Modular (mono) FM synth build

I’ve never built a modular, and the options are paralysing. So I’m asking for learned opinions in selecting a module (x6) that has both LFO capability up to audio rates, and a VCO.
Intention is to patch these in the usual FM modulator/carrier algorithmic ways, have two or three Envelope modules, generic Filter and Amp. If anyone has experience in modular FM, I’d love to hear your advice and experiences.

I’d been visiting old DX7’s, newer OpSix’s etc for over a year now, but wasn’t clicking with anything, particularly in respect to the hardware. So I decided to look into building a simple 4 to 6 operator modular rack, which would fulfill my simple “mono” FM synth needs.

edit: budget ~US$1,500

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This could get really pricey really quickly. If you want pure VCOs, check out Intellijel’s Rubicon 2 and Dixie II+. You could get that set times 3 I suppose? Really pricey though. Or just a bunch of Dixies. The Rubicon has thru-zero, linear, and exponential FM. I think Perfect Circuit has a video on YouTube where they rack up four Rubicons and a matrix mixer. I’m not sure Modular is the best approach unless you have a lot of funds.


Id recommend a test build in VCV or mirack first.
Build the system, see if it does what you want. Then price it up.

As said, it will get mega expensive very quickly.

Cant say enough good things about mirack, the sound quality is impressive.

(Ive just built my first semi modular synth, good fun)


I can’t say, money is no issue, but funds (and time) are available.
I’m not looking for high end boutique modules either, more so utility grade. Could end up being a failed ‘proof of concept’.

Thanks for the pointers. Searching now :+1:

Yeah, you’re not going to be able to build something useable in any modular format for under $1500, even if you DIY a lot of it. You need to think about a case/power as well as modules. Also - and keep in mind I’m not an expert - you will need some VCA modules in addition to the envelopes to control the amplitude of each “operator”. That’s an additional 3-6 modules you are adding, unless you plan on buying all-in-one Env/VCA combo modules.

If money is no object, I’d go with 6X Dixie II for oscillators, 3X Doepfer A-142-2 Dual Envelope Controlled VCAs, and shop around for a filter that you like. As many people say, filters are a very personal choice. You’re getting up around $2K with that setup.

Good luck!


I built what’s primarily used as an FM mono synth with Dixie II+, Generate 3 and Filter 8 and I enjoy it but I think Akemie’s Castle might be worth checking out


Oh I almost forgot, you are gong to need a bunch of mixers in order to generate different “algorhythms” or combinations of operators. Add on another few hundred $ for those.
And patch cables. And some mults. And house plants.


Or Humble Audio Quad Operator and a nice filter. OP seems to want 6OP though…

This will not end well.

You should find someone who has a couple of oscillators that have FM inputs, and patch with them for a bit. It’s quite possible you won’t like it at all.


Consider the Lyra-8. I’m not sure its modulations are actually FM, and I’m pretty sure it’s oscillators are more square than sine, but I find that it feels a lot like an analog FM machine. Fixed algorithm, but analog controls still deliver infinite variability.

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I’d like to ask some more fundamental questions - what aspect of existing FM hardware were you not clicking with and why do you consider this to be a feasible solution? Why does it need to be in the analog domain? Why 6 operators? Have you played around with a similar setup?

I do enjoy playing around with analog linear FM, but 6 analog ops is a total mess for anything other than chaotic feedback networks unless you start thinking about implementing proper CV control and a patching matrix, at which point it gets too expensive to be worthwhile. You might gain some tactile controls over a DX7 but it’s going to take you a lot longer to tune ops, dial in proper ratios, scale values, switch algorithms or do anything else that you’d take for granted with a standard DX style FM synth.

You’re probably better off starting with a dedicated Buchla-style complex oscillator like a DPO or one of the new Tiptop Buchla oscillators and seeing how much you can get out of it before scaling up or even looking at a couple of semi-modulars that have linear FM built in. The Minibrute 2s is super cheap and can do quite nice 2-op linear FM via the patch panel, and you can use waveshaping to get more complex timbres that you’d expect from a higher operator count in the digital domain.


Yeh should mention that a mate has offered me a powered rack (free) with two multiples in it, and my budget commitment would probably stop around US$1,500

Eurghhh… looks like all flavours of headache.

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What size is the rack you’ll be using?

I’d echo the complex oscillator recommendation if you want a more structured approach to FM. I absolutely love playing with the Make Noise DPO. A complex oscillator would also take care of some of the extra utilities you’d need if you were patching two separate oscillators into each other (VCAs, etc.).

You might also consider getting something like the 0-Coast or another semi-modular to take care of the bunch of things at once, and then use your rack space to add complementary pieces as you get more experience and figure out what you like.

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Don’t attempt anything more than 2-op analog FM.

For that budget, get two of the new Bastl Pizza digital FM oscillators (each one has a main, ratio, and octave oscillator) and some support modules (envelopes, LFOs, VCAs, mixers).


I really like the Korg opsix, but It’s not holding my attention.
Modularly, 4 ops would be enough, 6 to replicate existing tones I’m after. Why modular? Comfort I suppose, working with knobs, the usual.
Essentially, it’s in my nature to experiment with novel solutions, and if this produces interesting versions of the tones I’m after, then the effort is totally worth it.

I would echo the 2op plus wave shaping/ wave folding direction. You’ll get a lot of mileage out of that with a small collection of utilities.

What are the tones you’re after?

Initially, DX7 Bass.
Then, searing moans of large metal structures falling into black holes. The usual really.

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Again, I think analog FM is cool and worthwhile, but you are not going to replicate existing DX style FM tones in the analog domain, even if you’re using linear FM. It responds in a very different manner. It’s a lot easier to get the Opsix to sound (huge scare quotes here) “modular” than it is to get a modular to sound like a 6-op FM synth. Maybe some anorak in here will come out of the woodwork to tell me they’ve built one and it works, who knows.

If you’re after novel tones, start out with a complex osc, but don’t expect to build a stable 6-operator analog FM voice that approximates what you’re used to in the digital domain.