This filter for receiving audio in telegraphy was found in a magazine 73 for radio amateurs, in an edition of the 80s. The circuit is tuned to 700 Hz and has a 200 Hz bandpass.
This circuit with a double-gate MOS transistor was found in a Popular Electronics from the 90's....
This configuration was found in the January 1992 Radio Electronics magazine. The frequency is set...
This circuit shows how to get 5 V under low current from an operating amplifier 741. I got this...