Audio processors are usually used in paging systems, in wireless intercom and the likes to amplify the microphone signal to a certain level.
This can be done by using either a compressor or a limiter circuit. Although a compressor has lower distortion characteristics, it is a very complicated circuit.
The limiter is simpler to construct, but it has a relatively high distortion level. Intermodulation distortion is high in a limiter circuit and in order to effectively use
a limiter, you have to suppress the intermodulation
interference as much as possible. This can be done by automatically changing the limit frequency according to the strength of signal input. The circuit featured here does just that.
This circuit has an amplifier with a very high input impedance. When the input signal is still low, the diodes do not conduct yet.
In this situation the limit frequency is still dependent on R1 and C1. Once the diodes conduct, (it happens when the input signal has increased),
the input impedance of the amplifier decreases thereby shifting the limit fequency to a higher value. The lower frequencies are then amplified less thus
making the audio signal more understandable. The signal processed this way is much better than the one which is just simply "clipped". This circuit
is also applicable to process music signals.
The values of C6, C7 and C8 are given in the following table according to the
application of the circuit.
Electronic Circuits volume 1.0 - Circuit Nr. 12
The complete data of the electronic circuit described above can be found in the following book and is available from Amazon.com.
Click on the image to view the book.