The circuit supplied by 2 9V batteries, a +/- 12V to 18V regulated power supply can also be used, althought this choice may induce line noise (hum).
Working life with a set of two 9V batteries exceeds 50 hours of continuous use, with both channels assembled.
The currunet draw is ~16mA from a +- 9V supply for 2 channels including the indicator led and the microphone's bias current.
The microphone supplied by a 9 to 15Kohm resistor which connected to the positive rail. Voltage gain with the current values of R3=511 Ohm , R4=9.09 KOhm is 18.8 (calculated).
Input's high pass filter (R2,C1) has low cut-off frenquency -3dB at 6Hz. H R2 (25.5 KOhm) και ο C1 (1 μF) τοποθετούν την απόκριση των χαμηλών (-6dB) στα 6Hz.
Rout is approximately R5 = 51 Ohm. If the next stage needs 600 Ohm output impedance you can use 620 Ohm.
The prototype board for 2 channels, etched on a epoxy board (FR4) laminate.
Any OpAmp with the same pinout with the NE5534 can be used.
The prototype pc board assembled with the higher quality component's that are available and OPA637BP used instead of NE5534.