Sound detected by electret microphone MIC1 is fed to IC1`s input through resistor R2, and capacitors Cl and C2. Resistors R2 and R5 determine the overall stage gain, while C2 partially determines the amplifier`s frequency response. To ensure proper operation, use a single-ended power supply. R3 and R4 simulate a null condition equalto half the power supply`s voltage at IC1`s noninverting input. The output of IC1 is transferred to emitter-follower amplifier Ql via volume control R6.
The high-Z-in/low-Z-out characteristic of the emitter-follower matches the moderately high-impedance output of IC1 to a low-impedance headphone load.