Low Distortion Audio Limiter

The level at which the audio limiter comes into action can be set with the LIMIT LEVEL trimmer potentiometer. When that level is exceeded, the output from the LIMITER-DETECTOR half of the op-amp (used as a comparator) turns the LED which causes the resistance of the photoresistor to decrease rapidly.
That is turn causes the gain of the LIMITER half of the op-amp to decrease. When the signal drops below the desired limiting level, the LED turns off, the resistance of the photoresistor increases, and the gain of the LIMITER op-amp returns to its normal level-that set by the combination of resistors R1 and R2.
A dual-polarity power supply (+/- 12 volts is desirable) is needed for the op-amp.

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Low Distortion Audio Limiter - Circuitos de Electronica