This passive loudness control was obtained in an old publication. The critical component is the tapped potentiometer that can be adapted with an external resistor network.
The circuit shown is from an old European publication. In it we have the way to activate a relay...
We found this circuit in a European publication from the 1990s. Transistors are still common types...
This circuit was found in an American documentation and can be added to common amplifiers of any...