The basis of this circuit is the HA2400 released at the time. The circuit is from a Radio Electronics magazine from March 1980. The power supply must be symmetrical at 15 V.
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....