I wrote up the basic specifications for my DIY modular synthesizer, and now it’s time to begin selecting the circuits. I’ve decided to select the components roughly as per the signal flow; so oscillators first!

My specifications were very basic:

• 2 x VCOs and preferably different types
• 1 x sub oscillator

But in reality there’s a lot more to it. Types of waveforms, stability, features such as hard sync, linear and exponential FM (frequency modulation), PWM (pulse-width-modulation), CV (control voltage) and audio connectivity etc.

I posted a shortlist of 9 oscillators on Facebook, and was given some great suggestions to add to that. So here’s the list, specs and my thoughts.

Disclaimer: I’m going for musicality over experimentation. I always like to start with some pure raw material and then apply the amount of grit and craziness I want. So for comparison here’s a video of the WMD / SSF Spectrum VCO which I bought. The video clearly demonstrates that this one fat oscillator (by subjective listening terms). Waveforms are crips and buzzy, and the subs are well presented. So this is my benchmark.

Also, just because I take products off my shopping list, doesn’t mean I don’t think they are any good. What I’m blogging here is more of a buyers thought process rather than a technical comparison, which is difficult unless you have the modules side by side.

Mutated: Braids

In terms of flexibility, nothing beats a digital oscillator with wavetables and other goodies. It’s fantastic that the Mutable Instruments Braids is available as a DIY kit, with Mouser shopping cart ready to purchase for parts. It can replicate different waveforms, do classic emulation, crazy FM, drums etc. Given Mutable Instrument’s reputation, the sound quality and sheer number of sounds the Braids can produce, it’s going to be hard not to include this.

Manufacturer link (original module not DIY “Mutated” version): http://mutable-instruments.net/modules/braids

Mutated: Elements

The Elements was also considered. It’s a more experimental module using modal synthesis. I’m going to cross it off the list right now, as it’s too experimental for me, costs more than the Braids and takes up more space. I also only want one digital oscillator for this project. For those into noise, soundscapes and interesting percussive sounds I’d recommend looking further into it.

Manufacturer link (original module not DIY “Mutated” version): http://mutable-instruments.net/modules/elements

NLC CEM3340 VCO PCB

This is a tricky one. I love the idea of having a CEM3340 based VCO, the chip used in some of my favourite polyphonic synthesizers such as the Oberheim OB-Xa. One potential downside is that you need an actual CEM3340 chip, which go for around AU$60 on eBay, although there may be some cheaper re-issues floating around. The killer for me is I cannot easily find a good demo, so that crosses this off this list as well.

Manufacturer link: http://nonlinearcircuits.blogspot.com.au/

Fonitronik TH 555-VCO

I had some good feedback on the Fonitronik offerings via Facebook. This particular design is based on Thomas Henry’s design using the common 555 timer, typically not used in many designs but common for drone circuits such as the Atari Punk Consoles. Again there’s not that many videos showing it’s capabilities, however Fonitronik have posted the one below.

My impressions are that it sounds quite decent, without having an extended low end like a Spectrum. Actually running it through a frequency analyser the 555-VCO was getting down around 47Hz, and the Spectrum just below and down to half of that with the sub. Now obviously this depends on what note you hitting and it may just be the demo isn’t adequate, but where it’s clear the Spectrum has an amazing low end capability it’s not as apparent for the 555. For reference I use Focal CMS 65 monitors with IK Multimedia ARCII room correction system, and this combo gives me an excellent low end response which I’ve been missing in the past (on a side note I’d highly recommend the ARCII after some acoustic treatment).

However, for a cheap oscillator you get a nice sounding sync, linear and exponential FM, pulse with PWM, triangle, saw and sine waves, it pretty much has all the basics covered. This makes it to my final round of selection.

Manufacturer link: N/A

Fonitronik TH X-4046 VCO

I cannot find a clean demo of this, but it sounds quite nice and beefy in this video, albeit though a filter and LPG (low-pass gate).

Feature wise it’s similar to the 555-VCO, although has a useful LFO mode and a “rampoid” wave with skew which varies between a ramp and triangle which is quite nice. Given the extra feature I’d say that this gets slight favour over the 555, and they both make the short list.

Manufacturer link: N/A

Haible Living VCO

I must say that there’s no issue regarding the sub bass on these oscillators! The Soundcloud demo below nicely demonstrates the range. The Living VCO is essentially three identical oscillators, with a single VCA and other features including a portamento and vibrato which is nice. There’s also a separate mixer board.

In terms of complexity it’s more complicated build than some of the other VCOs, and is a riskier build considering you’re building and testing 4 modules at once and there’s a massive parts count (although the SMD caps come flow soldered in!). But I do like the features on this and on top of the standard PWM and Sync you have linear detune, and the VCA can be driven.

The only downside here apart from complexity actually the number of oscillators, which is really project specific and not bad at all. Ideally I’d have 3 different oscillators, one digital and two analogue. So if I was going to choose this and have four oscillators, it would be one digital and three identical analog ones. This is still a good option given I get one VCA too. Also, one of my optional requirements was to run a couple voices independently so having a fourth oscillator would provide some nice 3 and 1 or 2 and 2 options. This makes the short list!

Manufacturer link: http://randomsource.net/haible/living_vcos

Befaco Even VCO

After a little searching I found the following video of the Spanish Befaco Even VCO. My impressions were that it sounds quite decent, and it had no issues produce bass well below 47Hz. OK so at this point I must admit I’m addicted to bass! My pursuit of a really deep fat sound was the reason I got into Eurorack modular, so I could select all the components of my desire.

There’s really nothing special feature wise about the Even VCO, but there’s nothing wrong with it either and it sounds quite good, although there’s some ringing noise on the sine and triangle outputs and it’s impossible to know what’s causing this. A quick read of a Muff Wiggler forum thread regarding opinions on the module has alleviated some concern. Also, all SMD components are pre-soldered which is handy.

So… I’m going to cross this of the list. If there was a cleaner demo it would’ve made the shortlist, but given the features are pretty standard and some doubt about the tuning (from reading more threads), it’s just fallen short of the mark.

Manufacturer link: https://www.befaco.org/en/even-vco/

Erica Syncable VCO

I was told to stay from this module from someone via Facebook that’s built a heap of Erica products (and recommends the others). The feedback was that it’s a convoluted build, there’s no sine wave output, no linear FM and no precision voltage reference, so it would detune if there’s noise on the power rails. Given the other options I’ll cross this one off early.

Manufacturer link: http://www.ericasynths.lv/en/shop/diy/diy-kits-1/syncable-vco/

Erica Polivoks VCO

I like the Polivoks range, although this VCO also gets knocked of the list early. Why? It doesn’t have individual waveform outputs, which for me is a mandatory feature for a VCO. I quite often use two at a time, and it’s handy having the option to use the others as modulation sources.

Manufacturer link: http://www.ericasynths.lv/en/shop/diy/diy-kits-1/diy-polivoks-vco-kit/

GMSN! Pure VCO

From the demo this doesn’t sound particularly good to my ears, so this gets scrapped at the first hurdle. It would’ve been great if there had been more and better demos.

Manufacturer link: https://au.gmsn.co.uk/collections/synth-diy-kits/products/gmsn-pure-vco-smd-pcb-panel

Neutron Sound Orgone Accumulator

This oscillator was recommended via Facebook as is a “unique complex digital oscillator built on a PJRC teensy 3.1 ARM processor”. It’s open source with all the code on GitHub which is pretty cool. I must say I was surprised by the sound of this thing, not only the deep bass but the different textures available. Also, congratulations to them on a wonderful demonstration, including the kick drums around the 11 minutes mark. At $65 for the PCB (and panel which I don’t need) it’s more expensive than most other modules here, but in terms of parts and build it’s through-hole not SMD and appears easier than the Mutable Braids.

Manufacturer link: http://neutron-sound.com/noa.html

MOTM-300 ULTRA VCO

The MOTM-300 is a VCO by Synthesis Technology, who make some fantastic products including the E440 filter which I own and love. It has a solid feature set including a couple of FM input. The audio demos on the link below however are pretty poor MP3s and don’t really demonstrate too much, and there’s no good YouTube demos either. Off the back of the E440 I’d almost buy this deaf, as I can trust that the quality would be good. The build and parts procurement doesn’t seem like either the easiest or too complex. I’m going to shortlist this for now to keep my options for analogue VCOs open.

Manufacturer link: http://synthtech.com/motm/300/

Frequency Central System X Oscillator DIY

This System X Oscillator from Frequency Central is based on the old Roland 100M. Looking at the PCB it’s a pretty easy build which is handy, but how does it sound? Quite decent I think, it Extends well in the bass without sounding the brightest (video quality perhaps?), but has some nice tones. It has a pretty standard feature set but no sine output. I’m going to shortlist this.

Manufacturer link: http://www.frequencycentral.co.uk/?page_id=249

The shortlist

After all the searching and playing through videos, looking at build guides etc, it’s time to choose my oscillators! This may be subject to procurement but I can worry about that at the time of purchase.

So here’s the shortlist:

• Mutated Braids digital oscillator
• Neutron Sound Orgone Accumulator digital oscillator
• Fonitronik TH 555-VCO
• Fonitronik TH X-4046 VCO
• Haible Living VCO (3 x VCOs and 1 x VCA)
• MOTM-300 ULTRA VCO
• Frequency Central System X Oscillator DIY

None of these have a dedicated sub output which is disappointing, although I can probably getting away with tuning an oscillator down an octave anyway.

Analogue oscillators (VCOs)

My favourite in terms of the sound is the Haible Living VCOs, although the complexity of the build and size are against it. Out of the standard modules, I prefer the X-4046 over the 555-VCO and System X VCO due to flexibility. The MOTM-300 I’ll pass on due to lack of good demos, although I wouldn’t mind picking one up in the future for my modular system. So it’s the Haible Living VCOs and / or the X-4046.

Digital oscillators

This was harder than I thought it would be. I really thought the Braids would be my favourite here, but something about Orgone Accumulator really appealed to me. The Braids has those simulations which are great but the build is more difficult and I preferred the sound of the Accumulator. Perhaps I should select both?

The final selection

Despite the time spent looking at these options, I still feel like I could spend many more hours comparing each of these modules. In the end, I picked the ones I liked the sound of! The winners are:

• Haible Living VCO (3 x VCOs and 1 x VCA)
• Neutron Sound Orgone Accumulator digital oscillator

Even though the Living VCO will be a tougher build, I’ll end up with 4 oscillators including the Accumulator and a VCA, so this gives me the option to design around two signal paths if feasible. Also, I think the silkiness of the Livings against the flexibility of the Accumulator will give a nice balance.

In the next post I’ll look at a second VCA, before I get onto the filters! I’ll also start looking into procurement (ideally I’d do one big parts order but I want to get building some modules before Christmas), and mocking up what the panel will look like. Till next time…