avc circuit schematic

AVC Automatic Volume Control Circuit

AVC – The featured circuit controls a volume line automatically. It delivers an output voltage of approximately 4 volts peak to peak. This voltage remains relatively constant by input voltages ranging from several hundred milivolts to serveral volts.

This electronic circuit is not highly recommended for Hi-Fi applications because its noise factor is way above the accepted level. However, it is very useful in tape recording of computer programs. The constant amplitude is desireble in such application.

The opamp A1 works as an input signal buffer. If we remove the diodes D1 and D2, this opamp will work as an amplifier. The DC bias of A1 is done over R4 and C5. This little trick allows the A2 to limit the DC level at its input to a maximum amplification of 100 times. The offset bias is relatively constant. The amplified signal is fed to the regulator diodes D1 and D2 over the transistor T1 and T2. Trimmer P1 controlls this sample signal. The higher this signal is, the higher is the current flowing through the diodes.

These regulator diodes have a non-linear curve. Their resistance decrease with increasing current. The input signal gets grounded more or less through the diodes. To put it in anothes way: the diodes work as an attenuator with increasing effect by increasing the current through the diodes.

AVC Circuit Schematic

avc circuit schematic

– LM358 (MC1458, TL082)
– 2SC733


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  • cezar

    Hi, P. Marian

    I will definitely build me one of these.

    Back in the early days of cheap portable Karaoke equipment, which could also be used for stage announcements and debates, we have a problem with feed back from the speakers voice level,and one of the operators adjust the volume level to avoid the nasty feed back, and we have tried various circuits to adress the Automatic Volume Level Control, and I have not come accross this one.

    It will be fun to do it all over again..

    Thank you very much.

  • Robert Bates

    This looks interesting where could I get a few CCA’s. I shall need a few as my problem is stereo.

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