KSTR
Major Contributor
I'm pretty sure we are seeing a GND topology error on that amplifier PCB. The field emitted from the AC feeds won't be the main culprit. While not twisted, the wires are still close enough to each other to produce only little magnetic field. Same goes for the field from the tranny itself (toroids can have issues with beams in certain directions).I often wonder if the engineers said they should move things but marketing said it had to be in that exact case. Did the engineers say to twist the A/C wires and management said "that's stupid and would cost a few cents more each"?
IMHO even within the given constraints, a simple redesign of the amp+supply PCB layout could yield 20dB better results.
. "Justify" is of course highly subjective.