Zoxnoxious analog synth VCV Rack demos

Thought I might start a thread with of demos for the Zoxnoxious analog synth modules I’ve developed. These are VCV Rack controlled hardware components, with dedicated Rack modules controlling hardware “cards”. Cards developed to date include a 3340 VCO, 3372 VCF/Signal Processor, and a backplane/VCA board. The backplane holds up to six of the previously mentioned cards.

Composition-wise I’m pretty poor at this stuff, so I’m kicking it off by pulling in a MIDI file of the Voltron theme song. Why Voltron? An 80s cartoon of a robot fighters that were next to useless individually until they combined to form Voltron? My campy take on it is the developed boards are similar: a VCO in isolation isn’t all that useful. Combine with other components and you’ve got a working synth. Form Voltron!

Hoping some months down the road to get this to the point others can use it. To paraphrase a unix quote: it’s user friendly right now, it’s just choosy about who its friends are.


Wow, that’s really impressive! You are a few short steps away from controlling an entire industrial automation production line with a VCV Rack patch :rofl:.


Why not set the bar high? just a few steps from there to world domination with a VCV Rack patch LOL

Reminds me of a scene from Babylon 5:

Londo Mollari (laughing sarcastically): While you’re at it, why don’t you eliminate the entire Narn homeworld?

Mr. Morden (dead serious): One thing at a time, Ambassador. One thing at a time.

Well, that’s a next-level hybrid setup. Guessing the advantage over Eurorack with something like Expert Sleepers and using this is the control you can have over the hardware using a dedicated VCV module?

The big thing here is, as you say, you’ve got full control over the hardware with Rack. No patch cords either. Loading a new patch is as you would do in Rack, just load a new file. Analog VCOs and VCFs configured. I played with a ton of different hard sync configurations yesterday. The 3340 has some interesting sync options. Doing that with Eurorack would be a pain, repatching everything left & right. And having the modules to actually do the control- trivial in VCV Rack with software. Which is good & bad, I mean it’s all about making happy mistakes, right? Here the happy mistakes can be recalled via saved files, undo, and everything you with VCV Rack. Fun stuff!

I’ve yet to start doing more stuff that would just be annoyingly tough to do with real hardware. I’ve got some ideas there but it’s not high on the priority list just yet. Since it’s all software controlled there are some fun options to play with.

This is an interesting idea.

I have some ‘exotic’ Eurorack sound sources, but it is the simple stuff that often sounds the best, and it is a toss-up between my 258T and Electrosmith 3340 as to which is the winner in my rack.

Hope you manage to get this to a release stage. Following!


Here’s another demo, just an arpeggio that I recorded a month back. Unfortunately this is about as complex as I get composition-wise. Still got a long ways to go. Audio is three VCOs, two going through a filter. I really like clocked for bringing in a variation every so many bars, that helps a lot. The resonance goes way too heavy about halfway through but whatevz.

Edit: sheesh, I threw the wrong video in there. Adding the correct one below that actually uses Clocked and Count Modula’s Super Arpeggiator.


Putting this short vid together as I’m considering doing a tech video on the 3340 VCO’s sync features. It’s kind of interesting in that hard sync doesn’t actually reset the oscillator. It’s not the most sonically pleasing video, just playing around with what the datasheet calls “positive hard sync” and “negative hard sync” and the phase between the two.

Sometime in the next month hopefully do something a bit more interesting than this with some sync’d VCO sweeps and scope shots / tech dives of the same.

Video shows the sync pulses on the upper trace and the waveform output on the bottom trace. Note a positive pulse only reverses a rising triangle, and a negative pulse only reverses a falling triangle.

Added a video demo of the AS3372 analog signal processor, and the VCV Rack interface for it. The 3372 is Alfa Rpar’s clone of the Curtis CEM3372, notable for being the filter in some classic polysynths: Oberheim Xpander, Sequential Prophet 600 & T8, Chroma Polaris and others. Alfa Rpar’s design added to the CEM3372 with a voltage controlled panning block. So in addition to the 4-pole analog filter you’ve got 6 (six?!) VCAs onboard. Fun!

Thought this might be interesting to share: analog 3340 VCO being manipulated with a series of sync pulses. The phase between the positive and negative sync pulses are modulated very slightly by an LFO. I won’t go into details as it dives pretty deep on how the 3340 handles sync. Needless to say it produces a pretty interesting effect. On the scope it looks somewhat like PWM on the saw wave.

Having too much fun making little bleepy bloopy things with this setup of just a few hardware oscillators and a filter. A bit of a wierd drone thing is this; Rack-wise it uses an LFO and sequencer to tweak pulse width on a VCO. Hardware-wise it’s got two analog VCOs and a VCF. The analog oscillators are at about the same pitch, with one of them modulating the filter. The filter output is routed back to PWM the VCO, creating a bit of a feedback loop since that VCO is modulating the filter.

Unrelated to the above and soon to be Rack-related, I’m wrapping up hardware design on a dual-VCO / VCF board. Once that board is designed it’ll need a VCV Rack module for a frontend. The board itself is an AS3394 (“synth-on-a-chip” as used in the Sequential Six Trak and many other synths) and an SSI2130 (modern VCO with thru-zero FM and other cool stuff). Should be quite the synth voice once it’s working.


Time to go back to circuit design 'cause this is about as good as it gets with me at the helm. Would love to see what an actual musician could do with this setup. All analog here: transistor noise source, VCOs, VCF, VCAs. That noise source uses a single transistor VCA which honestly kind of sucks. Pops every time it turns on, so take that as a win to use it to set tempo. LOL-- make your weakness your advantage!

Not so much a demo, but here’s what’s coming up module + hardware-wise. This is a combo is the AS3394 (“synth-on-a-chip”) and SSI2130 (modern synth VCO). The '3394 is a clone of the CEM3394, as used in the Sequential Six Trak and many other synths of the 80s. It’s a single VCO voice, with VCF and VCAs onboard. Being only a single VCO it’s a little thin. Hence the addition of the SSI2130. Introduced a few years back, the 2130 is a VCO with a lot of onboard features. Notably thru-zero FM, but it’s also got a mixer onboard for the pulse/saw/tri waves, a couple sync modes, and is really stable.

This is the most complex board I’ve done and uses two 8-channel DACs, some a number of switches for patching, and an AS3364 for additional VCAs. It will plug into the backplane in the same way as the other boards I’ve done (previous in this thread). One output is the synth voice (two VCOs going through the VCF) and the second output is just the SSI2130 VCO.

There will likely be a round or two or hardware debugging / revisions (fingers crossed not more than two!). While both the '2130 and '3394 are close to datasheet implementations, it’s by no means trivial getting it all wired up. Block diagram:

Some interesting features:

  • Thru Zero FM on the '2130. The AS3394, as modulator, can provide pulse/saw/tri waveforms. A secondary set of waveforms from the AS3394 are also available: sin, half sin, sin + half sin. Each go through independent VCAs for FM-depth so one could crossfade or mix between a primary and secondary wave. I see this as a cool feature.
  • Sync. The board offers soft sync at the modulation frequency. Hard sync is enabled at one suboctave (yeah, slightly odd). Each are independently switchable for soft sync/hard sync/soft + hard sync combinations. The combination gives an interesting mellow-hard-sync or harsh-soft-sync flavor. Either description fits.

Some other cool stuff as well. And it’s all analog audio path with analog modulation options. Digital control elements from VCV Rack. The good stuff. Next up is designing getting a layout for the above. UI isn’t exactly my forte, and with getting 2x VCOs, VCF, and modulation all presented it’s going to be a decent size layout.


So I lifted @henk.lasschuit 's patch from the One Voice thread. Ported to the Zoxnoxious analog synth with some rash modifications. The patch uses two sequences: so I patched each one to a separate VCO. Then made The Big Deal here to have the audio path VCO sync’d to a master VCO, master VCO not on the audio path. The audio path is from the sync’d VCO, through a VCF. What’s most interesting is the clock enables/disables sync and is also used to modulate the sync method on the Zoxnoxious 3340 VCO via Sync Phase. The 3340 has a positive and negative sync, which isn’t quite hard/soft sync despite that I labelled it as such on the module.

Apologies to Henk since this is quite an abuse of his original. His patch is pretty cool and you ought to checkout the original. It was a great starting point to showoff some of the sync functionality with the Zoxnoxioius '3340 module.


Just really went off the deep end getting noise out of this. Per the YouTube description, it’s “audio simulation of an exorcism of buzzing robotic bees”.

Synth geek patch info: it’s just a VCO and VCF/VCA. The VCO modulates filter cutoff & VCA panning. One channel of the VCF/VCA output modules the VCO pulse width, frequency, and provides a sync signal to the VCO. LFOs are there to vary the modulation depth.


Super great stuff! Congrats @brer_rabbit! Do you plan to make an open source DIY project of this with kits to build?

I’m trying to figure out the direction from here. The project is open source. Kicad files, Raspberry Pi app code, VCV Rack modules. All at GitHub - brer-rabbit/zoxnoxious: Analog synth / software interface . Some demo-y stuff on the YouTube channel. I’d like to get something into other people’s hands. I’m pretty sure anyone on this board has greater musical talent than myself, I’d like to see what someone can do with it. That said, there seems a very long road to getting there. My roadmap is something like:

  • Finish current SSI2130/AS3394 dual VCO/voice module
  • Rework AS3340 VCO. Some good updates coming there.
  • Redo backplane module so it’s actually a eurorack module
  • I just got a bunch of YM2203 Yamaha FM chips. A module with that would be cool.

That’s the hardware side of things. Software-wise I’d like to put a Rack module together that’d be a utility module for the Raspberry Pi. So you could actually do stuff like connect to a wifi point, get system updates, utility like stuff since the thing runs headless. Basically start to get the environment around it user friendly. And documentation. That’s sorely lacking. I wrote datasheet on the AS3340 module. I need something similar on the others and other docs.

But yeah- overall goal would be to get it in other people’s hands. It’s been well over 18 months of development getting this far. I’d like the project to reach some usable end state eventually!


Here’s a demo of the AS3394’s Waveselect modulation. Background: the 3394, a clone of the same chip used in many 1980s analog synthesizers, uses voltage on a single pin to select between tri, saw, and tri + saw waveforms. How about modulating this pin at audio rate? Here’s a demo of exactly that.

Interesting? Just noise? Curious to hear what you think.

This is one of the more unique (and possibly questionable!) functions of this board. It’s got some interesting functions that I plan to do videos of down the road. Definitely the most complicated board I’ve put together, this thing is a beast.


The “v0.2” version of this board is out to JLCPCB for prototype right now. Should be back within two weeks. I’m expecting this version to have the SSI2130 fully working. What’s that mean? A modern analog VCO with thru-zero FM, sync options, output mixer with VCAs, blah blah blah. That’s integrated with the AS3394 demo’d previously, the VCO/VCF/VCA chip. Combined it’ll be a pretty cool synth voice. And combined it’s way more complicated of a board than I’ve done previously. Geek numbers: 192 nets, 20 opamps, 16 DAC lines, 8 switches… this thing is a beast. Not a lot of free space on that 80x80 mm board.

Some demos coming up once it’s available. Til then, here’s some AS3394 synth fun: