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.
This is a Japanese commercial circuit found in a 1967 documentation. The transistors are from the...
This circuit was found in a Radio Electronics from October 1952. The circuit equalizes the sounds...
This circuit is from an old American publication, but it can be easily implemented with modern...