Posted on Feb 6, 2014

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 ha

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s 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.

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