Here is a low noise preamplifier design for dynamic microphones. The circuit is based on the uA739 IC from Fairchild Semiconductors. uA739 is a dual audio operational amplifier with high gain and excellent stability. Out of the two opamps available in the IC, only one is used here. The audio signals from the microphone are coupled to the non inverting input of IC1 through the capacitor C1 and resistor R1. C1 performs input DC decoupling. The R1, C2 network bypasses the unwanted high frequency signals from the microphone. A fraction of the output is fed back to the inverting input in order to prevent oscillations and ensure better stability. The input impedance of this circuit is around 50K.This amplifier can handle signal from 20Hz to 20 KHz, which makes it excellent for audio applications.
- The circuit can be assembled on a Vero board.
- The circuit can be powered from an 18V DC supply.
- Output and input ground must be properly decoupled for better performance.
- The IC1 must be mounted on a holder.