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.
This circuit is from Popular Electronics from the 90's. The circuit discharges the battery with a...
We found this time base circuit in a timer project from the August 1972 Radio Electronics...
This sonic switch, vox or sonic relay uses a Darlington of the time that has a gain of 10,000. The...