Circuit diagram of portable microphone preamplifier

This is a circuit diagram for a portable microphone preamplifier. This circuit consumes low current (about 2mA), so powering the circuit has a long battery life. Harmonic distortion at all frequencies is approximately 0.1%@1VRMS output. Maximum input voltage (level control cursor set to maximum) = 25mVRMS. Maximum input voltage (level control cursor set at the center) = 200mVRMS

This small portable microphone preamp circuit is based on a low-noise, high-gain two-stage PNP and NPN transistor amplifier that uses DC negative feedback through R6 to stabilize operating conditions very precisely. The output level is attenuated by P1, but at the same time, as the value of R5 increases, the stage gain decreases. This unusual connection to P1 helps achieve a high headroom input that can handle a wide range of input sources (0.2 to 200mVRMS for a 1VRMS output).

Portable Mic Preamp Parts List:

resistance:

R1, R2, R3=100K

R4=8K2

R5=68R

R6=6K8

R7, R8=1K

R9=150R

P1=2K2 linear potentiometer

capacitance:

C1=1uF/6.3V polyester

C2, C3, C4=100uF/25Velco

C5=22uF/25Velco

transistor:

Q1=BC560C

Q2=BC550C

other:

J1=jack socket (mono 3 or 6 mm.)