This video presents a simple automatic audio volume leveling circuit. The application that prompted this is a police/fire/emergency scanner. Often times, different services will have different volumes in the receiver - so adjusting for a comfortable listening level on one service/station will often lead to other services being too loud or too quiet. This circuit will automatically adjust the volume of each received signal based on the signal's peak amplitude. Similar circuits have been widely published, so there's really nothing new here - just a quick tutorial and demonstration of how this circuit works. An arrangement of capacitors and diodes are used to implement a peak detector to measure the input signal amplitude. The dynamic impedance of diodes is controlled/changed to adjust the signal level. The result is a leveling circuit that has a very wide input dynamic range with a near constant average output level.
Note that fellow amateur radio operator SM6XUN created a PCB for this project, which can be found at:
Here is the schematic for this board:
Note that you'll probably want to increase C4 to about 2.2uF....
Do you want to hear what this sounds like on a scanner, check out this followup video: