Eurorack is a type of modular synthesiser. In a non-modular synthesiser the various functional blocks that generate and modify sound (such as oscillators and filters) are preconfigured — they are connected in a predetermined way. In a modular synth these blocks are connected manually using patch cables. This makes them harder to use but more flexible. It also makes them expandable; if you need another oscillator you just buy another oscillator module. There are several different modular synth formats. Eurorack is popular because it is open, the modules are physically small and there are many hundreds of modules to choose from, from a large number of manufacturers.

A fixed architecture synth. The oscillators, filters and envelopes are pre-connected.
A fixed architecture synth. The oscillators, filters, amplifiers and envelopes are pre-connected.


A modular synth. The oscillators, filters, envelopes, amplifiers and other gadgets are manually connected with patch chords. Each sound can have it's own synth architecture.
A modular synth. The oscillators, filters, envelopes, amplifiers and other gadgets are manually connected with patch chords. Each sound can have its own synth architecture.


Modular synth fans have a tendency to go slightly overboard.
Another modular synth. Modular synth fans have a tendency to go slightly overboard.



Some Holistic Considerations

When it comes to designing a system I found it very helpful to use a “modular planner” website. There are a couple of different ones out there but I’ve found to be the best by far. It has a huge database of modules that you can drag onto a rack so you can try out different layouts. It tallies the current draw and cost of your picks so it gives you helpful ballpark figures for these important metrics. As an added bonus it allows you to show off your creation to others.

The Eurorack format specification was defined by Doepfer and it can be found on their web page:
Technical Details :
Mechanical Details :

We’ve established that I’m building a fairly traditional subtractive analogue synth: an East Coast style modular. Modular synthesizers were pioneered in the 1960s by Bob Moog on the American east coast and Don Buchla on the American west coast.
Actually it seems that most countries had at least one crazy bearded guy who built a modular synth before everyone else in the world, but no-one cares about those guys. Anyway, an East Coast (Moog) instrument is usually played by means of a keyboard captained by a serious man in a suit.

All inhibitions are lost at a lively east coast performance.
All inhibitions are lost at a lively east coast performance.

A West Coast (Buchla) instrument is usually played by injecting psychedelic drugs in the audience and waving various extremities in the general vicinity of the synth.

A west coast player jams on the Buchla Breath Controller.
A west coast player jams on the Buchla Breath Controller.

As a further illustration of the difference I will now list the modules that I planned to get at one point for my system, as well as the modules for a possible West Coast type system that I picked out on

East Coast :
Doepfer A-190-2 MIDI/CV Interface
Doepfer A-110 VCO
Doepfer A-118 Noise Generator
Doepfer A-182-1 Switched Multiple
Doepfer A-140 ADSR
Doepfer A-132-3 Dual DVCA
Doepfer A-138 Mixer
Doepfer A-114 Ring Modulator
Doepfer A-148 Dual Sample&Hold
TipTop Audio Z3000 Oscillator
TipTop Audio Z4000 ADSR

An East Coast type modular.
An East Coast type modular.

West Coast :
Harvestman Piston Honda
4ms Euro Noise Swash
Make Noise Maths
Make Noise dual Prismatic Oscillator
Make Noise Pressure Points
Malekko Heavy Industry Assmaster
WMD Micro Hadron Collider
Harvestman Zorlon Cannon
Synthetic Sound Labs Modulation Orgy
Bananalogue Serge 3P

A West Coast modular.
A West Coast modular.

See what I mean? Did I just pick the modules with the weirdest looks and strangest names on Maybe I did. But I’m not that far off. And now I’ve managed to make a system I haven’t even bought yet seem a little boring…

Ok, ok. The main difference between the two styles lies in how sound is generated and shaped. The East coast style is to generate sounds using simple waveforms and shape them using filters and envelopes. The West coast style is to use advanced modulation techniques to generate harmonically complex sounds directly. And then drugs.

Anyway, I looked around for “starter” systems a bit but found surprisingly little. TipTop Audio has a “Happy Ending” kit which seems to be just an enclosure with a power-supply. Pittsburgh Modular has the “Foundation” system which looks nice and Doepfer has a “MiniSystem” which looks similar to the Foundation but is quite a bit cheaper. I chose to go with Doepfer as my “default” — if there is a Doepfer module that solves my problem, I’ll go with that unless I have a compelling reason not to.

Why Doepfer? They pioneered the Eurorack format. They are “competitively priced” (because “cheap” sounds cheap). They’ve been around for ages and have loads of modules. When I emailed them to ask if their 6U-enclosure could be split into two 3U halves I got a reply from Doctor Dieter Doepfer himself (it was “no,” in case you were wondering (but more politely put)). And yes, yes, I know that Americans did it first, but let’s face facts : a modular analogue synthesiser is a very German instrument.

An italian masquerading as a german weakens my point.
An italian masquerading as a german weakens my point.

The Enclosure

The Doepfer MiniSystem looked cool but I want to rack-mount my system so the cheap wooden box (the A-100LC6) it comes in would have to be replaced. I’ve basically settled on a A-100G6 which is a 6U (so it has two rows for modules) metal enclosure with rack-ears. It comes with a 1200mA power supply (which seems to be plenty) and a common bus for the two rows. It looks sturdy with decent room for expansion, should the fancy take me.

A lonely A-100G6 awaiting tennants.
A lonely A-100G6 awaiting tennants.

So the way the Eurorack system works is that you have a “bus” which supplies power (+12V and -12V, +5V and ground), CV and Gate signals. Not all modules use all the signals. Many modules don’t need the +5V pin and the standard Doepfer power supply doesn’t even supply power to that pin without an extra adapter. None of the modules I’m considering needs 5V so I’ll do without the adapter. One (and only one!) module on the bus can use the internal CV and Gate signals as outputs. In my case it will be a MIDI-CV interface module. Other modules may use the internal CV and Gate signals as inputs and in this case several can do that at the same time. All other connections have to be made using patch cables in the front of the system.

Each module is connected to the bus using a ribbon cable and apparently these cables are often incorrectly built. If the cable is incorrect or connected the wrong way you may end up letting the blue smoke out of the module you’re connecting (that’s bad). I’ll be sure to inspect each cable carefully before using it.

MIDI-CV Interface

Doepfer has a couple of options here that boil down to combinations of these two questions:

A) Do you need a USB connection or is a regular MIDI connector enough?


B) Do you need MIDI Clock?

A built-in USB-MIDI interface is a little more flexible than the traditional DIN-type MIDI connectors since you won’t need an extra adapter when controlling the system from a computer. All interfaces have regular MIDI connectors (the 5-pin DIN type) so if you want to connect a keyboard or an old sequencer or something you should be set either way. In my case I already have a USB-MIDI dongle so I think I can save a few bucks by skipping the USB option.

MIDI Clock is a bit trickier. The primary use is to synchronise any sequencers you have in your modular system with external sources. MIDI Clock could be potentially useful even if you don’t have a sequencer module though. It seems nice to have a clock source available in the system, for example to synchronise an LFO with the song tempo. Unfortunately the price difference between a simple MIDI-CV (MIDI-CV/Gate) interface and one that can also handle MIDI Clock (a MIDI-CV/Gate/Sync) is pretty substantial.

I haven’t really decided yet. I imagine that my needs are pretty basic so my current pick is a Doepfer A-190-2 MIDI-CV/Gate interface. It is a cheap, no-frills interface. However, Doepfer has the A-190-4 MIDI-CV/Gate/Sync interface coming out this spring and I may end up feeling that I can’t live without MIDI Clock. It’s my first module and the Euro-quicksand is already pulling me in!

The Doepfer A-190-2
The Doepfer A-190-2


Oscillators are one of the two big ticket items (the other being filters). There is no limit to the amount of crazy you can find here. I’m trying to put some upper bound on the craziness of my initial system so my picks are pretty traditional (you’ll hear this again!).

My first pick is a Doepfer A-110 Standard VCO. It goes for about 140 Euros and is a pretty basic oscillator that outputs sine, triangle, sawtooth and square waves on separate outputs. The output frequency goes from 15Hz to 8kHz which goes as “basic” I guess — not great. The A-110 has two CV inputs controlling the pitch so you can do basic frequency modulation. The pulse width of the square wave if also CV-controllable. Finally, the A-110 has a Sync input which can be used to hard-sync it to another oscillator. The A-110 oscillator may need up to 20 minutes of warming up before the tuning becomes stable — welcome to analogue land!

The Doepfer A-110 Oscillator.
The Doepfer A-110 Oscillator.

So the A-110 is basic and has what I need. However, a modular synth with a single oscillator is pretty boring, you’ll need at least two to get things interesting. Initially I planned on getting two A-110s. There are things to be said in favour of having two oscillators of the same type: they are more likely to track similarly across the octaves and detuning them to make a sound fatter is probably more likely to have the intended effect.

I had basically settled for two A-110s when I started to look at which LFO I wanted. The thing about the LFO is that the frequency has to be voltage controllable. One of my favourite effects is sweeping the LFO rate with an envelope (imagine creaking ice or the sound of a mooring cable being strained) so I definitely want a CV-controllable LFO rate. The basic Doepfer LFOs (A-145, A-146) does not have this but the A-147 VCLFO does. When I was digging around looking for an LFO it struck me (well, it struck someone who wrote about it on a forum via which it then struck me) that if I got an oscillator that could go down into really low (like a tenth of a Hertz) frequencies I could use that as an LFO and when I’m not using it as an LFO I’d have a free oscillator!

Ok, so “free” turned out not to be what this extra oscillator was going to be. Since the A-110 doesn’t go below 15Hz it is not a viable LFO-replacement candidate. This is when I found the TipTop Audio Z3000 Smart VCO Mark II. On paper the Z3000 is a lot more powerful. It does everything the A-110 does — square, sine, saw and triangle waves on separate outs, hard sync, CV-controllable PWM — and it has some extra goodies to tempt you away from your liquidity. It has a frequency counter display which allows you to set the oscillator frequency exactly and it can be used to measure the frequency of external signals as well. The Z3000 frequency range is 0.7Hz to 30kHz. It has a waveshaping input, which does something, and a Hard Sync Modulation input, which does something else. It’s significantly more expensive than the A-110 at about 220 Euro but if I replace one A-110 and can save on buying a dedicated LFO then it’s a wash. Right? Right? What happened next was typical.

The TipTop Audio Z3000 Smart VCO mk2.

What happened was that I read the manual to the Z3000. Never read the manual. At first a particular module is something you want. If you then read the manual the module becomes something you need. It’s a “Yesterday I didn’t know that it existed and today I cannot live without it” kind of thing. I like the possibility of setting exact, matched frequencies when using a pair of oscillators as parallel sound sources. So the Z3000 looks like it would be particularly well matched by another Z3000. So maybe two Z3000 oscillators and no A-110s…

It would be nice to have one of each because I’m curious about how pronounced the difference is between different oscillators — the naive assumption would be that for instance a sawtooth wave would sound much the same on all oscillators. On the other hand I do have a budget and the beauty of the modular system is that I can get another oscillator later if I want to.

Finally, I need a noise source. Maybe not strictly speaking an oscillator but as a sound-generator I’ll deal with it under this heading as well. Noise is a lot more versatile than you would think but I don’t need anything fancy so I’m going with the Doepfer A-118 Noise/Random module. It has outputs for white noise, coloured noise (high or lowpass filtered noise) and random voltage. The random voltage output is a low-pass filtered version of the coloured noise so that you get a lower rate of change which is useful for CV (rather than audio).

The Doepfer A-118 Noise Module.
The Doepfer A-118 Noise Module.

To summarise the sound-generating section:
2 TipTop Audio Z3000 MKII Oscillators
1 Doepfer A-118 Noise/Random


I absolutely want to be able to use an external sound source with my system. I’d quite like to compare the Nord Lead oscillators using the Nord Lead internal filter to the same oscillators using the modular filters for example. Or to use Thor as an oscillator with MIDI-control of the modular envelopes. Part of the point is figuring out what stuff actually sounds like. And part of the point is building grotesque Frankenstein patches.

There are various “external input” modules available (like the Doepfer A-119) but as far as I can understand I won’t need one. The modular should be able to accept a line-level input from my mixer directly.

Low Frequency Oscillator

I may possibly have motivated the step up to the posher TipTop oscillators by claiming a savings from making do without a dedicated LFO. But that was many paragraphs ago and the sacrifice has served its purpose. Of course I need a dedicated LFO! I’ll go basic here as well since my oscillators do support LFO frequencies and I think I should be able to get an LFO out of Reason via the second CV output of the MIDI-CV interface. The Doepfer A-145 looks decent enough and costs a reasonable 65 Euros. It generates sine, square, triangle, sawtooth and inverted sawtooth waves and the cycle can be synchronised via a reset input. The frequency range goes from somewhere around 0.01Hz to 4-5kHz.

The Doepfer A-145 LFO.
The Doepfer A-145 LFO.

I like using stepped random LFO waveforms for pads but those are not provided by the VCOs or LFOs I’m considering. Because of this I’m matching the A-118 Noise generator with an A-148. The A-148 is a Sample and Hold module; it has a sample input and a trigger input and will sample the level of the input whenever a trigger arrives and hold that level on the output until the next trigger arrives. Sample. And hold. Pretty simple. With this I can connect the noise source to the sample input and a square wave from an LFO to the trigger input and Chewbacca! : a stepped random waveform with the rate of change controlled by the LFO! It’s magic! Expensive, possibly pointless magic…

The Doepfer A-148 Sample & Hold.
The Doepfer A-148 Sample & Hold.

LFO summary:
1 Doepfer A-145 LFO
1 Doepfer A-148 Dual S&H


Filters. This is where the magic happens. Where the rubber meets the road. Where sane people take leave of their senses.

A filter should be fat. If you think that means that the module should be physically rotund then I must disappoint you. It means that the filter should sound fat. Is that even more confusing? Yes. Yes it is.

Disregarding the dubious practice of describing the character of sound using words from the kitchen you would think that two filters with the same specifications (filter type and attenuation slope) would sound alike but there is a whole host of other parameters that define the sound of a filter. How does it phase shift the filtered frequencies? How does it sound when overdriven? Is it resonant and how does the resonance behave? Can it self-oscillate and where? This is an area where you can find dozens of different filters that claim to sound like (or be inspired by) the filter on classic synths. Doepfer alone has 14 different filters currently available, including the A-103 18dB low-pass filter (based on the legendary TB303 filter), the A-106-6 multi-mode filter (based on the legendary Oberheim Xpander filter), the A-124 12dB multi-mode filter (based on the legendary EDS Wasp filter) and the A-105 24dB low-pass filter (using the legendary SSM2044 filter chip). I think you see by now that the most important property of a filter is that it is legend… wait for it…

The gold standard when it comes to analogue filters is the legendary Moog 24dB low-pass filter. This time the moniker is not exaggerated; we all remember the tale of how the Moog 24dB low-pass filter slayed that dragon and rescued the princess.

While I could never afford an actual Moog filter (not that they even make Eurorack modules) it would be tantamount to sacrilege to not include a 24dB low-pass as the primary filter in an analogue synth. I’ve picked the TipTop Audio Z2040 filter as my main filter. It is, as required, a 24dB low-pass filter and is based on the legendary Prophet 5 filter, with voltage controlled resonance and cutoff frequency. It has a two built in VCAs which is pretty nice seeing as separate VCAs are quite expensive. One VCA controls the input or output gain and one controls the cutoff modulation amount. This last one is a bit of a head-scratcher but I think it can be used to amplify or attenuate the action of an envelope or LFO on the cutoff frequency. In other words, suppose that an LFO controls the cutoff; we could then fade the amount with which the LFO controls the cutoff by connecting an envelope to the cutoff modulation amount input. Does that make sense? I’ll be sure to experiment a bit with this feature to see if I can figure it out.

The TipTop Audio Z2040 4-Pole Voltage-Controlled Filter.
The TipTop Audio Z2040 4-Pole Voltage-Controlled Filter.

Now, I don’t think that I can get away with having only a low-pass filter if I’m going through with my Synth Secrets project so I’m going to need a multi-mode filter as well. I’ve somewhat randomly picked the Doepfer A-106-5 SEM filter. It’s quite cheap at 100 Euros or so and has a 12db high-pass/low-pass/notch and a 12dB band-pass filter on separate outputs. The type of filtering on the high/low/notch output is controlled by a knob. This filter is not capable of self-oscillation but I’ve got that covered with the Z2040.

The Doepfer A-106-5 SEM Voltage-Controlled Filter.
The Doepfer A-106-5 SEM Voltage-Controlled Multi-Mode Filter.

I’m quite curious about how different my two filter choices sound.

Filter summary:
1 TipTop Audio Z2040 24dB low-pass filter
1 Doepfer A-106-5 SEM 12db multi-mode filter


I’ll need at least two envelopes: one for the filter and one for the volume. I’m mixing it up a little by picking a Doepfer A-140 ADSR and a TipTop Audio Z4000 Envelope Generator. The A-140 is a basic ADSR envelope. It has two outputs for the envelope signal (so it can control two other units simultaneously) and one which outputs the inverted envelope signal. It also has a re-trigger input which restarts the envelope generation every time it goes high and the gate signal is also high.

The Doepfer A-140 ADSR Envelope.
The Doepfer A-140 ADSR Envelope.

The Z4000 is also an ADSR envelope but with some extra bells and whistles. In addition to the knobs controlling the attack, decay and release lengths and the sustain level (like on the A-140) the Z4000 also has voltage control inputs for those parameters, which is a rather useful feature. It can invert the envelope but also add or subtract a level offset from it. This module also has a trig-button that manually triggers the envelope, and a switch to select a logarithmic or exponential attack slope.

The TipTop Audio Z4000 ADSR Envelope.
The TipTop Audio Z4000 ADSR Envelope.

Envelope summary:
1 TipTop Audio Z4000 Envelope Generator
1 Doepfer A-140 ADSR


I’m going to need a bunch of utility modules to make this system usable and moderately flexible.

First I’ll need some Voltage Controlled Amplifiers. I will try to make do with two dedicated VCAs (since the Z3000 oscillator has an in-built VCA) and am going with the Doepfer A-132-3 DVCA. This is a dual VCA with separately switchable control scales: linear (for CV amplification) and exponential (for audio amplification).

The Doepfer A-132-3 Dual Voltage Controlled Amplifier.
The Doepfer A-132-3 Dual Voltage Controlled Amplifier.

A non-Dalekable modular is no use to anyone so I’m including the Doepfer A-114 Dual Ring Modulator.

The Doepfer A-114 Dalek. Sorry, the Dual Ring Modulator.
The Doepfer A-114 Dalek. Sorry, Dual Ring Modulator.

It is sometimes necessary to connect an output to more than one input. You might want the LFO to affect the filter cutoff of both oscillators for example. To do this you need a “one-in, many-out” module called a Multiple and I’ve picked the Doepfer A-182-1 Switched Multiple module for this task. The simplest multiple is just a bunch of jacks that are all connected to each other to form a bus. The A-182-1 multiple has two busses and eight jacks with switches that can connect each jack to bus 1, bus 2 or disconnect it from both busses. This makes it a little more flexible than a fixed multiple.

The Doepfer A-182-1 Switched Multiple.
The Doepfer A-182-1 Switched Multiple.

When using more than one oscillator or other sound source the audio outputs will need to be mixed at some point. I could do this with an external mixer but I think the system will be a lot easier to work with if it has at least a simple audio mixer built-in. The Doepfer A-138b Mixer is a simple four-channel mixer which should do the job. The ‘b’ means that the pots are exponential which is what you want for audio. Consequently there is a A-138a as well on which the pots are linear and more suitable for CV mixing but I don’t think I’ll need that (famous last words).

The Doepfer A-138b 4 Channel Exponential Mixer.
The Doepfer A-138b 4 Channel Exponential Mixer.

Utility summary
1 Doepfer A-182-1 Switched Multiple
1 Doepfer A-138b Mixer
1 Doepfer A-132-3 Dual Voltage Controlled Amplifier
1 Doepfer A-114 Dual Ring Modulator