6: It is often convenient to use a balanced microphone preamp as a line input by using a suitable balanced attenuator, typically 20 to 30 dB. The input impedance of the mic input stage will be 1 to 2K for appropriate mic loading, and this constrains the resistor values possible. Keeping the overall input impedance to at least 10K means that the divider impedance must be fairly high, with a lot of Johnson noise, so the total noise performance balanced is almost always inferior to a dedicated line input amplifier. CMRR is determined by the attenuator tolerances and will probably be much inferior to the basic mic amp, which usually relies on inherent differential action rather than component matching. Fig 14a shows a bad way to do it; the differential signal is attenuated, but not the common-mode, so CMRR is degraded even if the resistors are accurate. Fig 14b attenuates differential and common-mode signals by the same amount, so CMRR is preserved, or at any rate no worse than resistor tolerances make it.
