Voltage level detector (CB21504E)

This circuit was found in a 1966 EEE. The circuit can be implemented with modern general-purpose transistors.

Automatic gain control for audio (CB20640E)
This circuit was found in an American publication called Ham Radio. The circuit uses still current...
Phase Splitter (CB4239E)
From an audio signal, this circuit gets two signals of the same amplitude and opposite phases....
Low frequency linear modulator (CB7305E)
Found in an Electronic Design from 1968 this circuit generates a linear sawtooth signal modulated...