Three suggested systems
Newcomers who are just getting their feet wet with modular synthesis often ask for summaries of how to build a starter system. There are a few things to understand about that.
Everybody's needs are different, and customization is a big part of the point of having a modular system. If you want just someone else's idea of what should go into a synthesizer, and you want to play synthesizer music as soon as possible without needing to think about how the machine works, you can get that more cheaply and easily by buying a ready-made non-modular synthesizer, which could be analog or digital according to taste. The only reason to pay the higher price tag and do the extra work with modular gear would be if you wanted to make your own decisions and not have it forced upon you by the designer. So it may be counterproductive after deciding you want to buy a modular, to then turn around and ask someone else to choose your modules. It's certainly not reasonable to expect anyone to produce a single golden recommendation of what the best starter system should look like for everyone, with the best of everything and suitable for every beginner.
This is also why I don't endorse the suggestion sometimes heard that new hobbyists ought to buy "semi-modular" gear "just to start" (i.e. a single large module combining the functions of a complete small modular synthsizer). That's often the worst of both worlds. The rest of this article is written on the assumption that we really are talking about a modular system as such; I've already written elsewhere about why you ought to buy something else instead of a modular system at all.
There are no such things as "beginner" modules and "expert" modules. Beginners and experts use the same modules. At most we might say that a beginner probably hasn't spent as much money yet, and so a beginner probably has fewer modules. But your skill level doesn't determine which modules you use. The idea sometimes floated that someone might buy beginner modules and then later replace them with expert modules, is an illusion: when mid-level hobbyists sell modules it's usually just because they want the money to buy other modules, not because they've outgrown their first modules in terms of skill level.
So with all that in mind, despite the foolishness of suggesting what someone else's modular system should look like, I'm going to do that anyway. Here are three suggestions on how to build a complete system. They all feature North Coast Synthesis modules, for obvious reasons. Some day, if the company survives long enough, I'd like to have a wide enough product line to offer a complete system made entirely of my own modules, but until then, it's also necessary to have some from other manufacturers, so I'm describing a selection of those modules as well. Remember, these are only suggestions. Use all of the suggested modules; use some of them; or use none of them. A big part of the fun of modular is making your own choices about which modules to include in your own system.
Prices described here are approximate, for estimating purposes only, and based on buying the modules new.
A small analog instrument
This system is intended as a manually-controlled single-voice instrument, playable with a keyboard or other MIDI controller. As shown it'll have a classic analog synth sound.
MIDI interface: The basic suggestion is the Doepfer A-190-2 Low-cost MIDI Interface (US$125). This requires a standard DIN MIDI connection, such as from a typical MIDI keyboard. If you want to control it from a computer instead, you could swap it out for something like the Doepfer A-190-3, which features a USB port. An upgrade, especially if expanding the rest of the system to run more than one simultaneous voice, would be to switch to the Mutable Instruments Yarns, which is a deluxe MIDI interface with sequencing features (US$360).
ADSR envelope generators: To play complete notes as a lead instrument you really need full ADSR envelopes; don't be fooled by "AD" and "AR" multi-envelope modules which are only at their best when making percussion sounds. I'm suggesting my own product, the MSK 012 Transistor ADSR, which is a nice basic envelope generator with a good sound to it (US$160). I suggest two, because being able to control amplitude and filter cutoff with separate envelopes really expands the sonic possibilities a lot, and you need two to get what people imagine as the classic analog-synth sound. But if trying heard to economize you could cut back to just one ADSR envelope. Even fancier envelope generators also exist, including ADSRs with voltage control over the steps, and other more complicated envelope shapes, but in a small system like this one I think going past two basic ADSRs would be overkill.
Multiple module: Even in a small system it's necessary to have a way of sending one output to more than one input. I have shown an Intellijel buffered multiple (US$75) in the picture but it would be reasonable to swap it out for some other manufacturer's, or to use an unbuffered "passive" multiple and save a few dollars.
Oscillator: I'm suggesting an Intellijel Dixie 2+ (US$230). This is a workhorse analog VCO with the features people typically want, from a good Canadian company that I like to support. The most obvious way to upgrade this would be to switch to a dual analog oscillator, like the North Coast Synthesis Middle Path (US$500). You could also just get a second Dixie 2+. Either way, you'd be well on your way to a two-voice synthesizer, or could just build a more elaborate sound in a single voice. But depending on the kind of sound you want to achieve, another idea would be to swap the analog Dixie for a digital oscillator like the Klavis Twin Waves (US$250) or Mutable Instruments Braids (discontinued by the original manufacturer, but probably available on the used market or in someone else's version based on the published plans).
LFO: Adding modulation to the voltage-controlled parameters of oscillator and filter modules really makes the sound come alive, and you need a low-frequency oscillator for that. I'm suggesting my own MSK 010 Fixed Sine Bank (US$205) as an LFO module; with eight independent outputs it can keep many things moving at once.
CV processing: It's nice to be able to adjust the levels of control voltages, as well as to inject an added voltage at certain points. I'm suggesting one Doepfer A-183-2 Offset/Attenuator module (US$50), for tasks like scaling the LFO outputs to apply to other modules. This module may not be absolutely necessary in such a small system, but it will become more important as the system grows and is nice to have right from the start. The mixer I'm suggesting (see below) does some of the same things - but it's a shame to tie up the mixer just for CV processing when you might want it for other things. I suggested the Doepfer unit as easy to find, but other manufacturers make similar things; the Manhatten Analog Control Voltage Processor (US$85) is one with a built-in slew rate limiter, which can be a nice feature to have. The North Coast Synthesis MSK 008 Dual VC Octave Switch (US$205) is another CV processor worth mentioning, but it does not directly replace the scaling and offset features of these modules.
VCA: Some form of voltage-controlled amplifier is really necessary for any music that consists of separated notes; otherwise there's no way to make the oscillator go silent in between the notes. I picked the Doepfer A-132-1 (US$80) as the bare minimum for this system, but it's easy to find other options which cost more money and space and provide a variety of extra features.
Filter: The timbre of a typical analog synth is defined primarily by the filter, not by the oscillator. Consider a saxophone as an analogy: although you do need a decent reed (original source of sound, oscillator) to play the sax properly, the body of the instrument (sound-shaping component, filter) is what primarily defines the tone. I'm suggesting my own North Coast MSK 009 Coiler VCF (US$205); it has a typical analog-synth sound with some unique character of its own, and it has three different outputs and two inputs to offer a lot of sonic options for a small system.
Mixer: A basic modular four-knob mixer is nice to have. Depending on the patch, it could be used to combine control signals from multiple sources for more complex modulation, or to build up an audio signal from multiple sources - for instance, two outputs of the filter. I like the North Coast MSK 011 Transistor Mixer (US$160), which can optionally add distortion at some knob settings.
Modulargrid's estimated price for the modules alone to build this system is about US$1500. It's reasonable to expect that a case, power supply, patch cables, and some kind of MIDI controller to plug into the A-190-2 will add a few hundred more, bringing the total price tag for the complete, ready to play system to just under US$2000. These modules fill 58HP of horizontal space; I've drawn them in a standard 84HP-width rack row, with 26HP of space for future expansion.
The drone machine
"I want to make drone music" is what people say when they mean "I was traumatized by childhood piano lessons and want nothing to do with a synthesizer that has a keyboard." But, never mind, it is a common request, and modular synths do lend themselves well to drone music. Here's a suggested system for that. It was tough to fit inside one 84HP row, so I'm also offering some suggestions on expanding it futher, but even this one row can make a lot of interesting textures.
Intellijel Shapeshifter (US$530): This is a digital multi-oscillator with a wide range of sounds all by itself. Other good options might include the North Coast Middle Path (a little smaller and cheaper at 24HP and US$500, but analog instead of digital); the Klavis Twin Waves (US$250) as a smaller and cheaper dual digital oscillator; or any basic oscillator if you're prepared to get two or three of them and a mixer.
Make Noise Erbe-Verb (US$489): Although called a "reverb," it's understood that this is not intended to accurately simulate room acoustics. It's more of an abstracted reverb that puts its own stamp on the sound. I think some kind of reverb or effects unit is a good idea for drones, but there are many other options including multi-effect DSP units like the Erica Black Hole DSP2 (US$350); electromechanical spring reverbs like the Intellijel Springray II (US$240); and even the various successors to the now-discontinued Mutable Instruments Clouds, for granular audio processing.
MSK 010 Fixed Sine Bank (US$210): Drones have to keep moving or they'll become boring, and especially with modules like the Shapeshifter or Erbe-Verb in the mix, each of which has many modulation inputs, it's important to have many independent modulation sources. It's also important, and often overlooked, to have smoothly varying waveforms; any sharp corners in the LFO signal will produce audible transients in the drone. That makes the triangle waveforms of most variable-frequency analog LFOs less than idea for modulating drone patches. My multi-oscillator is especially intended for racks like this one that require many smooth modulations at once.
MSK 007 Leapfrog VCF (US$400): The very sharp frequency response and controllable resonance of this filter gives it a smooth rounded sound that's nice for shaping the spectrum of a drone; and it works well as a sine oscillator too, further supporting whatever sound you're getting from the Shapeshifter.
MSK 011 Transistor Mixer (US$160): Useful for mixing the outputs of the other modules into a final audio signal, or for utility mixing of modulation voltages, depending on the patch.
MSK 008 Dual VC Octave Switch (US$205): Although some drones might include octave switching as such (or switches of other intervals, by attenuating the output), I also imagine using this module in this system to do mid-side decoding for stereo: create two differently-processed versions of a similar drone signal using the rest of the system, and then feed them into the "octave switch" to create a stereo signal with the different frequency and phase components spread between left and right.
Those modules just fill the 84HP row. If I were suggesting a bigger system for drones, I might suggest adding a multiple for general use; a noise source like the SSF Quantum Rainbow 2 (US$150); one or two more oscillators and effects units; a multi-CV sequencing/control module like the Mutable Instruments Frames (US$360); or a matrix mixer like the Doepfer A-138m (US$200).
The Modulargrid price estimate for the modules shown is about US$2000; add another US$300 or so for the case and power supply to make it playable.
Many people want to build rhythm tracks with a modular synth, and here's a one-row system suggestion for that.
Clock and control: I'm suggesting the ALM Busy Circuits "Pamela's New Workout" (US$275) as a clock source and pattern generator. Even by itself it can generate interesting patterns, but in the picture I've also added the Intellijel Steppy (US$240), which allows programming more elaborate sequences. Another option would be to replace these modules with a MIDI interface and control the synth from a computer or other MIDI control device. The TipTop Trigger Riot (US$500) (billed as a "groove composer") is also worth a look as a sequencer/controller for percussion patterns. Going in a different direction, the Mutable Instruments Grids (US$230) is a sort of self-controlled drum pattern generator: it contains many patterns built in, and morphs between them under voltage control.
Envelopes: I usually don't recommend simplified multi-envelopes (less than ADSR) like the Doepfer A-142-4 (US$150) quad envelope shown here, but this is the kind of rack where it makes sense to use such a module. As well as controlling amplitude envelopes, the outputs from this module can be applied to the input of a resonating filter like the Leapfrog to "ping" it for an organic percussion sound.
LFO: Because this synth rack is intended to play several independent percussion voices at once, there's a need to modulate them independently, and the North Coast MSK 010 Fixed Sine Bank (US$205) is great for that.
Noise source: Cymbals, snares, and related sounds benefit from having a noise source, and the SSF Quantum Rainbow 2 (US$150) with its multiple outputs can be the noise source for several different drum sounds at once in a single patch.
CV processing: The switches on the MSK 008 Dual VC Octave Switch (US$205) are good for performance control of parameters like filter cutoff, to change the sound in a coordinated way at a flip of the switch. It's also useful as a general CV processor, for instance to mix modulating signals for a more complicated sound.
Filters: I've suggested one of each of the North Coast Synthesis filters here (Leapfrog VCF, US$400; Coiler VCF, US$205). The Leapfrog, in particular, is good for "pinging" to create a drum sound; the Coiler would more likely be used with noise input from the Quantum Rainbow for wider-spectrum percussion sounds. In a rack like this one that's intended to be multi-voice, it's nice to have multiple independent filters.
VCA/mixer: The Intellijel Linix (US$330) combines several VCAs and a mixer; it's not the smallest way to get multiple VCA functions in general, but as a combined module it would work well in a multi-voice percussion synth like this one. Individual channels can be patched out for general-purpose VCA use while the main output is still functioning as the final mixer.
The Modulargrid estimate for the modules shown in this row comes to just over US$2300; add another US$300 or so for case and power supply to make it playable.