5 Pack 2x RCA Stereo Converter to 3.5mm Mono by Keple, Audio Adapter, Y Splitter, 3.5mm One Ring Male Jack Plug to 2 x RCA Phono Female Connection | Keple.com
Hosa GPM-179 Mini Stereo to Mono 1/4 inch Headphone Adapter | Sweetwater
Mono-to-Stereo 3.5mm 2-Conductor to 3-Cond Adapter
50-20795 - Multicomp Pro - Stereo to Mono Signal Converter
Binaural Synthesizer/Mono to Stereo Converter for Small Speaker/Phones – Electronic Circuit Diagram
Single Stereo to Dual Mono Audio Converter Mono out retains L/R input isolation | eBay