This filter for the 300 Hz to 3 kHz range was found in an American documentation from the 90's. The operational amplifier is a double 741.
The circuit shown is from the National Semiconductor FET manual. Both operational and FETs admit...
This circuit is a version of the previous one, but with full wave control. The diodes must be...
This circuit is from the time when cars used contact breakers. This 80's version measures the dwell...