This circuit with 24 dB per octave attenuation was found in the November 1973 Radio Electronics magazine. The components are for 1 kHz but can be recalculated for other frequencies.
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...