AGC Dynamic Microphone

Dynamic microphones have a fantastic dynamic range, easily handling whispers to rock concerts. The signal level range coming out of a dynamic microphone can be huge and an AGC amplifier can prove quite handy. This project employs an old hobby dynamic microphone mounted in a film canister with the AGC amplifier tucked inside. The LED/photocell is an ordinary LED glued to the top of a small CdS cell from a night light, covered with black 'liquid tape'. The top of the LED was sanded flat to make it easier to glue to the flat cell face. The pair is visible to the left of the output level potentiometer, above the op-amp.

The AGC range is over 60 dB and the maximum gain is set by the 22 megaohm across the photocell. Combined with the dynamic range of the sound recorder, the total range will extend from a whisper to a roar. The resistor in series with the LED sets the sensitivity of the AGC and that value may be increased if the loop is unstable, especially if the LED is a high-intensity type.

The loop is very slow and the performance is fantastic. This microphone should allow for adjustment-free recording or listening without overloading, even next to a jet! Many other types of microphones will simply start clipping. The prototype is mounted on a stand for general laboratory experimentation.