Hi Dweller,
Balanced ARC amps which don’t provide RCA inputs are unusual in that respect, and are the only amplifier designs I am aware of that will not work in a reasonable manner when used with RCA-to-XLR adapters.
That fact has been well established in past threads here. Conceptually, at least, it seems as if the input stage of those amps, rather than deriving a differential pair of output signals from whatever difference in voltage exists between the two signal pins on the XLR connector (pins 2 and 3), even if one of those pins is grounded as it would be by an adapter, instead provides the signal on pin 2 to one half of the balanced internal signal path, and the signal on pin 3 to the other half.
An adapter would provide the single unbalanced signal to pin 2, and zero volts (ground) to pin 3. The ARC architecture would therefore result in one half of the balanced signal path not receiving a signal, in turn causing the maximum possible voltage across the primary winding of the output transformer to be not much more than half of what it should be. And since for a given load resistance power is proportional to voltage squared, maximum power capability would be reduced to not much more than 25% of what it should be. And distortion would increase, because of both the limited power capability and the lack of cancellation of certain forms of distortion that would normally occur if both halves of the balanced signal path were processing a signal.
Regards,
-- Al
