This circuit was originally intended for reading movie soundtracks. The circuit is from 1967 and was found in an English publication. The sensor is semiconductor type.
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...