Portable Microphone Preamplifier by BC550C

Portable Microphone Preamplifier

High headroom input circuitry
9V Battery powered

This circuit is mainly intended to provide common home stereo amplifiers with a microphone input. The battery supply is a good compromise: in this manner the input circuit is free from mains low frequency hum pick-up and connection to the amplifier is more simple, due to the absence of mains cable and power supply.
Using a stereo microphone the circuit must be doubled. In this case, two separate level controls are better than a dual-ganged stereo potentiometer.
Low current drawing (about 2mA) ensures a long battery life.


P1______________2K2 Linear Potentiometer

R1,R2,R3______100K 1/4W Resistors
R4______________8K2 1/4W Resistor
R5_____________68R 1/4W Resistor
R6______________6K8 1/4W Resistor
R7,R8___________1K 1/4W Resistors
R9____________150R 1/4W Resistor

C1______________1µF 63V Polyester Capacitor
C2,C3,C4______100µF 25V Electrolytic Capacitors
C5_____________22µF 25V Electrolytic Capacitor

Q1___________BC560C 45V 100mA Low noise High gain PNP Transistor
Q2___________BC550C 45V 100mA Low noise High gain NPN Transistor

J1_____________Jack socket (Mono 3 or 6 mm.)

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