This circuit is a four-quadrant multiplier using an LM3900. The circuit was found in the July 1988 Radio Electronics magazine.
The integrated circuits used in this configuration are not common today. The circuit is from the...
In the figure we have the way to activate a red LED that needs 1.7 V or even others with higher...
Originally in the 1992 publication, this amplifier was recommended for use with record players....