This variable filter that lets frequencies from 500 Hz to 1500 Hz pass through was found in a Popular Electronics from the 90's. The operational amplifiers are the 741.
This circuit is from a 1967 documentation. It consists of a sensor for memories used at the time....
We found this circuit in the November 1992 issue of Radio Electronics magazine. The circuit has...
Found in EEE documentation, this circuit can have the duration and frequency adjusted separately....