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.

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.

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 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.

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).

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

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).

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

Ah, web forums. Wonderful. Liberating. Fountains of wisdom. Kindergarten. Give man a paint brush and man shall create great works of art and culture. Give man anonymity and man shall create a drawing of a truly epic penis.

Web forums are actually a great aid to beginners and veterans alike. Most people are very helpful and happy to give advice and opinions. The premier web forum for modular synths has to be www.muffwiggler.com (yes, I know). Trawling around there can easily eat away most of your afternoons if you let it. You can find discussions on everything from synth vanity shots to the inner workings of a particular filter implementation. Many builders of synth modules read and post there and you can even find support forums for some manufacturers hosted on the site.

Another site which can be useful is www.gearslutz.com. I don’t think I have to point out that this synth business is depressingly male-dominated, do I? You do find perhaps an above-average share of muppets at this site but as long as you stay away from topics that attract the tin-foil-hatted crowd — “is analogue better than digital,” or “which is most important, the preamp or the microphone” for example — you can get great help here as well.

So I’ve been studying the MIDI-CV conversion some more and found some references to a company called Expert Sleepers. Untangling what their product was and what it does and what the benefits were took a little time but the end result is that the cost of my interface just doubled. What you don’t know can’t bankrupt you, eh?

I’ll start with the conclusion because I’m sure you’re impatient types, and the rest of it is a bit involved. I’ll be getting an Expert Sleepers ES3 ADAT-CV converter module instead of a MIDI-CV converter. The ES3 module connects to my computer using ADAT and gives me 8 analogue outputs in the modular. The outputs can be used to get audio as well as CV into the modular. Controlling the ES3 at the computer end is a Rack Extension to Reason called the Silent Way Voice Controller which allows me to send basically anything in Reason to the modular, including gate and CV from modules in the Reason rack. The downside to this is that I need an ADAT interface since my sound card doesn’t have one.

Sound and CV signals are both represented by voltages in an analogue synth. In the Eurorack system audio signals go from -5V to +5V, CV signals go from -2.5V to +8V and gate signals go from 0V to +5V. These are close enough that you can connect an audio output to a CV input and vice-versa without exploding things and this flexibility is one of the beauties of a modular synth.

Expert Sleepers made the (like all great ideas rather obvious in hindsight) observation that since audio and CV signals are so similar, a sound card should be able to act like a CV-generator as well as a sound generator. They have produced a suite of DAW plug-ins called “Silent Way” that generate CV/Gate signals using a sound card which allows users to use spare audio outputs as CV generators. This means that you can control a modular synth (or any CV-controlled instrument) without having to buy a MIDI-CV converter.

In practice there was one problem with this scheme. Since audio consists of an alternating current signal — an AC signal — most audio interfaces are “AC-coupled” which means that they will block any non-alternating signal. They cannot output a constant voltage level. CV signals do not have the same alternating behaviour as audio, they have to maintain an even voltage level, and this means that they cannot be generated by a AC-coupled audio output. There are some audio interfaces that are “DC-coupled” which means that they can output constant voltage levels, and those interfaces worked with the Silent Way plug-ins. Audio outputs also have a tendency to be used for stuff, so it’s perhaps not terribly common that people have 3-4 extra unused audio outputs. To make the Silent Way system easier to use Expert Sleepers have produced a range of hardware modules that can generate DC-signals.

I say “range” and it sounds very structured but the initial impression is one of befuddlement. Expert Sleepers have 12 different modules with some only working as expansion modules that have to be connected to other modules in the range. But once you get your head around how the system works things become much clearer. All modules take an audio signal and does something with it. There are four basic modules that takes audio output from the computer and converts them modular-level voltages. Which one you want depends on the type of audio connection your computer has. The ES-1 takes up to 8 analogue audio inputs, the ES-3 takes 8 channels of digital audio through an ADAT input, and the ES-4 and ES-40 takes 2 channels of digital audio through a coaxial S/PDIF input. Both the ES-4 and the ES-40 multiplexes CV signals on top of the S/PDIF stereo signal. The ES-4 can handle 5 channels of 8-bit signals. The ES-40 doesn’t output anything but works as a base for a range of expansion modules.

To go the other way and bring signals from the modular into the computer there are two options. One is the ES-2 which takes modular-level inputs and converts them to line-level analogue outputs. The other is an expansion module to the ES-3 ADAT interface called ES-6 which converts 6 channels of modular-level signals into ADAT outputs.

There’s a bunch of expansion modules that connect to the base modules and provide extra inputs and outputs. The key thing to realise is that for audio, you want a full resolution signal (usually). For example. 16 bits per sample at 44.1kHz, or 24 bits per sample at 48kHz. But for CV this can be overkill. An 8-bit signal at 44.1kHz might be sufficient for envelopes etcetera (although this is debatable, it is possible to hear stepping effects with 8-bit resolution). For Gate signals it is _definitely_ overkill. A Gate is a binary signal so anything above 1 bit per sample is just pure waste. Expert Sleepers utilise this to get more channels out of the basic audio connections. For example, you can buy the ES-5 module which connects to a ES-3 ADAT module and uses one stereo pair of the 8 available inputs on the ES-3 as a “virtual channel” over which it provides 8 gate channels (and potentially even more with further expansion modules). The “virtual channels” require the audio to pass through a software plug-in for multiplexing while the normal channels can be used to get pure audio inputs into the modular directly.

The upshot of all this is that the Expert Sleepers system is expandable, very flexible and slightly confusing. Just like a modular synth!

I’m sold on the ES-3 ADAT module. It gives me 8 high-quality audio channels that can be flexibly used as audio, CV or gate outputs from the computer to the modular. The timing of the generated signals are sample-accurate; a lot better than an analogue MIDI-connection would allow. The Silent Way rack extension for Reason seems to make it really easy to use modules in the Reason rack as CV/Gate sources in the analogue modular. If I want a clock signal or an LFO or even an envelope I should be able to generate those in Reason and get them into the modular with a high level of accuracy.

Finding an affordable audio interface with an ADAT port was a little tricky. ADAT interfaces are mostly present only in the fancier models and those invariably contains posh preamps that I don’t want to pay for since I won’t be using them. I happen to have an old Yamaha O2R digital mixing desk sitting in storage (which is a fancy way of saying that it sits in the shed, under a bed, bringing my rhymin’ skills some cred). It has an abundance of connectivity, including four ADAT ports (two in and two out), as well as a gaggle of preamps. So a simple ADAT in/out module would get me both 8 channels of digital output from the computer to the modular via an ES-3, as well as 8 channels of digital inputs with decent preamps from the O2R.

I finally found the USBStreamer B through this thread at GearSlutz. It is a small, simple and cheap USB-ADAT interface with a TOSLink input and output. Perfect! When Andrew Ostler from Expert Sleepers posted on MuffWiggler that he had tried the USBStreamer B with the ES-3 and it worked I was sold.

Getting the ES-3 and the USBStreamer B does have some budgetary implications since this combination is significantly more expensive than a simple MIDI-CV interface. For this reason I’ve decided to sacrifice the LFO (again). I’ll just have to live with the LFOs in Reason for the time being. I’m also going to skip the Ring Modulator in my initial system, it is not really essential and can be added later if I so desire.

I intended to wait until the attic renovations had finished but that was never really going to happen was it? I had a cold and felt sorry for myself and talked myself into pulling the trigger on my first modular system as comfort. The final system looks like this:

After checking prices at escapefromnoise.com, thomann.de and www.schneidersladen.de I found out that the prices were pretty much the same at all places but Schneidersladen had everything in stock. They seem to have a pretty good reputation so I put in the order with them. Apart from the modules and the enclosure I also ordered some blanking-panels to protect the innards of the synth from unwanted creatures. And I needed a set of patch-cables as well of course.

I’m unsure if Schneidersladen prefer to ship the modules mounted in the enclosure (which seems like the safest way) or not, so I provided them with the layout above just in case. Now it’s just a matter of clicking “reload” on the FedEx tracking page every 5 minutes for the next couple of days.

Today I got a partial delivery from Schneidersladen. I’ve gotten everything except the oscillators, the multiple and the S&H modules. I haven’t had time to do anything other than unpack everything. The modules were not mounted in the enclosure, which is perhaps just as well. I’m looking forward to installing them. First impressions? The modules are sooo cute. They are smaller than expected but look really nice.

No oscillators are kind of a drag. At least I have the noise generator and a self-oscillating filter available so I can make some noise. Fortunately the USBStreamer arrived on the same day so if everything works as planned then I should be able to put some sounds into the system from Reason as well.

Now if only I had a table to work on and some spare time!

I finally had the time to put the stuff I received together. The good news is that mounting the modules was a piece of cake. Every module arrived with a pre-attached ribbon cable and mounting screws. The A100-G6 enclosure is sturdy and well made with a power switch and an IEC socket on the back. I mounted all the modules and the blanking panels and hooked up the A-138 Mixer output to a small Behringer Eurorack (!) mixer that I had lying around and I was ready to go.

The bad news is that the USBStreamer B did not want to talk to the ES-3 module. The ES-3 would just flicker all its I/O LEDs and the big LED (which I think is a clock sync indicator) didn’t light up. Its hard to tell if the problem lies with the USBStreamer or with the ES-3 but I found one person with a similar problem and he/she could talk to the ES-3 with another ADAT interface so it is tempting to suspect the USBStreamer. Support has been requested and we’ll see how it pans out. This means the only means of triggering anything in the system is through the Z4000 ADSR trigger button. Since the oscillators are on order, testing the delivered modules was more interesting than expected.

I had to be a little creative to get the system to make some noise but I could wire up a simple patch that used the ADSR envelopes with a VCA and the noise generator to create a simple snare-type sound by using the trigger button on the Z4000 ADSR. The Z2040 filter self-oscillates nicely without any input so I could turn up the resonance to max and use the noise generator low-frequency output to control the frequency. This made the modular bleep and bloop on its own accord which was kind of cute.

Both the multiple and the S&H module were also missing from this first shipment so the patching options are definitely limited. The A-106-5 filter has two outputs, one band-pass filter output and one low-pass/high-pass output. I could use this as a kind of multiple so I could feed back the tone from the Z2040 filter into itself and use the Z2040 gain knob to distort the result to dirty things up a bit.

That turned out to be quite entertaining. With the resulting feedback loop I didn’t even have to have the resonance of the Z2040 turned up and I could control the pitch using the frequency knob on the A-106-5. Limitations begets creativity!

I have to say that I could probably spend a weekend playing with this thing, sans oscillators, if only I had the S&H and the multiple. Limitations can be creative but in this case I think I’ll need just a little bit more flexibility.

My first modular advice : Get a multiple. Oscillators are optional. Multiples are not.

My second modular advice : Get twice as many patch cables as you think you’ll need!

My oscillators have still not arrived but I managed to get the USBStreamer B to work properly by reflashing its ADAT firmware. This required a rather sordid excursion into Windows 7, which I will spare you, but the upshot is that the ES-3 now works as intended. I’ve written a guide on how to integrate a modular synth with Reason using the ES-3 here.

Happily this means that I can trigger the envelopes from Reason devices as well as using the modular filters on sounds from Reason. First impressions: the Z2040 sounds pretty nice, especially when you turn up the gain knob so it starts to overdrive.

Remember how I wrote that “I won’t be digging into any of the esoteric math modules…“? It turns out that I was wrong and that I will be digging into, specifically, an esoteric math module. This is what happens when you order a spanking new toy and a vital part — the oscillators — takes a month longer to arrive than the rest of it. You can rationalise pretty much anything. I’m getting a Maths module from Make Noise. It’s a flexible little thing that can work as a multiple, LFO, sub-oscillator and envelope among other things. It also seems to be just the right kind of puzzling.

On the oscillator side I finally got bored waiting for the Z3000s (they would be 2-3 weeks more). I found a Z3000 in stock at Escape from noise and decided to let my sudden craving for a crazier oscillator to set the agenda. Schneidersladen had a Make Noise DPO in stock so I switched my order there from the two Z3000 oscillators to the DPO, with is a dual oscillator module, and ordered one Z3000 from Escape From Noise. The upshot is that my modular lost a Z3000 but gained a DPO. +1 oscillator! The Z3000 should arrive any day while the DPO and the missing Doepfer modules will arrive hopefully next week. Luckily for me I’m running out of rack space so there’s a natural barrier to further indulgence.

I’ll write some more about the Maths and DPO modules as I get them. You may notice that both of them are present in my parodical West Coast example system. I’m quickly seeing how addictive this hobby really is. How the hell can my brain confuse synth modules with nuts and berries? Collecting Eurorack modules won’t help me get through the winter without starving! The collecting instinct is truly a fucked up piece of neurological wiring…

If you go on a Eurorack forum and ask people to name one essential module that everyone needs in their rack I can almost guarantee that the most common suggestion would be for Maths from Make Noise. I think it is popular for three main reasons: it is very flexible, it is compositional (meaning that it consists of a bunch of basic functions that can be combined to create more advanced things) and it is a bit inscrutable — very much like modular synthesisers in general!

Make Noise describes the Maths variously as a signal generator and as an analogue computer which can be impressive and unenlightening in equal measure.

So what the heck is it? Think of it as the Swiss Army Knife of your rack; it does a little bit of everything. It can generate envelopes. It can repeat those envelopes, creating an oscillator. It can scale, invert and combine voltages in various ways. And it makes you feel clever.

Maths consists of four separate channels that can affect an input signal in various ways. The input channel jacks are “normalled” which means that they have an internal connection that is broken when a plug is inserted. This is used to provide one function when the input is connected and another function when the input is unconnected.

When the inputs are unconnected channels 1 and 4 can generate a simple Attack-Decay envelope with adjustable attack and decay times. The envelope can be triggered on a CV pulse (like a normal envelope) or cycle continuously while the Cycle button is engaged or when the CV Cycle input is high. The length of the envelope varies from the glacial (about 2 cycles per hour) to the quite fast (1kHz) so it can function both as an LFO and as a sub-oscillator.

When the Signal input of channel 1 or 4 is connected then the that channel functions as an envelope follower.

Channel 1 has a curious but useful feature where the End Of Rise (EOR) output generates a pulse signal when the channel 1 envelope has reached its maximum value. This can be used as a clock signal or a pulse wave with the length of the envelope determining the frequency. Channel 4 has a similar feature but it generates the pulse at the End Of Cycle (EOC) output when the channel 4 envelope has reached the end of one whole cycle.

Channels 2 and 3 are less sophisticated. Channel 2 generates a constant +10V signal and channel 3 generates a constant +5V signal. All channels pass through knob-controlled “attenuverters” which are circuits that can amplify, attenuate or invert a signal. This can be used to adjust the output voltage from any channel so, for example, if you need a +2V offset signal you could pick channel 2 or 3 and adjust the channel attenuverter until the channel outputs the desired offset. If a signal is sent to a channel input then the attenuverter for that channel affects the incoming signal instead (as expected).

All channels are by default routed to three logic function buses called OR, SUM and INV. This may sound complicated but isn’t really.

OR means that the highest voltage is present at the output. For example, if the channel 1 input is 0V, channel 2 is 1V, channel 3 is 0V and channel 4 is 3V then the OR output would be 3V. If the channel 2 input rises to 4V and the others stay the same then the OR output will change to 4V.

SUM is the sum of the input voltages. In the example above the SUM output would be 4V (1V + 3V) initially and then 7V (4V + 3V) after the channel 2 input has risen. Since the attenuverters can invert signals you can also use this with the SUM bus to subtract one signal from another.

INV is the inverted SUM, so it would start at -4V and end up at -7V.

So far a bit tricky, but not mind-meltingly hard. The fun stuff begins when you start to combine the different channels, feeding them back into each other. If you want to understand what goes on with that stuff I highly recommend getting an oscilloscope. The Maths manual has lots of examples of things you can do with the module: ADSR envelopes, signal peak detection, signal rectification and many others.

I bought a Maths primarily because it can generate envelopes, audio signals and perform signal amplification and inversion. It packs a lot of functionality into a reasonably sized and priced module. And yes, it makes me feel like a rocket scientist when I have to plan my envelope using pen and paper instead of just clicking somewhere.

Make Noise modules have a very peculiar look that some people love, some people hate and some people feel ambivalent about. Sort of like absolutely anything then. I’m in the ambivalent camp. In my eyes some modules, like the Dual Prismatic Oscillator, looks great and some modules look confusing or downright ugly. I didn’t really like the look of the Maths module but fortunately a company called Grayscale offers alternate panels for many of the Make Noise modules so I ordered a Maths panel from them. I must admit that the unadulterated Maths module looked a lot better in real life than it does in photos, so I could probably have lived with the original panel, but the Grayscale alternate is a lot neater and easier to understand.