Low Noise Portable Mic Preamp

This is the circuit diagram of portable mic preamp. This circuit consumes low current supply (about 2mA), so will have a long battery life for supplying the circuit. Harmonic distortion is about 0.1% @ 1V RMS output for all frequencies. Maximum input voltage (level control cursor set at maximum) = 25mV RMS. Maximum input voltage (level control cursor set at center position) = 200mV RMS

This small portable mic preamp circuit is based on a low noise, high gain two stage PNP and NPN transistor amplifier, using DC negative feedback through R6 to stabilize the working conditions quite precisely. Output level is attenuated by P1 but, at the same time, the stage gain is lowered due to the increased value of R5. This unusual connection of P1, helps in obtaining a high headroom input, allowing to cope with a wide range of input sources (0.2 to 200mV RMS for 1V RMS output).

Portable Mic Preamp Parts List:

Resistors:
R1,R2,R3 = 100K
R4 = 8K2
R5 = 68R
R6 = 6K8
R7,R8 = 1K
R9 = 150R
P1 = 2K2 Linear Potentiometer

Capacitors:
C1 = 1uF/6.3V polyester
C2,C3,C4 = 100uF/25V elco
C5 = 22uF/25V elco

Transistors:
Q1 = BC560C
Q2 = BC550C

Misc:
J1 = Jack socket (Mono 3 or 6 mm.)