Chk that Tranny
 

Aside laser SPDIF madness, COAX seems preferable but an expensively
niche, as laser cables are equally TeeVee candy-wrapper territory and
closest to semi-common to computers, although, unfortunately,
difficult in the latter instance to pair for good audio processing
chips to actual bargains in obscure brands (possibly to include kits
for those with an O-scope and ability to take it to the next level),
however, eventually got everything reseated to 5V power and laser
cords, either of which easily affect a couple of units now mostly
synch'd without 24-bit scrambled digits over glitch and drop-out
issues.

Leaving a hum then on one two SPDIF receivers, rather "buffers" from
microphone vacuum tubes to finally filter the SPDIF output. One
buffer I'd stuck into a spare 12V 1000mamp transformer after the
original factory unit burnt out. Why not -- pulled my box of spare
transformers and the first I grabbed was better suited, smaller,
250mamps more, clean without any hum to it. So, did a double-check on
polarity, voltages, while at it, which revealed that beef and heft to
the first replacement, regardless its stamped specs, it was putting
out 15V. No damage for running the plate outputs moderately,
regardless, even though preamp circuits are less finicky if any more
costly to screw about for replacements, than SPDIF digital to analogue
boxes.

I'd turned down the buffer outputs to the A/B power amp to negate most
of the hum, that "first watt" in a low-volume concessions/efficiency,
anyway, but wouldn't have liked any eventuality for burning out a
preamp buffer circuit because of an errant 15V overrating. A check on
those spare transformer voltages nicely turned out even better than
any expectation from rated labels stuck on by their "UL-listing" from
a manufacturer.