are completely equivalent pads on the circuit board, while IN- and OUT- also pads on the board but physically further away on the ground plane.)
answer: as long as it connects at one point only, or the same point of both channels, it doesn't seem to matter at all.
additional: the noise is a 60 Hz waveform, a positive spike and a negative spike once a cycle. It appears to originate from the current flowing through the transformer secondary as it is briefly switched on by the rectifier diodes twice...