The circuit shown is a microphone speech compressor built around operational amplifier LM324. This amplifier circuit exhibits gain and also continually adjusts that gain to maintain a constant amplitude output signal, regardless of the amount of signal coming from the microphone.
The microphone is coupled to the LM324 Operational Amplifier IC which has 4 separate units within single DIP package, however only one is required the others are simply left disconnected, refer manufacturer’s data sheet for pinout and more information Datasheet ). The amplified output is sampled, rectified, and used to charge a capacitor, which sustains a smoothed DC voltage, which is representative of the average signal amplitude.
This voltage is used to control the gate of the N channel JFET BF245A as Q1 ( MPF102 /105, J310, and 2N3189 all also work in place of BF245A) and its “ON” resistance. Because the JFET is connected in the feedback path of the LM324, it can be easily seen that the overall circuit gain can be “controlled”. The circuit performs best with the +5v voltage regulator, suitably de-coupled with 2.2uf electrolytic capacitors. Higher supply voltages seem to cause distortion. The 47k-ohm resistive voltage divider R7 and R8 is used to establish an operating point, connected to the non-inverting input to LM324.