Most pro-audio microphones provide a balanced output -- this is intended to provide improved noise rejection, and provided the output and line impedances are appropriately balanced, it does. However, with conventional differential-to-single-ended conversion stages, source and line impedance imbalances degrade the noise rejection ability of the system. Transformers don't suffer from this, but have problems of their own (magnetic coupling, size/weight, low frequency response), so the Whitlock bootstrapped topology has come into use for differential-to-single-ended conversion, at least at line levels:
Unfortunately, though, no microphone amp yet has adopted the Whitlock bootstrap for an internal differential-to-single-ended conversion function. However, at least some pro-audio microphone amplifiers are designed to be fully differential -- i.e. they have a differential input and a differential output. The intuitive thinking is that since a fully differential amplifier doesn't have a CMRR, it would not impact the ability of a downstream stage to reject the common mode signal, so cascading a fully differential microphone preamp with a line receiver is used to provide a high-CMRR balanced microphone input.
However, assuming a Whitlock bootstrapped line receiver (such as a THAT120x) is used for the differential-to-single-ended conversion function, does putting a fully differential preamp in front of it negate the ability of the Whitlock bootstrapped topology to maintain a high CMRR in the face of inbalanced input and line impedances?
