The function of this automatic volume control circuit is to amplify signals without distorting its dynamic compression. The amplitude differences in the signal are levelled off and the disturbing effect vanish. With this tehnique, overcompensation in the volume is avoided. The circuit is used to automatically control the volume levels of cassette recorders, audio tape recorders, amplifiers or radio devices.
The AVC circuit works this way: the FET (t1) is used as a variable resistor. The resistance between the drain and source of T1 can be between 150Ω and infinite. It is parallel resistor to R2 and togheter with R3 controls the gain of OP amp A1 (around 20 dB). The following OP amp A2 is wired as an amplifier with P1 as its gain control. The negative part of the output signal coming from A2 is rectified and fed to the gate of T1.
Small variations on the signal amplitude does not influence the amplification since the FET has a short delay caused by R6. The opposite effect is also slow because of the discharge time of C1. Both effects result to a smooth regulation of the signal amplitude, making this automatic volume control circuit so effective.
The signal voltage at the gate of T1 must be as low as possible to influence the drain-source resistance. To achieve this, the voltage divider R1, R4 is set at the input line to attenuate the signal by 40 dB. This tehnique enables signals up to 1 Veff to be processed without a problem and with distorsions level below 0.5%. The signal-noise ratio is over 70dB by an input voltage of 1Veff. The losses at the attenuator is compensated by the amplification through Ai and IC2.
The high pass filter made of C2, R7 prevents the bass signals from influencing the regulation. With proper dimensioning of these components , the cutoff frequency of this filter can be adapted to individual needs. Signals below the threshold set by P1 are amplified by around 18dB.
The automatic volume control circuit need a symmetrical supply of +/- 15 volts and consumes around 7mA.
Automatic volume control parts placement layout
AVC supply connections