Kosmodrome Phase 2

Slightly revised 1 Dec 2021

It’s been over a year since I set out a plan for what I then called Kosmodrome, and after a while, Kosmodrome Phase 1, since after all, I wasn’t about to stop there. And over a year is well over how long I thought it would take me to do it, and well under how long it will take me: Phase 1, as planned (more or less, there were adjustments to the plan, nothing earth shaking) is nearly complete, but there are two modules left. There’s the second envelope generator, which probably will be a copy of the first one. I have a PCB and a panel and most if not all of the parts, so I could knock one out in a few hours, but I want to take a little more time with the first one before committing myself to another of the same. And then there’s the second VCO, which is not going to be a copy of the first one — unless I change my mind — but a new design, one I’ve got drawn up but still need to breadboard in its entirety before going ahead with building it. So that will be some time.

And then it’ll be done, and then what?

Never mind what, how about why

I suppose the first question is, does there need to be a then what? After all, I now have a nice full-featured synthesizer, a Gray Meanie. It has pretty much all of Kosmodrome’s functionality and more. Is there a point to continuing with Kosmodrome?

I think there is, and it’s at least in part this: Building Kosmodrome isn’t a means to get synth functionality. It’s an end in itself. I got started building synth modules in large part because it was fun, and designing and planning and testing and building are still something I enjoy a lot. The point of Kosmodrome is not “I want a synth”, it’s “I want to build a synth”. A personal, idiosyncratic synth.

So yes, let’s keep going.

What’s needed

Phase 1 was informed by the Moog Model 15. Do I look to the System 35 for Phase 2? Nah. The System 35 was pretty much more of the same of the modules in the Model 15. More oscillators, more VCAs, et cetera. I’d rather expand the breadth as well as the depth.

In fact, to some extent, Phase 2 is informed by the Gray Meanie — or, really, the ARP 2600 I started with circa 1975. Besides the VCOs, VCF, VCA, and envelope generators, that synth had stuff like a preamp, an envelope follower, a ring modulator, a sample and hold, a spring reverb. To this day I feel like an analog synth without those things is not quite complete.

What else? I have several modules I hope to build soon that aren’t in the Phase 1 plan, and some are prime candidates for a Phase 2 plan. Such as a joystick module, an attenuverters module. The VCO I’m planning is actually intended as part of a VCO system, and the other VCOs in that system are going to need to be built, so they’ll be in Phase 2 too. Other stuff in the works is more “exotic”, about which more later.

Some fairly basic things are still missing. I considered a clock module for Phase 1, decided to leave it out. It’s going to be in Phase 2. Utilities like voltage inverters, lag processors, CV to gate and trigger converters aren’t that exciting, but should be there. Sequencer? I have a sequencer, predating Kosmodrome and so far still ensconced in Kosmogenesis, but maybe it should graduate to Kosmodrome. Or maybe it should be repackaged as a stand alone object, not taking up 40 cm of Kosmodrome rails.

Arpeggiator? MIDI to gate? Precision adder? And, yes, there are VCAs, an LFO, some mixers and multiples, but more of them would be good.

Mostly pretty basic stuff, if some of it’s less basic than the Phase 1 modules. But what about the exotics? The off-the-beaten-path, slightly weird, or downright wacky stuff? I’ll be making some of them, too, and putting them in the case, but I think they should be on a whimsical basis, not part of a plan.

A plan

What is the plan, then? Right now Kosmodrome is two 75 cm (ish) rows. I’ve built (I think) 137.5 cm of modules (including 15 cm not in the Phase 1 plan) with another 15 cm remaining in the Phase 1 plan, plus another 10 cm imminent and 7.5 cm likely in the next couple months (the latter two not in the Phase 1 plan either). Total of all that, 170 cm. For those just tuning in, what I’ve built is

  1. Attenuators
  2. Audio mixer
  3. DC mixer
  4. Dual VCA
  5. EG
  6. Fixed filter bank
  7. Joystick
  8. LFO
  10. MIDI/CV
  11. Neuron/Diff Rect
  12. Noise
  13. Passive/buffered multiples
  14. Sallen-Key VCF
  15. Stereo output
  16. Transistor ladder filter
  17. VC wave shaper

with another VCO and another EG planned, plus forthcoming not-in-the-plan stuff I’ll reveal later.

The plan is to add another three 75 cm (ish) rows. Which, by the way, is a whole topic of its own. I have materials for a one-row expansion to the present case, an angled bottom row — I bought them before the weather got cold but haven’t used them yet, and now I don’t know when the garage workshop will be warm enough to use it comfortably again. The next two rows will be either a further expansion of the case or a separate one, and by that time I’ll have to make up my mind whether to relocate Kosmodrome to the basement, because I’m running out of room upstairs. The basement gets cold in winter, too, though not like the garage! But it has space.

Anyway, 225 more cm. Reserve a row’s worth, 75 cm, of that space for spur of the moment modules and devote the other 150 cm to a planned set. That 170 cm of existing and soon-to-be-existing stuff consists of 23 modules, averaging to 7.4 cm each. So 150 cm ought to be about 20 modules. But to be conservative, I’ll plan 15.

Here’s the list:

  1. Attenuverters
  2. Audio mixer
  3. *Clock
  4. *CV to gate/trigger
  5. DC mixer
  6. *Envelope follower
  7. *LFO (simpler, perhaps dual)
  8. *Multiple (precision buffered)
  9. *Ring modulator
  10. *Sample and hold
  11. *Secondary oscillator #1
  12. *Secondary oscillator #2
  13. *Spring reverb
  14. VCA (dual)
  15. *Voltage processor

Some of these are already designed and ready to be fabricated, or are simply copies of Phase 1 modules. The rest — the ones with the asterisks — need to be selected or designed. I’m working on that.

Phase 1 is drawing to an end. Phase 2 is on!

New module specifics (or slightly less vagues)


A clock doesn’t need to be more than a simple square wave oscillator, but a microcontroller-based module has some advantages. For one thing, an Arduino has a much more accurate clock than you can make with a 555 timer. For another, it lends itself well to providing arbitrary clock divisions. There are some Nano based clock module designs around, though none I really like as is, as well as some metronome designs that could be adapted.

CV to gate/trigger

Some envelope follower designs I’ve seen (see below) include circuitry to generate a gate and trigger along with the envelope. That’s useful, but too useful, I think, to limit to that one use. So I plan to relegate the CV to gate/trigger functionality to its own module. A simple comparator followed by a gate-to-trigger circuit.

Envelope follower

I’ve been looking into these. Again, I don’t know of any DIY ones on the market that I’m really attracted to, but a scratch design won’t be hard. The ARP 2600 design looks good, but I want to breadboard several before deciding. The module will also include a preamp section for mic and instrument inputs, with a preamp output so it can be used with other modules as well.

LFO (simpler, perhaps dual)

A second MFOS VCLFO would be overkill at this point, but your basic manually controlled LFO is something I could use one or two more of. Nothing I’ve found on the market has the versatility of the Kassutronics shape variable LFO, so I expect I’ll make something based on that.

Multiple (precision buffered)

Something I learned early on is, sometimes 100k isn’t enough for input impedance. Not if you have a V/oct source with 1k resistors outside the loop on the outputs. So a dead simple buffered multiple with 1M inputs and in-the-loop outputs is a useful thing to have.

Ring modulator

Who doesn’t love a ring modulator? I like the looks of the CGS Active Real Ring Modulator, old school diodes and transformers with op amp stages to boost the output, null bleed through, and give more options for inputs. But there isn’t much point in including the preamp section, since that functionality will be in the envelope follower.

Sample and hold

Barton has a dual LFO and sample and hold module. But the LFOs don’t do all I want, and the S&H, well, not sure from a quick look at the schematics it’s set up the way I’d prefer. Anyway, there’s a design from René Schmitz worth trying out.

Secondary oscillator

I’m keeping a lid on this for now.

Spring reverb

This is looking like the only module in the Phase 2 plan for which I’m leaning toward a commercial PCB, aside from a second dual MFOS VCA. I certainly don’t know enough about spring reverbs to be confident of designing anything, and there are a few commercial options that look reasonable. I think the Music Thing reverb is most likely the way I’ll go.

Voltage processor

A simple utility, but useful. I’m looking at the ARP 2600 voltage processor, with two inverting voltage source/mixers and a lag processor. The thing that bugs me about that is the lack of non inverting outputs, but those are easy enough to add.

And then?

Trouble is, even with Phase 1 incomplete, I’m considering post-Phase 2 possibilities. Like breaking away from the East Coast subtractive paradigm. Some complex oscillators and waveshapers, maybe? Something else?

And Kosmodrome, Phases 1 and 2, has no percussion modules. There should be some. There should be a whole percussion subsystem, a subsidiary project. KosmoDrum. Oh no, now it has a name.

Better get busy.

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