Available as an assembled Module and as a DIY project.
This Module is currently available.
Unity Gain Mixer
The MST Unity Gain Mixer adds input signals together and combines them for a summed output. For example, if you put in a 1V source and a 3V source, the output will be 4V. It’s mainly used for CV, but it can also be used to combine audio signals together.
A unity gain mixer is handy in several ways:
Features:
http://www.synthrotek.com/products/mst-eurorack-modules/mst-unity-gain-mixer/
These merchants probably sell this module. Huh?