This circuit with the HA2400 was found in the March 1980 Radio Electronics magazine. The circuit has digital-gain control and must be powered by a symmetrical 15 V source.
The presented circuit monitors a line indicating if it is busy if there is an extension. The...
This high-powered megaphone circuit is from a Radio Electronics from December 1964. Note the use...
This linear circuit was found in a 1965 Electronic Design. The transistors are from that time....