In this circuit of an old American documentation, the input impedance is the same as the output impedance. The circuit has a sensitivity of 1 Vpp at 75 ohms.
This telephone circuit was found on a 90's EDN. It keeps a flashing LED to indicate that there was a...
This circuit is from an Electronic Design from the 90's. The main integrated circuit is uncommon...
I found this circuit in a 1992 Electronics Experimenter Handbook. The circuit can be easily...