The next big “Learning Modular Synthesis” project is Eurorack Expansion. The idea behind it is that you already have a semi-modular synth such as a Moog Mother-32 or perhaps a small modular system, and now you’re wondering what to add next. I’ll be covering a wide range of modules one at a time, demonstrating how they work, what they’re particularly good at, how you would go about interfacing them with your current system, and what new sonic and performance possibilities they would open up for you. The Eurorack Expansion course covers three different modules in each major “category” (such as oscillators, filters, et cetera), and then an ongoing web series of hopefully weekly movies covering additional modules.

But as the saying goes, you need to walk before you can run out and starting adding a second oscillator, more envelopes, an effects unit, et cetera. Therefore, for this series I’ve built up a core set of deceptively boring yet essential modules that will make it easier to interface the Mother (or whatever you’re using as your core) with the fun new modules you’re dying to try out. In this post I’ll spell out what I’m using for that core and why. Note  that these are not the only modules you could use – there are plenty of excellent choices available in each category – but I want to get you thinking about what you might need, and what features to look for.

The images describe the initial version of this rack; I have since swapped out a few modules which are mentioned in the text. Click here for a link to the current version of the rack on ModularGrid.

Case & Power

4ms Row Power 40I recently released an introductory course for the Mother-32, which included moving it out of its own enclosure into a Eurorack case. For that course, I used an old Monorocket 104 hp x 6 U skiff. Unfortunately, that skiff-style case is too shallow for some of the modules I want to include in the Eurorack Expansion series. Therefore, I’m going back to the enclosure I used for the original Learning Modular Synthesis course: an open-back 6U 19” desktop studio rack with a pair of 84hp wide Synthrotek rails + rack ears (spray-painted black) with threaded inserts. Going back to a smaller case meant I had less space for support modules; that’s reflected in some of my module choices below.

For the original course, I used a 4ms Row Power 40 to power the case, and was happy with how it worked. It uses an external 15v switching power supply (such as those used by laptop computers), and regulates + filters it down to the +/-12v and +5v required by Eurorack modules. This “hybrid” approach is becoming more common. Rated power output is 1.5 amps on the +12 and +5 volt rails and 1.25 amps on the -12v rail.

When planning your system, remember to always have at least a 20% safety margin between what your particular combination of modules requires, and what your power supply can deliver. Simulating this system on Modular Grid shows the core needs 970ma on the +12v power rail (and much less on -12). Running the math using the Row Power 40, 1500ma (1.5 amps) x 80% = 1200ma, leaving up to 1200 – 970 = 230ma on the +12v power rail for the module I’ll be reviewing in each movie: plenty of room, unless I started getting into power-hungry tube modules.

MDLR Case Bus BoardsClean power is also essential to a properly functioning modular synth. To this end, I’m excited that more companies are starting to offer power distribution bus boards that include additional on-board filtering. Therefore, I originally replaced the flying bus cables I used previously with a pair of MDLR Case Eurorack Busboards. These have lots of power connectors for modules (20 per board!), make it easy to connect to the main power supply of your choice (I ran standard 16-pin Eurorack power cables from the Row Power 40 to a free connector on each board), and even have on-board +5v regulators in the event your power supply only provides +/-12v. They’re also a great deal at 24 euro each, shipping from Holland. I’m considering replacing all of the bus boards in my main system with these. I’ve since exchanged these for a MIDI Hardware POWDIS heavy copper bus board and the Genus Modu Euro Header Adaptor system.

Notes In

Not all semi-modular synths have a MIDI interface built in; the Moog Mother-32 does, including an extra control voltage output that can be assigned to different MIDI controls such as the mod wheel. However, the MIDI controller I’m using – an Akai MAX25 – has a lot of additional control sources, including key velocity, aftertouch, and programmable sliders. I want to take advantage of these too while playing my modular, so I added an Expert Sleepers FH-1 ‘faderHost’ to the Eurorack Expansion system. It has eight outputs, which I’ve assigned to pitch CV, gate, velocity, aftertouch, two of the assignable sliders, and a built-in LFO (which I can control from the other two sliders on the MAX25). I wrote a separate article on programming the FH-1 to do what you want for your own system.

Expert Sleepers FH-1 'faderHost" + Distings; Malekko Performance Buffered MultI’m a big fan of using a buffered multiple for pitch control voltages inside my modular (this article demonstrates and explains why). Therefore, I added a Malekko Performance Buffered Mult to this system, to drive both the Moog and any other VCOs I might add. I also followed the calibration procedures for both the FH-1 and the Mother-32 to make sure they gave me an accurate foundation to build on. (To match the voltage ranges required by different VCOs, I’m also including an Expert Sleepers Disting to use as a precision adder – I’ll talk more about the Disting below.)

To split the less-critical gate signal, I’ll be using a Black Magic Modular MonoMult. I also happen to like their cables, especially since I color-code the cables in my patches to make them easier to follow. I’ve also started using Hosa Hopscotch cables to perform simple 1:2 splits of audio and modulation signals.

Sound Out

On the output side, many users just plug the very high level signals from their modular straight into their mixer, and the use a combination of input pads & trims plus channel faders to reduce the signal down to the level they need – and that works fine, as long as your mixer has enough headroom. I personally prefer bringing the signal into my mixer at a “normal” level so I don’t have to keep remembering to treat those channels differently, as well as to preserve headroom (as sometimes I like to slam things really loud).

Therefore, I like to include a dedicated output module in my racks. The one I use most often is a WMD Pro Output. It has a headphone amp (great for monitoring without a mixer) as well as unbalanced -10 (“line level”) and balanced +4 outputs. I use the latter, as it helps suppress noise induced by external power transformers.

Mixing and Matching Signals

I strongly believe every system – regardless of its size – benefits from having one or more utility mixers in it. By utility, I mean a module that can not only attenuate and combine signals, but one that can also invert them if necessary, as well as add a steady positive or negative offset voltage to the result. Among other things, this allows you to adjust the voltage range of any one output to match what you need as an input to another module.

utility mixer, envelopes, and VCAsTo fill that roll, I initially used a Happy Nerding 3xMIA that features six channels grouped together as three pairs. Each channel can be attenuated, inverted, or replaced with an offset voltage; pairs are sub-mixed together, and can be cascaded down the chain to the pair below it. And, it’s only 6hp wide! In later movies added onto the course, I swapped out the 3xMIA for an Erogenous Tones LEVIT8, because I found its ability to amplify – not just attenuate – signals to be very important. And even more recently, I’ve started using the Frap Tools 321 as it’s both compact and versatile.

Envelope Issues

It’s a trend in small synths these days – including semi-modular systems – to provide a single envelope generator, often switchable between AD and AR modes. Personally, I prefer different envelopes for my filter and amplifier: I like to use an AD or an ADSR shape with a low sustain level to “pluck” the filter, and then keep the amplifier open on sustained notes using an AR or another ADSR with a relatively high sustain level. On top of that, I might want to modulate another parameter or two with a different envelope or LFO.

To fill this roll, I’ve been using the Roland 540: It contains two ADSRs that can be looped as well as a voltage-controlled LFO with initial delay and multiple waveshapes inside one package (16 hp wide). I also happen to like the sound of its envelope shape: it has the classic logarithmic attack and exponential decay + release that I feel sounds more natural in many applications.

More VCAs

When a user shares their modular system in a forum and asks “what am I missing?”, many race to chime in “more VCAs!” In general, these allow you to shape the amplitude of other signals – such as LFO or FM depth – as you route them around your system. In the specific case of the Mother-32 I’m using as my “starter synth,” there is also no external audio input for the Mother’s own VCA – meaning if you want to try out a different filter with this system, you need to route it through the Mother-32s existing filter (Moog’s tip: set it to High Pass, and turn both the cutoff and resonance all the way down). The Mother’s own VCA is also hard-wired to its envelope: You can add to this envelope’s effect on the VCA, but you can’t replace it using the Mother’s patch panel.

Therefore, you’re eventually going to want to add more VCAs to your system. I prefer ones that can be switched or continuously dialed between linear and exponential response, and which also have an offset control so you can “open up” the VCA to hear what’s going through it without having to send it a voltage – akin to the Mother’s VCA Mode switch. For this system I chose an Intellijel uVCA II, which ticks all of those boxes and also is only 6hp wide.

What About a Make Noise Maths?

Many modular users – including those building their systems around a semi-modular core like the Moog Mother-32 – add a Make Noise Maths as one of their first expansion modules. A Maths can act as a utility mixer…or a pair of simple AD or AR envelope generators…or a pair of LFOs…or a slew limiter to smooth out signals…and much more. If you are a bit technical and are comfortable translating the Maths’ raw capabilities into the functions you require, then you will probably love this module or something in a similar vein such as the classic Serge Dual Universal Slope Generator; many do. On the other hand, if you are relatively new to the concepts in modular synthesis, you may find it easier to have dedicated modules for the functions you need – at least starting out.

(Tweaky side note: As hinted above, I have a personal preference for logarithmic attacks and exponential decays for my envelopes; the Maths is continuously variable from log for both, through linear, to expo for both – not quite the combination that is my first choice. But if you’re using very fast attacks, this distinction doesn’t matter, and envelopes is just a small part of what a Maths can do. )

Distings: The “I Wish I Had Another…” Module

When you get into modular synthesis, you will find that you always wish you had one more module to finish a patch – one more envelope, one more LFO, one more VCO, or an esoteric utility module that might be tied up elsewhere in the patch. To save money and space, I turn to the Expert Sleepers Disting (4hp and well under $200 each) to solve this problem.

The Disting is a chameleon module that can change personalities. The Mark 1 and 2 models supported 16 different functions, shown in the chart below:

Expert Sleepers table of first 16 modes

The Mark 3 and 4 models contain over 50 functions (and growing!), including envelopes, filters, effects, audio and MIDI file players, and more.

Quite often, I’ll be using one of the Distings in this rack to transpose oscillators that use a different voltage reference for Middle C than the Mother-32 does (this is an issue in any modular system). The other will be used to fill in gaps as needed, such as a second VCO (mode 4C has really nice FM, and both 4C and 4D have huge-sounding sawtooth waves), a sample and hold, an extra LFO or filter or envelope, and more. Proving that I practice what I preach, I have five(!) Distings in my personal system to cover these “I wish I had another” situations.

Mordax DATA moduleVisual Aids: The Mordax DATA

If you’ve already seen a few of the modular movies I’ve created, you know I believe in connecting what you’re hearing to an understanding of what’s going on with the actual voltages and audio signals inside a modular system. Therefore, I’ve included a Mordax DATA in this rack, so I could constantly show you what the signals inside a patch are doing. For me, this is how you really grasp what’s going on with a waveshaper, resonance in a filter, and so forth.

The DATA is also much more than just a modular oscilloscope – it can also measure voltages and frequency (great for tuning or tweaking modules), generate its own oscillator and clock outputs, and it has its own spectral analyzer and spectrograph displays – so it’s well worth checking out.

(I’ll also be supplementing the DATA with the Spectre software to display harmonic spectrums.)

And Now: You’re Ready to Get Started!

Once you have a collection of seemingly mundane but essential modules similar to those above to cover your interfacing needs, I feel like you’re finally ready to start adding more exotic modules to your system. (I know; you wanted to get to the fun stuff first – I’m that well-meaning parent that insists you finish your vegetables before you get dessert.) That’s what the hole in the system above is for: to plug in a different module each movie. In the Learning Modular Synthesis: Eurorack Expansion course that will appear on and LinkedIn Learning this summer, I’m going to dive into a variety of modules in each basic category for you to consider:

modules to be explored in the Eurorack Expansion course

The idea behind the selection of modules above is to give you a taste for the variety of choices available in each basic category. For example:

Since there’s far more modules in the Eurorack world than just the 18 mentioned above, I’m going to keep creating movies of additional modules and releasing them through the Learning Modular web site, including adding them to the version of the Eurorack Expansion course streamed through this web site. So stay tuned; it’s going to be a fun couple of years while we explore different ways of expanding the sonic territory you can explore with your modular synth.