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Moog Subharmonicon - EXCLUSIVE FULL REVIEW

Semi-modular Analogue Synthesizer By Gordon Reid

Moog Subharmonicon

Moog love to push boundaries, and the Subharmonicon may be their most unusual synth yet.

If you purchased an Engineering Workshop ticket for Moogfest 2018, you had the opportunity to build a previously unseen Moog device called the Subharmonicon. Inspired by elements of the Trautonium and the Rhythmicon (both from the early 1930s) this eschewed almost all conventional wisdom about how to design an analogue synth and sequencer. Instead, it offered a curious sound generator based upon fundamentals and sub-harmonics and coupled this to a compositional concept developed by a chap named Joseph Schillinger who, amongst other things, derived rhythmic phrases by dividing a master clock using various integers and then adding the results to create polyrhythms.

Today, the Subharmonicon has been considerably enhanced to create the, umm... Subharmonicon. In truth, this should come as no surprise. The trend was started when, in 2015, Moog released the previous year's workshop project, the Werkstatt. And although the BFAM synth/sequencer built at the 2016 Moogfest has never reappeared in commercial form, the following year's DFAM was released in 2018 and has since become a firm hit with fans of the unusual.

Sound Generation

The Subharmonicon has an analogue signal path that begins with two oscillators. Each of these comprises a conventional pitch generator (the fundamental) plus two sub-harmonic generators that produce sounds ranging from the fundamental pitch 'f' down to f/16 (see 'What Are Sub-harmonics?' box). Three waveform options are offered: sawtooth waves for all three partials, square waves for all three partials, and a hybrid mode of a square wave for the fundamental and sawtooth waves for the sub-harmonics. When the last of these modes is selected, each oscillator's SUB1 sawtooth then provides the source for pulse-width modulation of the fundamental's square wave. The resulting PWM isn't slow enough for the usual chorused sounds unless the fundamental is in the deep bass and, since the driving waveform is a sawtooth rather than the more usual sine or triangle waves, there are bumps in the resulting sound. Happily, you can overcome these limitations by directing an external LFO to the appropriate inputs in the patchbay (see box) to create a much wider and nicer range of PWM sounds.

A mixer allows you to determine the contributions from each of the six sources before the audio is passed to the filter. As is common on Moog's synths, low to moderate signal levels will be passed cleanly while higher levels will distort euphonically. The filter itself is the expected 24dB/octave low-pass ladder, with controls for cutoff frequency and resonance, and it will happily oscillate with resonance values above two o'clock or so. The signal then reaches the audio signal amplifier, which offers nothing more than a Volume control.

It's easy to describe the LFOs and other modulators on the Subharmonicon because there aren't any. If you want to create modulation effects you have to use external devices and direct their outputs to the appropriate inputs in the patchbay. There are, however, two contour generators hard-wired to the VCF and the audio VCA respectively, although these offer nothing more than Attack and Decay phases or, when a Gate is applied and sustained, Attack, Sustain and Release phases. The filter contour is bipolar, with a fastest Attack time quoted as a snappy 1ms, and a slowest quoted as 10s. Its Decay/Release times range from 5ms to 10s. In a novel twist, the contour is simultaneously both single-triggering (during the Attack phase) and multi-triggering (during the Sustain and Release phases). It would be hard to describe in a few words what the benefit of this is; I'll have to confine myself to saying that, when using complex polyrhythms, you can make the filter do things that would be difficult if not impossible to obtain from conventional devices. The amplifier's contour has the same facilities, with the exception that it can only be applied with positive polarity.

The Sequencers

The Subharmonicon's sound generator has been married to two identical four-step sequencers and, unless patched otherwise, the first of these drives VCO1 while the second drives VCO2. You might wonder how much use just four steps might be, especially since you can't chain them into an eight-step sequence or use one sequencer to modulate the other to obtain a 16-step sequence, but the Subharmonicon is all about rhythm rather than melody. It works something like this...

Each sequencer appears to offer nothing more than four small bipolar pots to adjust the CV generated, plus four LEDs to tell you which step is active at any given moment. However, you'll find buttons in the VCO1 and VCO2 sections that determine whether each sequencer controls the pitches of the associated oscillator's fundamental, or its SUB1 or its SUB2 partials, or any combination of these. If you activate just the OSC button, all three partials are affected conventionally and their relationships remain constant. However, if you activate either or both of the SUB buttons, the sequencer values control the integer by which the fundamental is divided to create the sub-harmonics. As you can imagine, this generates very different results.

Like the Mother 32 and the DFAM, the Subharmonicon can be removed from its case, turning it into a 60HP Eurorack module.Like the Mother 32 and the DFAM, the Subharmonicon can be removed from its case, turning it into a 60HP Eurorack module.

Two further facilities determine the notes generated by the sequencers. The first of these is a range switch that offers three settings of ±5 octaves, ±2 octaves and ±1 octave, and this limits the maximum offset generated by each of the sequencers' pots. In a nice touch, it's possible to obtain the full ±5V range from the outputs in the patchbay while restricting the voltages applied to the oscillators to either of the narrower ranges. This means that the sequencers can modulate external 10Vp-p inputs fully while playing notes within a more easily programmed range on the Subharmonicon itself. The second is a quantiser. When this is used, the outputs from both oscillators (and, optionally, the CVs generated by the sequencers at their SEQ1 and SEQ2 outputs) can be quantised in one of four ways: chromatically with equal and just temperaments, and diatonically with equal and just temperaments.

The sequencers are driven by four derivatives (called Rhythms) obtained from a single master clock whose speed is set by the Tempo knob. Its range is 1/3Hz (20bpm) to 50Hz (3000bpm) but, just as the frequencies of the sub-harmonics are determined by f/n, the frequencies of the Rhythms are also determined by f/n where f is now the master clock frequency and n is again an integer from 1 to 16. The four pots in the Polyrhythm section determine the integer used to create each Rhythm, and the buttons beneath these determine which four-step sequence is driven by that Rhythm. Each Rhythm can drive either sequencer, or both, or neither, as you choose, which is much more flexible than the system provided on the original Moogfest version.

The final group of sequencer controls comprises five buttons that turn out to be considerably more complex and capable than they first appear. OK, the action of the Play button is self-evident; press it once to start the sequencers and press it a second time to stop them. But next to this, the Trigger button has two functions. If the EG button is off, it generates a Gate, holding the contours at their Sustain levels until released. But if the EG button is On, it generates a Trigger, reinitialising the contours, but only if they have already reached their Sustain or Decay/Release phases. The EG button also has three functions of its own. When it's Off, triggers are not generated on sequencer steps, so you obtain a legato sequence; when it's On, triggers are generated on every sequencer step; and if you press and hold it for a second or two, it generates a permanent Gate that holds the contours at their maximum levels until you press and hold it again to release them. To the right of this, the Reset button reinitialises the Rhythm generators to their starting positions and both of the sequencers to Step 1. However, if you keep it depressed, it will hold the sequencers at Step 1 even though the contours will continue to be triggered as the Polyrhythm plays, thus generating a rhythm without changing pitch. Finally, pressing Next moves the sequencers forward one step whether they're stopped or running. This is particularly useful when the sequencers are stopped because it allows you to tune the steps accurately before pressing Play.

Playing The Subharmonicon

The commercial Subharmonicon is a big step forward from the version built at Moogfest. The addition of square and PWM waveforms considerably enriches its palette, and the quantisers make it much easier to obtain melodic results. Then there's the replacement of the three-position switches that determined the original's Attack options (1ms, 100ms, or 1s) with fully adjustable knobs, which is an obvious improvement. But the description above should have made it clear that it's still not a conventional synthesizer even though it will, if pushed, produce a small but powerful range of lead and bass sounds. Indeed, if you tune its six partials correctly you can create some huge patches that are highly reminiscent of those obtained from large modular synths with six or more oscillators. What's more, when used this way the bumps in the internally generated PWM are masked so you can achieve even richer sounds without adverse side effects.

I'm delighted that the company is willing to push the boundaries to create something that reaches beyond the sounds and sequences that have pervaded popular music for the past half century.

But that's not what the Subharmonicon is all about (at best it's an hors d'oeuvre before the main course of polyrhythms) and a selection of factory sounds provided in the manual and on overlays provides a good starting point for understanding the sound engine, the sequencers and the patch bay. Blank charts are also provided in the manual (which is always a nice touch when there are no patch memories) as is a full two-page block diagram. I wish that all manufacturers would provide such a diagram as a matter of course. To misquote an old adage, when learning a new synth a picture can be worth a thousand words.

Having gotten to grips with everything (which didn't take long) I found that experimenting was great fun. But I obtained the best and most musical results when I decided beforehand what I was trying to achieve. A piece of paper was invaluable when doing this because, while it's easy to work out in your head what's happening with Rhythm frequencies of, say, f and f/2, it's much harder to envisage the outputs from a pair of sequences based on divisions of, say, f/3, f/7, f/9 and f/13, let alone how these might interact when you switch Rhythms in and out of the polyrhythm. As you might imagine, things can get quite complex and, unless you're careful, quite chaotic. Mind you, serendipitous chaos might be your target, in which case go for it!

Published June 2020