This mixer is great for mixing CV sources. I use it to mix LFO’s. I has a offset with negative 5 volt or positive 5 volts. A switch for attenuation (6db) a summing output and a inverted summing output. Handy little tool to have. It’s a bit difficult to calibrate unless you have the right gear, you will need a precise multi meter.
https://www.haraldswerk.de/Mixer/Utility_Mixer_I/Utility_Mixer_I.html
Difficulty to build : Medium, but you do need a good multi meter and some center dented potentiometers for this one