Atmasphere 2-28-2018Ralph, I’ve never been able to find a schematic for any of ARC’s amps which only provide balanced inputs, such as the recent Reference series amps, but I’ve had the suspicion that instead of using differential stages they basically have a separate signal path through the amp (up to the output transformer primary) for each of the two signals in the balanced signal pair they receive for each channel. That would be consistent with a very dramatic reduction of power capability as well as an increase in distortion if the amp were to be provided with unbalanced inputs via RCA-to-XLR adapters or adapter cables, as was found to be the case with the Ref 150 used by the OP in this thread we had participated in some time ago. I believe it would also be consistent with low CMRR, due to the gain and other characteristics of the two paths not matching precisely.
Its probably not the amp that is causing you to hear the cables so much as the preamp. But there is a recent period of ARC amps that had me scratching my head. I’m not sure which models they are. They were out when Kalvin Dahl (with whom I went to school) was still at ARC (about 2-3 years ago). Apparently the amp has a very low CMRR (Common Mode Rejection Ratio) so it only has balanced inputs. Apparently also if you try to run it single-ended the power goes down and the distortion goes up.
I can’t think of a good reason for a low CMRR in a differential amplifier (which is what these amps use). You wind up leaving performance on the table (I’ve been designing differential circuits since the mid 1980s).
I have no idea why they might have chosen that kind of approach, and I can’t say for sure that they did, but if I am correct in suspecting that they did so it would seem to explain some or all of the things you mentioned about their amps that you referred to above.
Best regards,
-- Al