With this circuit found in an American documentation from 1969 it is possible to vary the selectivity (Q factor) of a tuned circuit. The operational is from that time.
This circuit, from the 90's, is intended for operation with vinyl record players of the time, as...
Using a zener diode it is possible to change the linearity of the scale of an analog instrument by...
Found in American documentation from the 1980s, this circuit prolongs the ringing of a doorbell or chime...