We found this simple toy organ circuit in the February 1968 issue of Radio Electronics magazine. The transistors were common at the time and the transformer was from a transistor radio output.
This circuit is a traditional mixer version with the operational 741 and appears several times on this...
This common circuit using operational amplifiers was found in the 1993 issue of Radio Electronics Special...
This configuration was found in old documentation and is valid for most general purpose NPN...