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Simple microphone preamplifier

Simple microphone preamplifier

This is a simple microphone preamplifier circuit which you can use between your microphone and stereo amplifier. This circuit amplifier microphone suitable for use with normal home stereo amplifier line/CD/aux/tape inputs.
The circuit is a simple one transistor amplifier with amplification of about 30-40 dB (depends on transitor, temperature and voltage). The dynamic mic input is just a simple one transistor amplifier circuit with nothing special in it. LED D1 is in the circuit to show that the circuit operates. The voltage drop caused by LED (around 1.8V for RED led) has been taten in account when designing the amplifier circuit built around Q1. Resistor R4 and capacitor C5 make a filter to filter out possible noise from battery or other power source which is used to feed this circuit. Capacitors C1, C2 and C3 are used to block the DC bias on Q1 base to flow out of microphone input to microphone (the polarity of all capactors is straigh line = + and curved line = -).

Electret microphone input has a resistor R1 fo feeding current through electret microphone capsule when it is connected to the electret microphone input. Electret microphone needs some current (about 1 mA) flowing through it to operate, because there is a small amplifier circuit inside the microphone capsule. This circuit is suitable for all typical cheap electret capsules which available from any electronic component shop. Because electret microphones have higher signal level output, it is quite easy to overdrive the amplifier when you shout to electret microphone.

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