Stereo Mini 3.5mm to Mono 1/4" Jack - Phone to Mixer - Left/Right Summing Cable
The Stereo-to-Mono Summing Cable That No One Makes | Late Reflections – the Silent Sky Studios Blog
Summing Cables - Pro Co Sound
Amazon.com: 6 Ft Castline Gold 3.5mm TRS to 1/4" TS Mogami 2549 Stereo to Mono Summing Cable with Resistors : Musical Instruments
Universal Audio Apollo 8 & The SUM Cable | Pro Audio LA - YouTube
Pro Audio L.A. - The SUM Cable's passive design gives you a pristine signal path directly from your convertor. The DB-25 version turns interfaces like the Lynx Aurora, Apogee Symphony or Apollo