Getting started with Eurorack – part 2

Posted on Apr 11, 2016 by rutgervlek

Build a rock boys!

I know a few of you have been eagerly awaiting part two, so here it is! And let’s start with a picture… the MiniMoog! It’s the mother of synths, the sound that became iconic, everyone wants one, and the price for a vintage Mini keeps rising. Nearly all of its internals have been cloned and reincarnated in various Eurorack modules. I won’t point you to any specific brands or module types, but I’ll help you understand what you need to recreate the architecture of a MiniMoog as the basis of a Eurorack system. It’s simple and a great way to learn and expand from there.


A brief glance at the MiniMoog shows you 3 VCOs, a noise source, a mixer, a VCF (voltage controlled filter), two EGs (envelope generators) and a VCA. That’s basically the audio path of the MiniMoog, so if you want to do this in Eurorack, this is your shopping list:

  • 3x VCO 1x noise source (white and pink)
  • 1x 4-channel mixer
  • 1x VCF (low pass transistor ladder-filter)
  • 2x EG (envelope generator)
  • 1x VCA (audio)

Also, you want to be able to control it through a keyboard, or DAW. So add a MIDI-to-CV convertor to the list as well (unless you have a keyboard that puts out CV and gate). The MiniMoog also has a slew-generator on the keyboard CV voltage, which is used to create portamento effects. Some MIDI-to-CV converters have it built in, some don’t (in which case you can buy it as a separate module).

Now we get to the hidden parts of the MiniMoog, that you don’t see from the frontpanel, but are crucial to its capabilities. First of all (and still a bit visible from the outside): the third oscillator doubles as an LFO. You may be able to find a Eurorack module that can do this too, but I’d recommend going for one (or two) separate LFO modules. The output of the Mini’s ‘LFO’ is mixed (with a balance control) with the output of the noise generator into a modulation path for the VCOs and/or filter. To recreate this this you need an additional (CV) mixer with level controls or a balance knob. Also don’t forget how the Mini’s modulation wheel allows you to control the depth of modulation in this path (from LFO-noise mixer to VCO and filter). If you put a VCA in this path, you can feed the modulation wheel voltage (usually coming from the MIDI-to-CV convertor) into the CV input for depth control. You could do with one shared VCA for both VCO and filter modulation, but it makes more sense to have one for each.


An overview of the MiniMoog’s architecture recreated with Eurorack modules

Depending on your choice of filter and envelope generator, you may find you still lack control over the envelope depth to the filter. A small mixer between EG and filter would give you this, and it would also allow you to modulate the filter with many other signals simultaneously.

Lastly, you may need to route the output of one module to inputs on several other modules (for instance the noise, which goes into the main audio mixer, as well as into the modulation mixer). Somehow you need to split the signal, and this can be done in several ways:

  • there are patch cables for Eurorack that allow you to insert another cable on top of its connector (stackable)
  • so called passive multiples provide an input connected to several outputs
  • active multiples do the same, but buffer the signal such that you don’t lose signal when you need to divide it over lots of modules (otherwise you may notice the signal weakens a bit)

So let’s update the shopping list:

  • 3x VCO
  • 1x noise source (white and pink)
  • 1x 4-channel (audio) mixer
  • 1x VCF (low pass transistor ladder-filter)
  • 2x EG (envelope generator)
  • MIDI-to-CV convertor (with portamento, or separate slew module)
  • 2x LFO
  • 3x VCA (1x audio, 2x CV)
  • 2x 2-channel (CV) mixer
  • a few multiples (active or passive)

Strictly speaking, if you wish to copy each and every feature of the MiniMoog, you need another few more modules. A master volume control, line-level output and headphone output stage might be very handy (can often be found combined in a single module). And the external input is still missing. The 4-channel audio mixer needs to become 5-channel, and you need some way of lifting an external input to typical Eurorack levels (between +5V and -5V). Again, you can find dedicated modules for this, if you wish (sometimes including additional features like a microphone input, or envelope follower)

MiniMoog on steroids

If you select modules crucial to the sound character of the MiniMoog carefully, I bet you can get very very close to the sound of a MiniMoog, and it’s functionality. But this is Eurorack, so now I’d like to show you the benefits of going modular. This Euro-Mini can do a whole lot more than you thought!

Features that are now possible with the same set of modules, but that you can’t find on an original MiniMoog are for instance:

  • PWM! Pulse Width modulation is a very common option on Eurorack VCOs, and really adds to the sound flavours available.
  • Pitch envelopes! The envelope generator can be routed to one or several pitch inputs of the VCOs
  • Oscillator Sync! (most Eurorack VCOs allow for this)
  • Tremolo and panning effects: LFOs can now be routed to the VCAs you have and you can even build stereo effects with these
  • Allowing VCOs to modulate each-other (cross-FM)
  • A world of additional and more dynamic modulations:
    • Allowing EGs to modulate the depth of your LFOs through a VCA (gradually increasing vibrato depth)
    • Allowing LFOs to modulate each-other
    • audio rate filter modulation with a VCO, and an LFO adding slow movement at the same time
    • vibrato to the VCO and slow movement (second LFO) in the filter independently
    • more expressive control from your keyboard (velocity, after-touch, pitch-bender)

And that’s just the obvious ones. While experimenting, you are bound to discover many more sonic possibilities.


A case of….

When you start assembling ideas about a Eurorack system you probably have already stumbled upon the modular grid website, and if not I highly recommend it. It allows you to drag and drop, save and share concepts for a modular with modules from nearly all manufacturers. However, at some point your virtual plans need to emerge in the real world. In other words: you need a rack or case and a power supply. There are plenty of wonderful cases and rack systems that can be bought ready for use, but it’s also a lot of fun to build your own unique enclosure for the instrument you’re so carefully preparing to assemble. I’ll walk you through the options.

Most Eurorack modules conform to the standardized height of 3U (height units), which is the same metric as the height units used in 19″ rack world. In recent years, people have found need for an additional height standard for smaller utility modules (mixers, multiples, control knob, etc) and this became the 1U “tiles” height: exactly a third in size from a normal Eurorack module. Width is expressed in similar measures, using HP (horizontal pitch). Most module widths are rounded to an even number, and 2 HP is in most cases already too small for comfort (unless it’s just a few sockets for a multiple). Commercially available cases often accommodate up to 84 or 104 HPs. It’s also good to know, that if you already have a 19″ rack system, there are 84 HP width rail systems available that can be 19″ mounted. Most cases are of the upright type, but “skiff”cases are becoming more popular. These are lying horizontally on a desktop surface and have a shallow depth. This is something to be aware of when selecting modules, as not all modules will fit in such a shallow enclosure (modules that fit well are usually indicated as “skiff-friendly”, or have a depth roughly below 40 mm).

Below you can see the case I decided to build for my modular (deliberately fairly small). It provides 2 rows of 84 HP width and 3U height, and a single 84 HP row of 1U hight (for utility tiles). I’ve bought rails (e.g. part number “BGT GB 29L” from Reichelt) and had a bit of help from a furniture craftsman to create the wooden side panels to my specifications. In the bottom of the case, I left additional room for a pair of spring reverb cans. The rails are mounted such that the panels of the modules will be slightly slanted (a few degrees) to make it easier to work with when standing (e.g. on stage). On the back plane you can see I fitted a power supply (connecting to a mains power switch in the side of the left wooden cheek), as well as two so called “distribution boards”. These help to deliver the power to the connectors on the back of the Eurorack modules with a ribbon cable per module.


mycase_psu mycase_rails

Power to the people

Nearly all Eurorack modules require power. This is usually delivered from one central power supply unit (PSU) in a Eurorack case and distributed to the back of the modules in it through a distribution board or cable. The Eurorack format prescribes it delivers both +12V and -12V DC. Some modules also require an extra +5V DC line. This is usually specified per module by manufacturers.

For PSUs there are generally three options:

  1. Linear power supplies. This is a type of power supply that uses a large(ish) transformer to step the high AC mains voltage (110V or 230V) down to a more manageable AC voltage. This is then rectified and regulated to +12V, -12V (and +5V optionally). Only use well-regulated linear power supplies, as non-regulated linear power supplies may induce unwanted noise and instability in your instrument. They can be bought ready-made or as DIY kits. In the latter case you’ll be working with dangerously high voltages though! Be warned. Because the transformer windings are usually determined based on the mains voltage in your country, these supplies don’t normally work on foreign voltages (so if you’re a touring musician….). Linear power supplies are not at all efficient. This means that part of the current they are processing is turned into heat, which is a factor that may effect temperature sensitive modules (like VCOs and filters).
  2. Switch mode power supplies. These regulate the high mains AC voltage by very high speed switching into +12V, -12V (and optionally +5V ) DC lines. This proces potentially injects high-frequency noise in the supply lines, which may become audible as noise from your modules. In the past this was reason to avoid them for analog musical instruments, but modern good quality designs easily outperform linear power supplies in terms of noise and stability. They are light-weight (no heavy transformer) and much more efficient than their linear counter parts. Altogether I’d say they are now the preferred way to go. They’re generally too complex for DIY, but an affordable ready-made unit with excellent noise-performance can easily be found online (for instance the Meanwell RT-65B, part number “MW RT-65B” at Reichelt). Hooking it up still requires you to handle mains voltages, so be careful!
  3. An external adapter (line lump). This is for most the safest and easiest route, though not always the cheapest. You buy a sealed adapter that converts the mains voltage to a manageable and safer voltage. This is then further regulated to the right voltages for Eurorack by a separate module in your Eurorack system (see for instance by Synthrotek).
If you start out with a +12V and -12V power supply and you later find you still need the +5V line, you can get special convertor boards that generate a +5V line from the +12V line (see for instance Volts by Mutable Instruments). Make sure your +12V line can deliver the additional current required for this though.

About voltage and current

By far the most frequently asked question about power supplies is: how much amps do I need? To understand this question you need to know a little about voltages and currents (expressed in ampères, or miliampères). A great analogy to understand the difference is to think about water in a pipe with a pump on one side and a tap on the other. The pump (PSU) puts the water under pressure (the voltage). If you open the tap a little, water flows out (the current). For some applications (e.g. a washing machine) you need to have a lot of water flowing, so you request a lot of current. If the pump cannot keep up with this current flow, the pressure (voltage) will drop, or the pump will overheat. Now let’s take it back to the Eurorack system. Your PSU needs to be able to provide a constant voltage +12V and -12V (and +5V) at the amount of current that your modules demand from it. You can simply sum the current draw that each manufacturer specifies for a module per power rail. Have a look at this example for a set of fictive modules:








120 mA

80 mA

230 mA

47 mA

477 mA


100 mA

80 mA

200 mA

47 mA

427 mA


21 mA

50 mA

(not used)

(not used)

71 mA

If this is your complete system, and you don’t expect expansion, make sure the total current draw per rail is well within the maximum current specified per rail for you PSU (for instance one specified for a maximum of 500mA on both 12V rails, and 100mA on the +5V rail). In practice though, you’d probably want to keep room for expansion or future modules that draw a bit more current. In that case you could go with 1A (=1000mA) capacity per 12V rail, and 500mA for the +5V. This leaves you plenty of room for the future.

Power distribution

While there’s no harm in connecting a module directly to the PSU, it’s most practical to have a power distribution board (a.k.a. bus board) or cable in your system. You can get both ready made or as DIY kit. As the power distribution system is only connected to the low-voltage part of the PSU it’s relatively safe to handle. Following the Doepfer Eurorack specifications, there are two types of power connectors you could find on the back of a module: either 10-pin or 16-pin. Both carry several parallel ground lines, +12V and -12V. The 16-pin connector carries an additional +5V line, as well as a CV and gate line. The purpose of the latter will become clear in the next section. Cables to go between your distribution board and the back of your modules can also easily made yourself, for instance with the following parts:

  • 2×5 pin male IDC connector (e.g. part number “PFL 10” from Reichelt)
  • 2×8 pin male IDC connector (e.g. part number “PFL 16” from Reichelt)
  • 10 or 16 pin ribbon cable (e.g. part number “AWG 28-10G 10M” or “AWG 28-16G 10M” from Reichelt)
Check cable orientation! I can’t say this often enough. The one big flaw in the Eurorack specifications is that it allows the power cable to be connected in reverse. This can cause permanent damage to the module when power is applied, as many users have unfortunately experienced. Luckily most modern modules (including River modules) are designed with built-in protection against this. So caution is only really nescessary with older modules. Did I already mention: check cable orientation?

Ground control to major synth

As you probably want to play your modular from a keyboard or computer program (Cubase, Logic, Live) you’ll probably also want a MIDI-to-CV convertor that allows your computer or keyboard to talk to your modular. In some cases, you can benefit from a keyboard with CV/gate outputs. These outputs send control signals about pitch (CV) and when a key is pressed (gate) directly to your modular. They usually also convert the action of a pitch bender for you, and can provide separate CV outputs for key velocity, after-touch and other controller data. If you mainly plan to control your modular from your DAW, and you already own a (multi channel) audio interface, have a look at the collection of interfacing modules made by Expert Sleepers. These convert audio, rather than MIDI for use with your modular and can provide timing precision no MIDI interface can come close to.


Playing it loud

After having fantasized about a modular, and perhaps being on the brink of buying all those lovely modules, have you thought about how you’re going to connect them? A fair supply of mono mini-jack patch cables would come in handy. And how are you going to get the sound out of your modular into the world? This is a good time to thing about adding a module that provides a headphone connection. Also, bear in mind that the connections commonly used in studios are of the full-size jack kind, and carry “line level” signal, while your Eurorack modular uses mini jacks and they carry a signal that’s way to hot to feed into a line input. The easy and cheap solution is to have a mixer or VCA as the last module in the chain and attenuate the signal there to within “line level” specifications. The nicer and more professional solution is to have a dedicated output module that converts the modular signal level to “line level”. These modules typically also help in metering the output level, and sometimes support connections for headphones and other devices as well. Similarly, but going in the opposite direction, there are dedicated modules for feeding microphones, line level signals (sound card outputs, other synths, iPods) and guitars into your modular.

Checklist for getting started

  • Case with 3U rails and screws
  • Power supply that delivers +12V and -12V (+5V optional) and sufficient current
  • Power distribution system (either board or cable)
  • Power cables (either 10-pin or 16-pin)
  • Patch cables (mono 3.5mm mini jack)
  • MIDI-to-CV convertor, or other form of interfacing
  • Audio output at line-level (optionally: microphone/line input, headphone ouput, etc)

If you are still with me at this point I hope to have sparked your enthusiasm with mine. Whatever you decide to (not) do with Eurorack, I hope this tutorial has been informative and fun to read so far. I’ll take a short break to recharge and see what information is still lacking and be back soon with another blog post.