audio - Mono to stereo circuit only if one cable is plugged in - Electrical Engineering Stack Exchange
Simple Stereo To Mono Circuit | Electronics Forum (Circuits, Projects and Microcontrollers)
Why are two 1 kΩ resistors used for this additive stereo to mono conversion? (For audio inputs to an amplifier) - Electrical Engineering Stack Exchange
Binaural Synthesizer/Mono to Stereo Converter for Small Speaker/Phones – Electronic Circuit Diagram