Optical isolator (CB9165E)

This circuit was found on an old EDN and can be used as a shield. Optical couplers are from the era and modern equivalents can be used.

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Digitally controlled audio switch (CB12748E)
This circuit can serve as a basis for a digital mixer or simply for a noiseless switch of audio...
Microphone mixer (CB7988E)
This traditional mixer configuration with a high input impedance was found in a 90’s Popular...
1-transistor radio (CB20247E)
This medium wave radio works with any general purpose NPN transistor. The diode must be made of...