This circuit is from a factory manual from the 80s. The circuit can be implemented with more modern operationals. Capacitor C1 is a low leakage type. The power supply must be symmetrical.
The transistors are 2N918, 2N3464 and other RF equivalents of the time for Q1. For Q2, 2N3399, 2N2894...
This circuit, which can be used as a shield, was found in a 90s Electronic Design. The circuit...
This interesting intercom circuit was obtained from an Argentine publication from the 1970s....