Ah, mixers and multiplexers/multiples. Not the most interesting modules, but very useful.
Yet again, I've reached into the excellent resource that is LookMumNoComputer for both the Mixer and Multiple schematics. Both of which are based around the well used TL072 dual, and TL074 quad op-amps (I bought a job lot of these for this project!).
Takes a single input and outputs a multiple copies of the that signal. Very useful when you have a single gate trigger, and want to use that gate to trigger multiple components.
Very much like a regular mixing desk; sums multiple inputs into a single output. Typically with control over the levels of each input. This module is very useful for blending CV signals into the VCF so that many different CV sources and influence the filter cut-off (eg key velocity + accent + envelope)
Not a huge amount more to say other than I've chosen to build active or buffered modules, which means they are powered (-12/+12v) and use op-amps to ensure the signal input and output signal strength is kept strong. You can achieve a similar effect with a passive module (not powered) but there is a good chance (particularly with the MUX) that the output signals will be weaker/lower - basic voltage divider characteristics.