How to Build a DIY Analog Passive Summing Box - YouTube
BUILD] "Unison Design" Passive Summi...
Eddy Bergman.com: Synthesizer Build part-17: MIXER and PASSIVE ATTENUATOR in one.
XPSum: A new product combining the XPatch with a Analog Summing Mixer — CB Electronics | Specialists in Studio Monitor Controllers, Digitally Controlled Analog Patchbays, Timecode, Remote
New York Dave's Passive Summing Mixer - Mute Switch | diyAudio