Case two part two

I’ve been doing more thinking, planning, designing. If this new case is going to be, conceptually anyway, a synth capable of standing alone and not just more modules added to the ones I’ve got, then maybe that synth needs a name. Kosmodrome? Maybe.

In deciding what to put in it I looked to the old Moog modulars for inspiration, and in particular the Model 15. My understanding is the Model 15 came out in 1973 along with the System 35 and the System 55 — what the difference between a Model and a System is, I don’t know. But the Model 15 was the entry level modular of the day, and seemed a good, ahem, model for me to base this synth on. Not that it’s going to be anything like a Moog clone, far from it — then again, its price isn’t going to be anything like a Moog modular’s either. But the basic functionality is based on that of the Model 15.

The Model 15 had:

  • (3) VCOs – (1) 921, (2) 921B, with a 921A driver
  • (2) 902 VCAs
  • (1) 904A VCF
  • (1) 907 fixed filter bank
  • (1) 910 power supply
  • (2) 911 ADSR envelope generator
  • (1) 923 noise/high-pass/low-pass filter
  • (1) 952 49-note (C-C) duophonic keyboard
  • (1) CP15 with mixer, multiples, attenuator

(This may have varied over the years.)

If I were trying to clone a Moog, that first line would present a problem. The 921 was a full featured oscillator while the 921B had fewer inputs and controls but was intended to be used with the 921A driver, which could for instance set the frequency and pulse width for multiple 921Bs simultaneously. Not too many DIY oscillators are set up like that. (The LMNC Performance Oscillator has a “link” connector for something like an oscillator driver, but so far no such driver has been released.) As for low frequency oscillators — there wasn’t one. The 921 and 921B could be switched to a low frequency range, which would then leave you with only one independently-controlled audio oscillator. I decided to go instead with two audio VCOs and a separate LFO.

The fixed filter bank is an interesting thing. You don’t find one often on modern synths, though they’re available in Eurorack and other formats. But it was there on most, I think, of the old Moog modulars. So let’s have one here.

The CP15 is a full-width panel with the power switch and several utilities. Instead I’ll have several utility modules.

The 1973 Model 15 mysteriously lacked a MIDI interface, possibly something to do with the fact MIDI didn’t exist for about another decade. I’ll have a MIDI to CV module here, and for the keyboard I can use one of my MIDI ones or the Keystep if I want CV directly for whatever reason. I currently don’t use MIDI much. I haven’t connected my modular to my computer at all. So far I just use it for the keyboard, so my needs are simple.

A MIDI controller can supply a clock signal which the MIDI-CV module can put on a jack. But for MIDI-less operation a simple clock module would be good to have.

I’m not clear on how one typically got signals out of a Moog modular though I know there were “trunk lines” jacks that were connected to a distribution panel on the back of the synth and that I suppose was the usual way. This synth will have an output module.

Putting it all together the modules list is:

  • (1) Attenuators (several in one module)
  • (1) Clock
  • (1) Fixed filter bank
  • (2) EG
  • (1) LFO
  • (1) MIDI/CV
  • (1) Mixer
  • (1) Multiples (several in one module)
  • (1)Noise
  • (1) Output
  • (1) Dual VCA
  • (1) VCF
  • (2) VCO

But which ones?


This is easy. Three passive attenuators: three pots, six jacks, some wire, front panel, boom.


Not decided yet. I’ll probably design something. I could just use a Gate Grinder but that’s 100 mm wide and I’m not sure I use its full functionality often enough to justify having one in each case right now. I’ve drawn up a 25 mm front panel that’ll be enough for a simple clock.

Fixed filter bank

I like the YuSynth FFB, except that Serge argued against octave-based formant filters and that makes sense to me. So… likely I’ll build the YuSynth, but with capacitor values changed for non octave frequency ratios. The amount of panel wiring involved is massive, so I expect I’ll design a panel PCB to plug into the pin sockets already provided for.


I spent a long time dithering over this, and the LMNC ADSR or some other EVGEN8 based design tempted me, but I finally chose to go with the YuSynth design here too, at least for one module. One thing that was holding me back was a lack of retriggerability (yes, spell check, that’s a word if I say it is), and then I found this mod. There’s a couple other trivial changes I want to look into besides. I’ll come up with an auxiliary PCB for the retrigger mod, and while I’m at it I’ll put board mount controls on it. I’ll defer decision on what the second envelope generator will be, but it might just be another copy of the YuSynth.


This also took me a while, but I’ve settled on the MFOS VC LFO. I actually bought a PCB for one months ago, but I also bought the Eurorack front panel for it, and having done so I don’t want to toss it out. So I’ll someday get around to building it Eurorack, and I’ll build another for this synth.


I’m kind of surprised how few DIY MIDI/CV modules are out there. Befaco has one, but I think it’s only available as a full kit with Eurorack panel I’d have to buy not only the main PCB but the Eurorack front panel and the panel components PCB, which are useless to me. LMNC has one, but it’s too powerful! It offers a lot of functionality I currently would not use, and its 100 mm width would be wasted on me. There’s not much else, but there’s the Erica Synths one. I believe their DIY stuff is no longer in production but they and SynthCube have stock, and anyway they’ve open sourced the design. So I might just get the board fabbed, especially if I decide their board’s form factor doesn’t work well with my Kosmo panel layout.


Mixers are easy enough to bang out, but there are some nice features in the MFOS Quad DC Mixer. Of which a spacious, usable front panel isn’t one! But I’ll just put a single unit behind a Kosmo panel. If I end up wanting more mixers, I’ll make more modules. I have some modifications in mind, like a fourth input and diodes on the bias control to give it a dead zone near center. Of course this will have to be a new PCB for me to design and have made.


This I already have, the PCB and panel anyway: Cory Torpin’s passive and buffered multiples. This brings up a point. I’ve decided I want a uniform visual appearance in this synth. So I’ll use Cory’s front panel, but I’ve designed an overlay for it to make it match, kind of, my other modules.


MFOS again, the Noise Cornucopia. There’s a modification to the noise source by Eddy Bergman that supposedly improves it. I’ll try that out; I can kludge it onto the MFOS PCB, I think.


I have this PCB too, the Stereo Out by Barton. There are a lot of panel components including five dual gang pots which are board mounted in his design, but they’re too close together for Kosmo. Wiring six mono jacks, one stereo jack, four single gang pots, and five dual gang pots on a panel that size doesn’t look like my idea of fun, so I think I’ll work up a panel PCB for this one too.


And another MFOS. Pretty much unmodified, this time.


I have the PCB for a Barton Sallen-Key VCF. Again there are board mounted pots too close together, but this time I think I can use the PCB with the first and third pots mounted, moving the middle pot to the bottom and panel mounting it.


I’ve shied away from circuits using specialized synthesis chips, for whatever reason, but I’m attracted to a 3340-based VCO. LMNC has one, and so does Kassutronics, and so do lots of people, but those two seem the most accessible to a DIY build. And I have problems with both of them. In a nutshell I like how Kassutronics handles the outputs but not their version of the controls and inputs, and vice versa for the LMNC. So I’m working on a hybrid design. As with the EG, I’m holding off on deciding what to do for the second one, though maybe by the time I build one I’ll have decided to build two.

So that’s the plan thus far. All subject to change of course. I’ve been drawing up front panels and it’s looking like this. Still room for modules beyond the Model 15 set.

2 thoughts on “Case two part two

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