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The Weird World of Doepfer

Doepfer's Strangest Eurorack Modules Explained

Robin Vincent · 01/16/25

Doepfer Musikelektronik is both the most influential and one of the most peculiar manufacturers in modern modular synthesis. The founder, Dieter Döpfer, has been developing synthesizer technology for decades. He built synths for Kraftwerk, developed the Sound Sampler for the Commodore 64 and released MIDI controllers of all kinds in the 1980s and 1990s. But it was his fascination with Moog and Roland modular synthesizers that led to the creation of the A-100 modular system in 1995—which to date is likely his greatest contribution to music technology. The A-100 was built from individual, patchable, CV-controlled synthesizer modules just like the Moog Modular and Roland System 100m. To keep costs down, he built it into a familiar racking format that was readily available at the time and often used for test equipment. The format was called Eurorack.

Now thirty years since he designed the original handful of modules for the A-100 system, Dieter has continued to develop and release new ideas and emulations in modular form. Currently, Perfect Circuit carries over 200 Doepfer products, and Doepfer lists around 200 Eurorack modules on its website. Every year, Doepfer seems to come up with more.

The catalogue is huge, unwieldy, and often difficult to penetrate. Doepfer modules are sometimes mistakenly dismissed as being a bit basic and utilitarian—but that's only because you haven't fought hard enough through their somewhat-dated website and rested your gaze upon some of the more outlying modules. Dieter has a love of creative and quirky modules that approach control voltage and synthesis in fascinating and inventive ways. In this article, we're going to check out some of the more esoteric modules that you perhaps won't find anywhere else.

The Trautonium Modules

A good place to start is with Doepfer's interest in the Trautonium. The Trautonium was an electronic musical instrument invented by Friedrich Trautwein in 1930s Berlin. Oskar Sala later enhanced it in the 1950s as the Mixtur-Trautonium. It consisted of an electronic sound generation unit controlled by applying a finger to a resistive wire, pushing some foot pedals and flicking some switches. The Trautonium Project led Doepfer to design a number of specialized modules that could be combined with existing A-100 modules to approximate the Trautonium experience.

Key modules were the A-177-2 Foot Controller Interface and the A-104 Trautonium Formant Filter, but these were not that unusual. More interesting were the A-113 Subharmonic Generator and the A-198 Manual Controller.

The A-113 is immediately impressive with its remarkable stack of two-digit displays and feels like something you'd find in a Delorean time machine. It's based on the subharmonic generator of the Mixtur-Trautonium and generates four signals derived by dividing the frequency of an input signal. Each of the four generators sets the division with the buttons and shows it on the display. A knob then controls the level. Footswitch inputs (patched from the A-177-2) can switch between different "mixtures" or divider settings, giving you some very dynamic control. Once you have it set up, you can store up to 50 presets.

The A-198 Manual Control Unit is a simplified version of the string and resistive wire arrangement used in the Trautonium. It's essentially a ribbon controller that uses a positional and pressure sensor to map the placement of your finger. This makes it far less fragile and expensive to maintain while giving it versatile control over any other Eurorack modules.

When you place your finger on the ribbon, a voltage is generated proportionally to the position, and another comes from how hard you are pressing. These voltages are translated through the A-198 TRC module. The ribbon is connected to the front via a USB cable. The top half of the module deals with position, and the bottom half with pressure. The Hold switch allows you to release your finger and the gate to remain open, and the Scale knob sets the range of voltage across the ribbon. For pressure, you can set a threshold at which the gate fires in order to manage the sensitivity.

It's a great way to bring some expressive control into your modular. It's also interesting to note that with the Hold switch open, the voltage will gradually fade over time in true analog fashion—as it's held in a buffered capacitor.

A-110-6 Trapezoid Thru-Zero Quadrature VCO

The A-110-6 Trapezoid Thru-Zero Quadrature VCO is one of Doepfer's most interesting dedicated oscillators. It may not have a very snappy name, but it is truly an unusual oscillator that generates all sorts of interesting shapes.

It has a trapezoid core as opposed to the more common triangle core, and it produces two waveforms set at a 90-degree phase shift. They are called trapezoid sine or TRASIN, and trapezoid cosine or TRACOS. From these two waveforms, you can derive similarly shifted pairs of triangle, sine, sawtooth and square waves, giving this module ten waveform outputs.

This can get really interesting in terms of FM and cross-modulation. The VCO is called "Thru-Zero", which is very popular at the moment, and what this means is that instead of trying (and failing) to generate negative frequencies when modulated, the voltage hits zero and then bounces in the opposite direction. This results in a much cleaner and more musical response to frequency modulation.

The tones you can get out of the A-110-6 are huge, complex and versatile. Or you can switch it into LFO mode and use it as an overfunctional source of modulation.

A-112 Sampler

The A-112 is an 8-bit sampler that can also be used as a wavetable oscillator. Remember the Commodore 64 Sound Sampler? This is that in modular form. It has a whopping 128kB of memory in two banks of 64kB, which gives you about 2 seconds of sampling at 32kHz for each. You can wind that down to 2kHz, giving you a deliciously lo-fi 30 seconds or increase it to 60kHz for about 1 second. The CV input can be used to pitch the samples, and you have a Gate input for triggering. You can record sound directly into the module or transfer across using a MIDI dump via the MIDI ports that look rather ugly right on the front. The memory has a battery, so the samples will stay in the module if you turn it off.

In wavetable mode, the memory is not accessed in sequence like when playing a sample but in pages. Each page contains a slice of the wavetable that can be spun as a waveform, giving you a Wavetable oscillator. Pages can be selected via the Wave CV input to let you scan through up to 256 waveforms within the wavetable, delivering that classic morphing wavetable sound.

You can also use the A-112 as an experimental 8-bit digital delay or pitch shifter. It's not trying to be clever or offer the functionality you often find in samplers, but it has a great vibe for lo-fi sounds and rough and ready sound generation.

A-196 Phase Locked Loop

No, I have no idea what this is either. Phase feels like an effect, and Loop could be sampling, but in either case, it's locked up in some way. Doepfer calls it a PLL, and somehow it can be three completely different things.

On the one hand, it's a VCO, on another it's a phase comparator, and if you have a third hand, then it can be a lowpass filter. All of it is locked into a closed feedback loop so that the internal output of the VCO gets compared to an external signal where the difference is turned into a negative, positive or zero outcome. This resultant voltage is smoothed out by the lowpass filter (which acts more like a slew) and fed back into the pitch control of the VCO.... And here's our feedback loop. So ultimately, the PLL compares the internal oscillator to an external oscillator and tries to match it up by feeding the internal oscillator the difference—right?

What's it for? Well, Doepfer calls it experimental, and it's really up to you how you approach it. In reality, it's a usable VCO packed full of a weird wonkiness that will thrill, surprise and occassionally frustrate you. It's great for effects, weird sync sounds, portamento and twisted bleeps and bloops. As it tries to follow your external VCO it generates all sorts of ticks, slips, syncs and wobbles, which can add a fascinating amount of texture and strangeness to your modular music. That sounds like it could be amazing, and I'm wondering why I don't have one.

A-189-1 VC Bit Modifier / Bit Cruncher

Another snappy title, but this one is really all about bit-crushing. The idea is that the module samples the analog input into digital data and then sends it back out reassembled as analog. That process introduces a certain amount of distortion depending on the bit depth and sample rate. As we crush those bits down to almost nothing, a shocking level of distortion is generated in the resulting output. The sample rate reduction distorts differently. The effect is somewhere between sync, FM, and phase shifting, as reducing the sample rate causes the loss of more and more high frequencies and drags down the perceived pitch and harmonics.

There are 16 different bit-crushing modes that affect how the signals are being crushed or how the bits are being processed. The algorithms are named and described in terms of their function in the manual, but they don't really describe how it makes a difference. The important thing to know is that each of the modes provides a different sound using a similar process, giving you loads of tonal possibilities from satisfying crush to alarmingly broken telephony equipment.

A-188-2 Tapped BBD

At last, a sensible name for a module based around a BBD Bucket Brigade delay circuit. Most BBD delays are relatively simple: you put something in, you set the delay time, feedback and wet/dry mix, and you get your signal back with a slightly crunchy echo following behind. We don't tend to think about the circuit or what's happening behind the front panel. Doepfer, on the other hand, likes to have everything out on the table so you can poke about in the innards and pull the maximum amount of possibility from the module.

The idea behind a Tapped BBD is that echoes are produced at six set stages of different sizes, which equates to different delay times with reference to the Delay setting. Each stage has its own output, which allows you to mix and process them separately from the original signal. The A-188-2 has two separate mixing units, allowing you to create two different mixed outputs containing all six stages. You would use short stages for chorus, flanging and Karplus Strong effects and longer stages for more traditional delay/echo effects. Or you could do both at once, taking the relevant outputs separately.

The A-188-2 makes working with delay very deliberate and focused and opens it up to some very interesting feedback loops and modulatable possibilities.

A-178 Theremin Control Voltage Source

Theremins work by using a player's hands to interrupt electromagnetic fields around two antennas. These interruptions cause a capacitance that can be measured and used to control the pitch and volume of the Theremin's oscillator. The A-178 only has the one antenna, but it works in a very similar way.

The idea is that you wave your hand around the antenna, which generates a voltage that you can use in your modular. The range is about 30-40cm. The CV output has an Offset knob so you can dial it into the range that you're after. You've also got a Gate output with a Threshold so you can adjust the sensitivity to new movements.

It's simple and effective. If you wanted to model a Theremin, then you'd need another one to control the VCA while the other controls the pitch. They need to be at least 40cm apart to not interfere with each other.

There's a similar module that uses light rather than an antenna. The A-179-2 has a built-in photoresistor that generates voltage depending on how much light is falling on the sensor. So you can wave your hand around in a similar way to the Theremin. The A-179-2 also comes with the photoresistor on the end of a 2-metre cable so you can move away from your modular and interact with it differently.

A-174-4 3D Joystick

Talking about hands-on control, the A-174-4 is a fascinating take on joystick control. It looks a little ridiculous in that it's big, fat and chunky, but this is for good reason. The A-174-4 works in three dimensions rather than most joysticks' more common XY dimensions.

The added dimension is that you can twist the joystick to control the Z parameter—that's why you need a good solid handful. As well as giving you individual dimensional outputs it can also be used as a quadraphonic panner by pointing the joystick into one of the four corners.

It may look a bit weird, but a 3D Joystick could be enormously useful for expression and multi-dimensional modulation.

A-174-2 Wheels

The A-174-2 is another usefully expressive device. It looks like the pitch and mod wheels you'd normally find on a synthesizer, and that is the general idea. Of course, in modular, you can patch CV to whatever you want, so you can use them in any way you wish.

Wheel 1 is spring-loaded, bipolar +-5.5V and returns to the centre like a traditional pitch-bend wheel. Wheel 2 isn't sprung, and simple outputs CV from 0V at the bottom to +8V at the top. Simple, useful, modulation wheels.

One oddity is that the bottom of the potentiometer pokes out to the side of the module, so you'll need room on the side. Why Doepfer couldn't extend the front panel to cover it is beyond me.

A-126-2 Frequency Shifter

A frequency shifter is different from a pitch shifter because it shifts the entire frequency spectrum of the incoming audio by the same amount, whereas in pitch shifting, the shift is proportional. So essentially, a frequency shifter adds or subtracts, whereas a pitch shifter multiplies or divides. This means that frequency shifting does not retain harmonic relationships, and so are used more as an effect and a generator of textual variations rather than musical changes or transposition.

On the A-126-2 you have coarse and fine frequency control over the amount of shift and a Mix knob to set a balance between how much up shift and down shift you are experiencing because both happen at the same time. There are CV controls over the knobs, and there's also a built-in VCA which can be controlled externally or via in internal envelope follower that tracks the incoming audio. There's also a sine and cosine output which is the output of the internal quadrature oscillator that is used to do the pitch shifting. Sounds a bit like we're running the A-110-6 VCO through the A-196 Phase Locked Loop, but it's probably not remotely similar.

The results are really interesting. It can add a lot of texture and movement and not just in the pitch. Varying the Mix control can radically change the perception of pitch change and create self-modulation and detuning effects. You can use it to add variation to percussive sounds or anything that sounds too static. It has the ability to conjure up inharmonic metallic sounds similar to FM. One particular use is to drop it into the feedback loop of a delay effect and use it to do strange things to the echoes. As with many weird modules, the key to success and satisfaction is experimentation.

A-152 Voltage Addressed Track & Hold / Switch

This one just rolls off the tongue, and the name is probably what makes people ignore this otherwise very useful module. The A-152 is a switching and Track & Hold module that ends up doing more than you think.

Basically, it's a module in three parts. In the switch section, you can feed it 8 inputs and control (or "address") which one is routed to a single output, or you can have a single input and control which of the 8 outputs it is routed to. It's bidirectional and uses the same "Addressed Switch I/O" sockets for both input and output.

In the Track & Hold section, you have one input and 8 outputs. The signal passes to the selected output unchanged until a new output is selected, at which point the voltage is held at the previous output at the time of the switch. As the selected output changes or gets modulated over CV the other outputs hold their voltages creating a cascade of voltages as the selection moves.

The last section is called the Digital Output, which is like individual gate outputs on a sequencer, giving a high gate for whichever output is selected. You can use it to trigger events, envelopes and modulations relevant to the outputs being selected.

All of it is controlled from a single CV input in combination with a manual address knob and some clocking. So, all the happenings are related, even if they are doing different things. There are endless possibilities, from simple sequence switching to audio rate sound generation, to shift registering polyphonic shapes and multiplexing modulation to a single destination. It's the sort of module you could explore indefinitely.

Doepfer is easily overlooked as a source of cheap utilities and standard modules, but dig a little deeper, and you will find some fascinating possibilities.