This microphone amplifier circuit electronic project is designed using MAT02 dual monolithic transistor and few passive electronic components .This microphone amplifier circuit allow use of microphones with different impedance and provides a voltage gain of 20 dB and 23.5 dB (amplification 10 and 15), selectable from a switch.
Preamplifier has two stages with direct coupling and reaction, and the input stage operates at very low collector current to minimize the noise.
Output impedance is approximately 70 ohms at a gain of 23.5 dB. Low power consumption (approx. 2.5 mA) and power from a 9V battery make it ideal for portable applications.
Since the resistor R3 is responsible for input impedance and a few other important features of preamplifier, with R3 = 220 k, the input impedance is 30 k.
Change the value of R3 to 6.8 k input impedance dropped to about 1 k.