This elaborate mixer is from an Electronics Today International from the 80s. Operational amplifiers are common and resistors with no value determine the gain of the amplification steps.
This is a commercial radio circuit from the 1960s. The circuit was found in a Japanese publication from...
The integrated circuit used in this 80's differential amplifier is a quadruple version of the 741...
This touch or proximity sensor circuit was found in a Popular Electronics from 1992. In the...