Passive multiples and friends
Passive multiples are often recommended as first synthesizer do-it-yourself projects. There are a few reasons for that: they're simple modules without too many things that can go wrong; they're useful modules that almost everybody needs; the parts don't cost a whole lot; and they're an opportunity to practice soldering.
Often the advice just starts and ends with "build some passive multiples for practice." There are a few kits on the market, but often the builder is just expected to sort of find parts somewhere and know how to hook them up. That may not be ideal for the builder who is a beginner, building a synthesizer module for the first time.
Here's my contribution: a set of instructions (PDF file) and a PCB panel design (ZIP file) for a 2HP Eurorack passive multiple design. You can send the ZIP file to any popular PCB contractor to get panels made up; source some appropriate jack sockets and wire; and go raw-dog building your own multiples.
I have serious reservations about so-called "passive" synthesizer modules. Many of them contain active components like diodes and so they're not really passive at all but rather unpowered. Terminology aside, I think users are inclined to put way too much emphasis on the coolness of a module not needing a power connection, and overlook the serious disadvantages of designing that way. Physics being what it is, energy has to come from somewhere, and requiring it to come from the output of the upstream powered module is usually not better than allowing each module to have a power connection of its own. With a few exceptions (passive multiples in particular being among them), a module being unpowered or "passive" is usually a negative thing that makes the module less useful; but it's often presented as a positive selling point.
I'm also not a big fan of modules that are 2HP in width. There needs to be enough space around a jack socket for the user's fingers to grab the plug, and 2HP modules next to each other put the sockets so close together that plugs on neighbouring modules are likely to interfere with each other. When you use a 2HP-width module it's effectively stealing some space from the modules on either side, and those other modules had better be designed with less cramped panels themselves or it won't work. As I say, your eyes are smaller than your fingers.
Nonetheless, I've added a bunch of other unpowered 2HP module designs to the "passive multiples" document; I call it Passive multiples and friends. By adding just a few more components as descibed in the instructions, instead of just building a passive multiple you can build a mixer; an OR combiner for gates and triggers; a fixed attenuator; a low-pass or high-pass filter; or an envelope follower. If you think unpowered 2HP modules are what you want, these design ideas may make it easier to build them. I've included notes on the issues to watch out for (in particular, regarding impedance) when connecting these modules to others in a patch. And, at the very least, it was fun drawing the wiring diagrams.
I've designed the PCB panel so that it's symmetric to a vertical flip. The silkscreen art on one side groups the jack sockets into two groups of four with one jack in each group marked as different; on the other side it has them in four groups of two. That way, if you buy a batch of these PCBs you can build modules with both kinds of connections and not need a second PCB design.
All in all, whether you agree with me about "passive" 2HP modules or not, I hope these designs are helpful!