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Analogue Systems Synthesis Modules

RS420 Octave Controller • RS100 MkII Low-pass Filter • RS370 Poly Harmonic Generator By Paul Nagle
Published May 2007

Analogue Systems Synthesis Modules

Analogue Systems' modules continue to develop and evolve. We take a look at a selection of the latest designs.

UK modular-synth manufacturers Analogue Systems certainly haven't amassed their impressive range of Integrator modules through inertia. Even when their output justifiably slows down, they take the opportunity to polish and refine existing gear, and having already revised their primary sound source (the VCO), they recently overhauled another synthesis stalwart, the low-pass Moog-style filter. But the most significant update involves the RS370 Polyphonic Harmonic Generator — which was already a module with surprising depth and power. Before we launch into an evaluation of these improvements, let's begin today's round-up with a look at the only module supplied for review this month that's not covered by an earlier SOS review.

RS420 Octave Controller

As a single point of contact for master tuning and octave selection of up to three VCOs, the RS240 recalls the famous Moog Oscillator Controller of yore. The RS420 has two merged CV inputs and provides up to three simultaneous CV outputs, the idea being that if your oscillators don't have octave switches of their own (as Analogue Systems' VCOs don't), you can conveniently perform octave selection from this one location.

Analogue Systems Synthesis Modules

Typically, your main keyboard or sequencer control voltage would be connected to the primary input. One use for the second CV input could be to connect an LFO and thus add global vibrato to the pitch of all your oscillators. Alternatively, you could connect a keyboard and sequencer to these inputs to obtain sequences that you could manually transpose.

Despite the presence of an arrow on the panel pointing from the octave switch to the CV1 output, this switch actually affects all three outputs simultaneously, for global transposition of plus or minus one octave. The two remaining CV outputs, CV2 and CV3, offer independent shifts of a further plus or minus two octaves. A master tuning control is a vital addition, necessary to globally tune all attached oscillators; its range is approximately one octave. Naturally, this assumes that the VCOs are in tune with each other to begin with! If you require global portamento, a slew control provides up to five seconds of lag to all output CVs.

As with all components of a modular system, there are a variety of ways to use even such an obvious module as this. Further examples might involve connection of an LFO (or two) to the input CVs. With some creative octave selection, you could obtain simultaneous modulation outputs that are exactly double speed, half speed, and so on.

Really, that's all I need to say about the RS420, other than to observe how refreshing it is to encounter such a straightforward module that works exactly as you'd expect.

RS100 MkII Low-pass Filter

Analogue Systems Synthesis ModulesThis is version two of the RS100 module, although there is no clue visually to its extensive internal reworking. In this version, the filter response has been significantly improved. The old RS100 had a response of approximately 30Hz up to 20kHz and, to my ears, sounded muffled and muddy when compared to another, splendid, AS filter, the RS110. The revised model now boasts a range of between 20Hz and 35kHz — and even higher with external CVs applied. It therefore has a markedly deeper bottom end and the whole filter sounds bolder, fatter and cleaner.

Apparently, the transistors and capacitors of the ladder filter are now fully matched and paired, which contributes greatly to the frequency response. There is also far less noise and less coloration of input signal than before, meaning that with resonance down and the filter fully open, the signal suffers no noticeable gain loss — something the original RS100 couldn't boast.

Thanks to heater circuits, Analogue Systems have eliminated many of the stability problems of the earlier module. This means that it is now possible to tune the resonance and play the resulting sine wave reliably as if it were an extra oscillator. That all this has been achieved without sacrificing the essential 'Moogy' character is laudable, although (as with all topics analogue) I am sure there will be those who steadfastly believe that the original, flawed design had to be better because it had more soul, or something.

RS370 Polyphonic Harmonic Generator

A quick recap should suffice here, before we move on to the 3.9 firmware update. For the full story on the RS370, check out my previous review in SOS June 2005.

Analogue Systems Synthesis Modules

The RS370 is a six-voice polyphonic digital synthesizer with four oscillators per voice, two LFOs and six envelopes — all software-generated. It also features a six-voice polyphonic MIDI-CV converter that receives its data on a single MIDI channel. Extensive MIDI and CV control is implemented, ready to modulate your patches into life. Sonically, perhaps the most interesting aspect of the RS370 is its additive synthesis facilities, giving the user control of 32 harmonics. Two monophonic modes go even further, providing a unique form of real-time harmonic content modification. The monophonic mode really requires the RS375 expander (also covered in the June 2005 issue), which was not supplied for review this time. Previously, when in polyphonic mode, Harmonic Wave Morphing offered a means of dynamically sweeping through conventional synth waveforms (sine, square, sawtooth) in the form of a simple wavetable. This feature has now blossomed into full-blown wavetable synthesis.

The latest version of the firmware is 3.9 and existing RS370 owners will be delighted to know that it's available as a free download from the Analogue Systems web site. Once installed, you'll notice that a new menu has appeared: Morph Table. This is where you'll find those original synth waveforms, along with 15 new wavetables based on the PPG 2.0. With names such as 'Digitana', 'Disharmonix' and 'Vocaloid', each of these wavetables feature 256 waveforms through which you can sweep forwards or backwards with an LFO, envelope, external CV or a chosen MIDI controller. This provides the RS370 with a wide range of PPG-like timbres, plus formant-type vocal tones and timbres that are harsh, digital, bright and buzzy. In this, its final incarnation (the operating system is now packed to the gills) the RS370 provides the kind of metallic and bell-like raw material that you simply don't expect to find lurking inside your modular synth! It's not all hard and digital either; start to layer and detune the oscillators and take advantage of the 'analogue drift' parameter, and you'll find it is possible to transform those crisp, icy tones into something warm and lush.

As I navigated the menu system, I must admit I found it just as cumbersome as I did in 2005, especially when it came to editing the multi-stage envelopes — one stage per page. However, there is a lot crammed in here, and with plenty of opportunities for external MIDI or CV control you should be able to devise some personal methods for minimising the menu-hopping.

Slight Revamp: RS95E VCO

Analogue Systems Synthesis Modules This is a minor redesign of Analogue Systems' voltage controlled oscillator. The enhancement this time is the inclusion of a 10-turn Vernier dial for precise numerical pitch adjustment. It's certainly a jolly nice dial. Other than that, the module is as it was when it was reviewed in the June 2002 issue of SOS.

Next I turned to the new factory sounds and noted that the new software didn't open up any additional memory locations. There are still only 46 of these, most of which now contain example patches, including a selection of organs, several analogue-type sounds, fizzing, shimmering pads, weird sound effects and so on. Favourites amongst the new factory sounds include 'Harmonic Choir', in which the mod wheel is used to move through the wavetable, altering the vocal timbre. The 'Glass Gallery' patch is another gorgeous wavetable morph with manual control of both the speed of morphing and the tuning of one of the oscillators. Many of the factory sounds benefit from patching the CV inputs or use either the mod wheel or other MIDI controllers to warp and bend the patch during performance. As starting points from which to explore the RS3790, all of them are useful, but as you grow in confidence you will surely yearn for more memories. Fortunately you can export your patches as MIDI system exclusive data. If you recall from the original review, the misleadingly-labelled MIDI Thru port is actually a soft Thru; it functions as a MIDI output for saving your data on a per-patch basis. When you load a patch in again, it drops into a buffer from which you can then save it to any location.

Incidentally, the MIDI port can now be used as a note overflow, courtesy of the new 'MIDI chain' parameter. So you could buy a second RS370 and create a 12-voice polyphonic synthesizer, if your budget stretches to it!

Analogue Systems have also made some text updates to the panel. The six trigger outputs have been augmented with alternate clock-related labels underneath. Thus, when you are processing MIDI clock, perhaps to generate an analogue clock with which to drive your sequencer or envelopes, you no longer have to remember which port carries the clock, as it's labelled for you.

Finally, there is a wider range of modulation destinations than before. I previously felt the LFOs needed more possibilities and they've now got nine destinations, including full access to the pitch of each of the four oscillators, as well as wavetable position or pulse width. There are 32 destinations for the analogue voltage inputs too, including all the envelope stages, LFO speeds, oscillator levels and tuning plus pulse width or harmonic wave-morphing. Also, up to 16 MIDI controls (or aftertouch or pitch-bend) can be routed to the same range of destinations, which is very flexible indeed.

Module Pricing

  • RS15 MkIII Case, £245.
  • RS95E Voltage Controlled Oscillator, £95.
  • RS100 MkII Low-pass Filter, £85.
  • RS370 Polyphonic Harmonic Generator, £595.
  • RS420 Octave Controller, £245.

All prices include VAT.

RS15 MkIII Case

Last up, Analogue Systems have improved the RS15 6U case supplied for this review. This version includes three rear-mounted LEDs, one for each power rail (+12v, -12v and +5v). These LEDs should all go out after power-up if everything is working correctly, which is rather a neat idea for diagnosing problems, unless you happen to tuck your RS15 away in a flightcase! The MkIII case has the capacity to handle up to 1.5 amps of module draw and comes with 14 AS power sockets and eight Doepfer sockets. It also has internal fuses to protect the modules.


Analogue Systems should be applauded for taking every opportunity to improve their existing range. In the case of the RS100 filter, the redesign is a significant improvement over the original version of the module. So if you wrote it off as indistinct, lacking in bass, or just requiring greater stability, the new version will be a breath of fresh air.

Analogue Systems Synthesis Modules

The RS420 is a useful addition to the AS range, especially given the lack of octave selection on the AS VCOs. Speaking of which, I did like the Vernier dials of the RS95E very much (see the 'Slight Revamp' box), but my opinion from the 2002 review stands: it's time to go back to the drawing board and redo this oscillator from scratch. Keep the great sound but create a less cluttered panel — with octave selectors.

Most interesting of all, the RS370 has been boosted by the addition of 15 digital wavetables and other refinements such as additional modulation destinations. The wavetables open up a world of retro digital tones with highly complex timbral movement — way beyond the previous capabilities of this already versatile module. With so much on offer, it's a shame the menu system remains so laborious, and those 46 memory patches are going to fill up pretty darn quickly. The great news for existing owners of the RS370 is that the firmware upgrade is free — and anyone previously tempted might feel the inclusion of wavetable synthesis tips the balance. If you want a polyphonic synth inside your analogue modular, this remains a uniquely powerful way to get one.

These modules are proof of Analogue Systems' ongoing commitment to the quality of their products. It's exactly the sort of attitude that means anyone looking to build or extend a Eurorack modular system should always consider AS when it comes to sourcing the components. 


  • RS370: Free update for existing owners; version 3.9 adds 15 new wavetables and implements PPG-style wavetable synthesis.
  • RS370: MIDI control and modulation routings extended in this version.
  • RS100: Lovely, clean-sounding low-pass ladder filter with a solid low end and top-end clarity.
  • RS420: Octave Controller does exactly what it says on the tin.


  • RS370: Still has only 46 patch locations.
  • RS370: Menu system remains laborious.
  • RS420: Octave Controller probably wouldn't be needed if AS oscillators had octave switches.


Although an expensive module, the RS370 has new wavetables that add spicy, unique polyphonic timbres to your modular. With its MIDI interface, the only barrier between it and your other analogue gear is the menu system, and even this can be negated (to a certain extent) using external MIDI and voltage control. Of the other modules bundled in this collection, the RS100 stands out as a marked improvement over the earlier version and is now recommended.


See 'Module Pricing' box.

Analogue Systems +44 (0)1726 850103.

+44 (0)1726 850103.