This circuit was found in an Experimenters Handbook of the 1990s. The purpose of this circuit is to keep constant the audio signal level of a system, increasing the lower signals.
The amplifier shown in the figure is from the manual of linear integrated circuits from Analog...
This circuit with transistors and low current was found in American documentation from the 1980s....
Found in a 1968 Texas Instruments documentation this circuit uses an SCR from that time. Modern...