This circuit was found in the April 1993 Electronics Now magazine. With it, it is possible to control the gain of an operational circuit with a JFET.
This filter for the 300 Hz to 3 kHz range was found in an American documentation from the 90's....
Radio Electronics published this circuit in a 90's edition. The circuit makes use of an uncommon...
We found this circuit in a 90s Popular Electronics. The operational amplifier can be replaced by...