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The sound card for a PC generally has a microphone input, speaker output and sometimes line inputs and outputs. The mic input is designed for dynamic microphones only with a impedance range of 200 to 600 ohms. With this circuit, the sound card can be used with a a common electret microphone. The circuit contains a composite amplifier using two transistors. The BC413B operates in common emitter to give a slight boost to the mic signal. This is followed by an emitter follower stage using the BC547C. This is necessary as the mic and circuit and battery will be at some distance from the sound card, the low output impedance of the circuit and screened cable ensuring a clean signal with minimum noise pickup.