The microphone preamplifier circuit design presented in this schematic use SSM2015 produced by Precision Monolithics Inc. (PMI) which offers a high amplification and low noise ( 1.3nV/√f).
Has been designed to work with differential input signals and can deliver 10 to 2000x amplification and it is term of R4 value. For R5 = R6 the amplification is A = (20000/R4)+3.5
The values from the schematic correspond to 1000x amplification. Noise signal ratio is 95dB.
Distorsions were 0.006% at 1kHz and 0dBV and lower than 0.01% at 10kHz. Bandwidth is 180kHz at 3V and 1kΩ impendance.
R3 value depends on selected amplification ( tabel 2 ):