Found in American documentation from 1968, this circuit has an acute cut-off of the frequency to which it is tuned and which depends on RC. Transistors may be modern equivalents of general use.
This circuit is from a 1995 technical documentation. The transistor can be a BC558 and the sensor...
Found in a Popular Electronics from the 90s, this circuit checks the presence of voltage on a...
The critical component of this amplifier is the output transformer. We found the circuit in an old...