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.
Using components from the era, this 90's EDN circuit is used in analog to digital conversion...
Found in an American publication from the 90s, this circuit allows the supply of 3, 6 and 9 V...
We found this circuit in an old Popular Electronics. The circuit is experimental, driving an LED...