Al, it is possible that they use this amp to invert signal, but why? What are they going to do with two signals of the same polarity?Hi Kijanki,
Not sure what you mean by this. I have not referred to two signals of the same polarity. What I am describing is a possibility that the amp has a fully balanced signal path, and when its RCA input is selected the signal from the RCA connector together with its inverse (generated by the INA134 used as an inverter) form a balanced pair of signals that is provided to the rest of the circuitry in the amp.
It seems much simpler just to use INA134 as differential amplifier and use SPDT switch to select between its output and RCA input.But if the amp is fully balanced, as has been claimed, this would not provide the rest of its circuitry with a balanced pair of signals.
In addition, summing of the signal when only half of it passes thru inverting op-amp compromises CMRR at higher frequencies.I have not referred to any signals being summed. But yes, if the amp is fully balanced and only one of the two signals provided to the rest of its circuitry goes through an inverter when the RCA input is used, that can be expected to result in some degree of sonic compromise. But arguably some degree of sonic compromise will result with any fully balanced amp if it is provided with an unbalanced input.
The bottom line is that my hypothesis would explain why the amp may be fully balanced, as has been claimed, while at the same time providing equal gains regardless of whether its RCA or XLR input is used, as the OP has reported finding.
Best regards,
-- Al