I bring up this thread because saw a circuit (though not a head amp) which used TLC271 (CMOS) unity gain buffer but which was bootstrapped (10M + 10M to ground in positive input, feedback from output via 1 uF + 100k resistor to the joint of 10M resistors) and had an input impedance of 1G (with the AC input, when no input the Zin was 20G). It was powered from +/-5V and used 3 mA current. Any problems with that kind of circuit as mic head amp? Can you avoid using the 1G resistor in this case (maybe not as the circuit was still AC coupled with 100 pF cap)?
(please put this thread to Mic Meta ...)