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Cwejman Synthesis Modules: Part 1

Modular Synth By Gordon Reid
Published November 2008

Wowa Cwejman is already in possession of a fine reputation for esoteric synth modules, but he hasn't run out of ideas yet. Join us as we take a tour of his latest creations...

Cwejman Synthesis ModulesPhoto: Mike CameronThe market for modular analogue synthesizers has never been healthier, and there has never been a wider selection from which to choose. From lunchbox–sized mini–modulars to the wall–sized instruments that you can assemble from large–format manufacturers, there's something for every requirement and every budget.

The most successful manufacturers of recent years are a British company, Analogue Systems, and its German equivalent, Doepfer Musikelektronik, both of which have sold small-format modules (Eurorack-compatible panels with 3.5mm I/O sockets) in quantities that would have seemed unimaginable when Moog, ARP and EMS were ploughing this furrow in the late 1960s and early 1970s. But despite the success of the Brits and the Germans, the engineering high–ground for small–format modular synthesizers belongs to a Swedish company, now known simply as 'Cwejman'.

From December 2006 to February 2007, I reviewed the first seven modules in the Cwejman modular series. I found that they each offered a number of facilities not found on equivalent products from elsewhere and that, while they did not in isolation constitute a particularly exotic synthesizer, integrating them within an existing modular synth would extend the range and quality of the sounds that you could obtain from it.

At the time, I also alluded to five modules in development. The prototypes of these arrived later in 2007, but I encountered a couple of problems with them and returned them without conducting the proposed review. Happily, Wowa Cwejman is a perfectionist who is very open to constructive criticism. It turned out that he had already identified areas in which he could improve the new modules, so he cancelled their launch and set about redesigning them. A few months later, the revised versions appeared and there were now 11 new modules rather than five. A hugely interesting bunch they are too. Forget conventional oscillators, straightforward filters, basic ADSR contour generators, simple LFOs and so on. We're now entering the realms of esoterica such as a frequency shifter, a four–phase generator, a spatial phaser, a quad voltage–controlled resonator and much, much more

VCO6 Multi–output Oscillator

Cwejman Synthesis ModulesThe VCO6 provides six waveforms simultaneously. The first three are straightforward: sine, triangle and sawtooth. The fourth is a sawtooth an octave higher than the previous waves, to which you can apply something akin to pulse width modulation. At low modulation amounts and speeds, it shifts between its basic pitch and the octave below; as you increase depth, it turns into a rich chorus; as you increase modulation rate, a ring–modulator–type sound emerges. The fifth and sixth waves are different types of positive and negative pulse wave, and neither is the typical pulse wave found on other analogue synths. You can apply PWM to these, with different modulation signals directed to each if you wish.

The VCO6 offers two frequency ranges (LFO and audio), three frequency modulation inputs (one offering DC– and AC–coupled FM, and two calibrated to 1V/octave that track over a huge range) plus two types of PWM, and sync. It creates tones that are subtly different from those obtained using a conventional analogue oscillator, especially if you mix some of the uncommon waveforms, and there's so much you can do with it that it's impossible to discuss all its features here. But despite its complexity, it retains the Cwejman signature: it's always clear and precise without ever becoming bland.

PH4 Four–phase Generator & MX4S VC Stereo Mixer

Cwejman Synthesis ModulesNext, let me introduce you to my new friend, the PH4 low–frequency modulator. This is, in essence, an LFO that generates a sine wave or a triangle wave with frequencies in the range 0.025Hz to 50Hz. You can control the waveform using CVs to affect the rate, the level, and the offset. (The last determines whether the wave is centred on 0V for conventional modulation effects, or elsewhere in the plus or minus 5V range for effects such as guitarist's vibrato, which is always positive). But the PH4 is no simple LFO, because it generates the resulting waveform at four phases: zero, 90, 180, and 270 degrees, which makes it a uniquely powerful module. To illustrate this, let me also introduce you to the MX4S Four–channel VC Stereo Mixer.

This, as its name implies, is a voltage controlled 4:2 mixer. It offers input level and pan controls for each channel, plus a single output level control. You can use it as a simple audio mixer if you wish, but you'll have much more fun if you experiment with its nine (yes, nine!) CV inputs. Four of these control the input level for each channel, four control the pan position for each channel, and one controls the overall output level.

So let's now return to the PH4. I took four outputs from the VCO6 and inserted them into the four channels of the MX4S. I then used each of the outputs from the PH4 to modulate the input levels of the MX4S, crossfading through each waveform in turn to create lovely, morphing timbres. Even better, I then directed the modulator's outputs to the pan inputs on the mixer, thus obtaining all four input waveforms simultaneously, but sweeping them endlessly in a circle within the stereo field. Changing the inputs from four waveforms derived from the same oscillator to complex sources such as sound effects CDs yielded even more startling results. We are stepping way, way beyond conventional analogue synthesis here, walking realms that even well-appointed monosynths can only admire from a distance.

SPH2 Spatial Phaser

Cwejman Synthesis ModulesSticking with the concept of phase, let's now turn to the SPH2. This is similar to the classic phasers of the 1970s, but rather than having just a couple of knobs or switches to control the nature of the effect, it has all its guts hanging out (figuratively speaking, of course). The audio passes through dual 14–stage phasers, and you can control the rate and depth of the internal LFO, the initial spectrum (the unmodulated position of the notches) and the amount of feedback (spelled 'feadback' on the panel!). Four CV inputs then allow you to control the internal LFO rate and the 'spectrum' — either the left and right individually, or with both channels locked to a single modulating input. Finally, there are two types of phasing on offer, called Spatial and Rotating. As far as I can tell, these are simply two phase relationships between the sweep of the spectral positions, one close to 'in phase' and the other 'out of phase'.

It sounds complex, doesn't it? Happily, you don't need to understand the engineering to create remarkable sounds with the SPH2. I took a single, attenuated audio signal from one output of the VCO6 and directed this to the audio 'in' on the phaser. I then used a single modulating signal from the PH4 as a spectrum CV, and set the phaser to Spatial mode. Within moments, I was creating the kinds of sounds that I haven't heard in years; sounds that I created on my EMS VCS3 when I had the time and energy to experiment with it.

In short, the SPH2 is perhaps the most productive sci–fi sound effects machine that I have encountered since 1970–something. Sure, it will certainly produce superb, musical phasing effects (and let's not overlook those!), but as a generator of heavily modulated sweeps, bleeps and bloops, it is simply unsurpassed. If you're hoping to work on the next series of Doctor Who, direct a noise source into one of these and I absolutely guarantee that you'll impress somebody!

DMF2 Two Modes Dual Filter

Cwejman Synthesis ModulesCwejman synthesizers are renowned for their filters, which offer very steep slopes (up to 36dB/octave) and do exactly what you ask them to, no matter how you abuse them. The DMF2 offers two of these, called Filter A and Filter B, and each offers a cutoff range of (notionally) 16Hz to 16kHz, 24dB/octave and 36dB/octave options, resonance that will self–oscillate at high values, two voltage control inputs for the cutoff frequency, and voltage control of the amount of resonance. There are also two modes available simultaneously: a standard low–pass mode, and a band–pass mode with a 6dB/octave high–pass slope and a 24dB/octave low–pass slope. When presented with an audio signal of low to moderate level, the filters exhibit the usual, precise Cwejman character. Then, as you increase the level, they enter a region that the company calls 'saturation', first adding edge to the sound, and then moving into filter distortion.

If the DMF2 simply offered two independent filters, it would be very desirable, but it has a further set of tricks up its electronic sleeves. There is a combined A+B audio input, a CV input that affects the cutoff frequency of both filters simultaneously, a combined A+B output for the low–pass mode, and a combined A+B output for the band–pass mode. Again, this suggests all manner of possibilities. Given that you can direct the output of one of the filters to the input of the other, you can create a 72dB/octave low–pass filter that totally silences the signal if you present it with pitches a couple of octaves above the cutoff frequency! More interestingly, you can use the two in series as a simple formant filter, controlling the cutoffs and Qs independently or (in the case of the cutoff frequencies) together or with a combination of independent and common CVs, as required! You can even use the DMF2 as a dual sine–wave oscillator. Unfortunately, I found that it did not track the incoming CV perfectly (it went a little flat as I played up the keyboard), but I am sure that this is just a simple matter of calibration.

RES4 Quad VC Resonator

Cwejman Synthesis ModulesThe idea of the formant filter now brings us to the RES4, which is designed, amongst other things, to synthesize acoustic instruments with well–defined resonances and to imitate human vowel sounds. It offers four filters in parallel, each with a centre frequency, a bandwidth control (from half an octave to a few cents) and a level control, all controllable using CVs. There are also master CV inputs for controlling the cutoff frequencies and levels of all four filters in unison, which is extremely useful. Oh yes and there are again two modes, band–pass and notch, which offer subtly different flavours.

Devices of this nature are often fairly 'sledgehammer' in their implementation, with slopes of (typically) 12dB/octave. The RES4 is far more precise, and you can create extremely sharp profiles with it, even sending the filters into self–oscillation at minimum bandwidth (i.e highest 'Q'). What's more, whereas competitors' formant–filter modules tend to offer three filters, the fourth in the RES4 makes it more powerful and more flexible. Mind you, you have to work at formant synthesis to get anything useful from it, but once you've discovered how to set up the RES4, you'll be making it say 'wow!' in no time at all. Then you'll discover that it can create all manner of sweeping, resonant effects. Then you'll discover that you can take the simultaneous outputs from the band–pass and notch modes and direct them to the MX4S to create unique spatial effects. Then you'll discover some of the effects made possible by inverting some of the CVs, and then oh heck, discover it for yourself!

FSH1 Frequency Shifter

Cwejman Synthesis ModulesFrequency shifters are strange devices. Used carelessly, they just create clangorous sounds that soon become boring. Used subtly, they can add interest to existing sounds, but without really justifying their expense. The FSH1 seeks to overcome this in a number of ways. Firstly, it splits the external audio into two signals, with the second phase–shifted by 90 degrees. Then it does the same to the internal sine-wave carrier, thus making it able to generate two frequency–shifted signals with different phases. Secondly, it can act as a traditional frequency shifter (often called a Bode frequency shifter, after the inventor of the technique) or as a ring modulator. Thirdly, it offers two carrier ranges, low (for relatively subtle effects) and audio (for extreme effects and the generation of clangorous tones), with two CV inputs for controlling the carrier frequency. Finally, it offers three outputs: one for each of the shifted signals, and one carrying a mix of the two, with CV control of the mix.

Using the FSH1 as a ring modulator is straightforward, and it creates all the expected tones. Using it as a frequency shifter is more complex. If you listen to the Mix output, the results are little different from ring modulation. But if you use just the 'Up' or 'Down' output, many more effects can be obtained. Alternatively, if you direct Up and Down to the left and right channels, many spatial effects are possible. The FSH1 does not offer instant gratification but, rather, it rewards patient experimentation.

VCFC Voltage Controlled Forward Compressor

Cwejman Synthesis ModulesThe frequency shifter may be a little arcane, but perhaps the most esoteric of these modules is the VCFC compressor. If used conventionally, it acts as a sophisticated but straightforward monophonic compressor/limiter, with knobs for the threshold, ratio, attack (a phenomenal 200 s at its fastest), release and make–up gain. There are also switches for 'hard' and 'soft' knee, conventional compression and less conventional auto gain modes (the latter of which acts a little like a limiter with a constant output level), and for 20dB of input gain, so that you can use the VCFC as a preamp for low–level signals presented to its inputs. There's even a side-chain, which permits all manner of sophisticated dynamic processing effects.

The range of CV controls is no less complete, with inputs for the mode, knee, attack and release, ratio and make–up gain. Finally, in addition to the audio inputs (3.5mm and quarter–inch) and output, there's an output for the internally generated envelope, which can be directed as a CV anywhere else within the synthesizer (or beyond).

To be honest, I have never seen a device quite like this, and the possibilities suggested by voltage control of all these parameters are many and varied. For example (and in common with many compressors) you can add a percussive front-end to many sounds, but you can also now control its character using CVs. This can be excellent for making otherwise bland sounds punch through a mix in unusual ways. Furthermore, directing the generated envelope to other modules such as filters and VCAs suggests even greater possibilities. At some point in the future, I will be very interested to know how people use the VCFC, whether as a conventional compressor/limiter, or as a more advanced creative tool.

INS2MX Dual Insert VCA

Cwejman Synthesis ModulesFinally, we come to the INS2MX. I must admit that this flummoxed me for a while, largely because, in common with all these products, there is no manual for it (and there seem to be no plans to produce any at present). Then I realised that the sockets marked VCA1 and VCA2 are the audio inputs to its two VCAs, and its operation became clear. In short, each half of the unit can be used as a standard VCA, but each also offers a side–chain with send and return sockets into which you can plug effects such as delay lines, phasers, reverbs or anything else you choose. You can direct any mix of the original and effected audio to the output, with level and mix controlled by knobs and/or CVs. Once you've got the hang of this, many additional possibilities become apparent. For example, Cwejman suggests that you could ignore the send socket and present two independent signals into the VCA inputs and their associated return sockets, thus using the module as a stereo, voltage controlled morphing mixer. Alternatively, you could use it to mix CVs, making it a hugely powerful modifier of modulation signals.


In this whistle–stop tour of 12 products, I have tried to give you a flavour of what they can do — not just in terms of technology, but also in terms of some of the patching and sound-generation possibilities that they represent. As you can tell, I was impressed and, with the exception of my comments regarding the rack's power sockets (see 'The Rack Case' box), I have no significant criticisms of any of them. Sure, there were occasional niggles, usually caused by the lack of documentation, but ultimately these were minor enough to not require any discussion. Indeed, everything performed faultlessly throughout this review, did everything that it claimed, and often exceeded my expectations. None of these modules is cheap (see the 'Pricing & Distribution' box) but they are top quality, and they can do things that other synthesizers cannot.

So now you have to decide which is your overriding purchasing criterion: price or performance. If money were no object, I wouldn't hesitate to construct a complete modular synth using a goodly selection of the bread–and–butter devices that I reviewed for SOS in 2006 and 2007, and all of the 11 reviewed here. If I couldn't afford that, I would certainly recommend many of these modules — the PH4, SPH2 and MX4S in particular — to add extra spice and excitement to any existing Eurorack system.

Pricing & Distribution

All Cwejman products for Europe are priced in Euros and distributed by Schneidersbuero in Germany. In the past, this may have been of concern to UK buyers but, in the modern era, there is no difference between ordering from Berlin and ordering from Basingstoke.

The prices, exclusive of local taxes, for the 11 modules in this review, and the rack case and its power supply, are listed below. Schneidersbuero point out that discounts are available with orders of over 1000 Euros.

  • ATT4 184.03 Euros
  • DMF2 419.33 Euros
  • FSH1 486.56 Euros
  • INS2MX 318.49 Euros
  • MX4S 503.36 Euros
  • NS4 318.49 Euros
  • PH4 318.49 Euros
  • RES4 520.17 Euros
  • SPH2 486.55 Euros
  • VCFC 486.56 Euros
  • VCO6 301.68 Euros
  • Rack case 377.31 Euros
  • Power supply 142.02 Euros

Module Pricing

The prices for the 11 modules in this review and the rack case and its power supply are listed below.

  • ATT4 $299.00.
  • DMF2 $590.00.
  • FSH1 $630.00.
  • INS–2MX $460.00.
  • MX4S $775.00.
  • NS4 $485.00.
  • PH4 $440.00.
  • RES4 $750.00.
  • SPH2 $725.00.
  • VCFC $725.00.
  • VCO6 $425.00.
  • Rack Case $695.00.
  • Power Supply $275.00.

The Rack Case

The modules were delivered in a newly designed Cwejman case with a dedicated external power supply unit. The case itself is a heavyweight 7U enclosure with room for two rows of modules plus a 1U strip that includes a master CV control section and five four–way multiples. This makes the whole system feel a bit like a Roland System 100M, which is a good thing. Unfortunately, the case is not suitable for use as a desktop unit because the power-supply sockets (a mains input plus a 'stacking' mains output) are on the back panel. What's more, there are numerous screw heads standing proud from the panel so, if you tried to lay it flat, you would scratch your desk horribly.

The external PSU is a fabulously robust affair. It delivers just 15V AC, but it's built like a much more powerful unit, and it feels far superior to the wall–warts that some manufacturers supply. However, I'm a little concerned by the choice of plug and socket used to deliver the power from the PSU to the rack. For some reason, Cwejman chose to use XLRs, so the synth's 15V power cable is indistinguishable from a short microphone cable or an AES/EBU digital audio cable. Oh yes, and while on the subject, there are two power switches: one on the PSU, and one on the front of the rack, and both have to be 'on' for anything to happen. That's weird.

NS4 Noise Generator

Ah yes noise. The NS4 generates three types of noise simultaneously — white, pink and something called 'red', which does not conform to the scientific definition, but is low–pass filtered at 200Hz or thereabouts. In addition to these, it also offers a fully controllable S&H (sample and hold) generator with knobs and CV control over rate (approximately 0.1Hz to 500Hz) and slew. This is ideal for creating random CVs that you can direct to oscillators and filters for the classic 'Welcome Back My Friends' effects so popular in the 1970s. However, you can also input an external audio signal and clock, and the NS4 will then derive its voltage samples from these, creating all manner of sophisticated effects such as pitch stepping, (quasi–) ring modulation, and gritty waveforms that sound like low bit–rate digital audio.

ATT4 Quad Attenuator

Despite the sophistication of the modules discussed in the main text, synthesizers need simple housekeeping modules that route signals around their architectures. The ATT4 is such a module, and offers four independent inputs, each with a level control knob and associated output. There are no CV inputs, but each channel has a level/bias shifter that converts ± 5V signals into 0V to +5V signals, which is a neat way of matching otherwise incompatible CV outputs and inputs. Usefully, the ATT4 is also transparent at audio frequencies, and the level/bias shifter imparts little, if any, tonal change to full–spectrum signals.


  • High–quality engineering.
  • Excellent ergonomics.
  • Great sound.
  • Unique facilities.


  • Not cheap.
  • No manuals.


To misquote an old TV advertisement, "They look good, they sound good and by golly they do you good". Cwejman modules currently represent the pinnacle of Eurorack synthesizer technology, and while quality costs a little more (well quite a lot more, sometimes) I wouldn't hesitate to recommend them.


See the 'Pricing & Distribution' box.

Schneidersbuero +49 30 9789 4131.