The operating principle is as follows:
Resistors R1, R3 is used to adjust the gain, which equals 25. To increase the gain by 75 to build a resistance of 68 ohms. It is not recommended to install too high gain, because it may affect the sound quality, although this depends on the microphone. Capacitors C5 and C6 are used for suppression of feeding.
This circuit has been tried on three microphones, one from SONY, and then two Chinese, everything works! With the correct assembly of the device starts to work immediately, and does not need tuning.
Parts list : R1, R3 : 220 R2, R4 : 1K5 R5, R6 : 6K2 R7 : 470 C1, C2, C3 : 0.1uF C3, C4, C7, C8 : 10uF/10V C5 : 470uF/15V DA1 : K548UN1