Portable Microphone Preamplifier
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 1uF 63V Polyester Capacitor
C2,C3,C4 100uF 25V Electrolytic Capacitors
C5 22uF 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.)
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.
The 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).
Harmonic distortion is about 0.1% @ 1V RMS output (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
Enclosing the circuit in a metal case is highly recommended.
Simply connect the output of this device to the Aux input of your amplifier through screened cable and suitable connectors.