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.
– LM358 (MC1458, TL082)