Matthew Skala (from North Coast Synthesis) that we keep linking to 
just posted a set of easy-to-build unpowered module designs, starting with eight holes in a narrow front panel and a wiring diagram for a passive multiple, and expanding that to a mixer, an OR combiner for gates and triggers, a fixed attenuator, a low-pass filter, a high-pass filter, and an envelope follower. The article includes a 2HP Eurorack frontpanel, but expanding to Kosmo is of course trivial.
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This is quite nice for educational purposes because it collects filters, voltage dividers etc. in one place and shows the applications. Only the colour-coding of the multiple’s front panel is a bit confusing, as it shows inputs in black, while the other modules use white for inputs. Maybe this is just in the PDF, but I have not checked the PCB files.
That is because all modules use the exact same panel, it has the 2x1<->3 silkscreen on one side, and 4x1<->1 on the other.
While it may be (slightly) confusing, it is nice for beginners because you always get at least 5 PCBs when ordering, so you can build 5 different modules with a single order, no waste (or more than 5 if you want, or just don’t make the mult, there are enough other modules in that PDF…)
I think there’s no PCB for this, all components are solder on the jack socket
Yes, all components are soldered directly to the jacks, but the front panel is a PCB…
You find the gerbers in the ZIP file.
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What is the harm in ordering 6 PCBs? Maybe I’m missing something here? Also, this is not so much about being a beginner, but that you will have to remember that the colour code is reversed one this one specific module. Perhaps I’m nitpicking here 
There is no harm if you can use them all, if not it’s just waste and pollution !
So being able to use the same panel for all modules is a great plus for a small minus…
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A passive decay envelope generator:
(play with different capacitor values for change the time of decay)
A passive matrix mixer from
http://www.mickeydelp.com/passive-matrix-mixer
Some passives and cool circuits:
Baxandall passive Filter (tone filter) and a passive mixer/attenuator :
Have fun !!
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Passive LPG/vca circuits with vactrols are my favorites ! (sorry for my dirty schematics)
This one look like the Mengqin LPG:
My builds, more vactrols integrated on a panel :
Dual Vactrol LPG:
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Attenuator, LPG, Warp, mute, switch, Fuzz …
All passive
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This isn’t really the right place to post this — this thread is about passive circuits.
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Feel free to make a new thread though, things are pretty lax around here!
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Ok, I’ll leave the v1 schematic of this same project (Opto-VCA), when it was still passive and functional! 
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Thank you, feel free to visit and comment on the new thread! ^^
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I was watching a video on taming the spring reverb in the b2600 grey meanie/blue marvin using a passive 3 band EQ. Does anyone have any suggestions for building one?
My other option is one of the active 5 band EQ boards from AliExpress but shipping has been hit or miss at the moment. 