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Korg Sigma Analogue Monosynth By Paul Ward
Published June 2001

a well‑preserved example of a Sigma, from the collection of SOS contributor Gordon Reid.a well‑preserved example of a Sigma, from the collection of SOS contributor Gordon Reid.

Is it a home organ‑style keyboard? Is it a real synth? Is it a complete crock? Paul Ward explores the Korg Sigma, an instrument with a serious identity crisis...

Synth history is littered with oddities, few of them odder than Korg's Sigma. Launched in 1979, and perhaps looking more like a home‑organ add‑on than a 'real' synth, the Sigma instantly found favour with players looking for a simpler alternative to knob‑laden analogues.

The Sigma is surely something of an ugly duckling. It certainly seems a little oversized for its ambitions. Whole expanses of front panel languish in glorious knoblessness, whilst there's so much action going on over by your left hand that you'd think a few controls had dropped off and rolled down there of their own volition. This schizophrenic charm goes right to the heart of the machine.

Split Personality

The oddly named 'Synthe' section is the Sigma's nod in the direction of real sound synthesis.The oddly named 'Synthe' section is the Sigma's nod in the direction of real sound synthesis.

The internal architecture of the essentially preset Sigma is divided into two discrete sections, consisting of the 'Synthe' sounds and the 'Instruments'. Over to the right of the front panel are the 11 grey Instrument tabs, while in the centre of the panel are eight white tabs associated with the Synthe section. These rocker‑tabs were clearly borrowed from the home organs of the late '70s. They actually do a good job of making changes quick and easy, but at a glance it can be difficult to see which ones are selected.

The Instrument tabs move up in pitch from left to right, starting with a 32' Electric Bass and ending with 4' Hammered Percussion. Highlights of the Instruments would have to be the Flute, with its ethereal quality, String, which is great as a screechy lead patch, the delicate Double Reed, and the bouncy Electric Bass. Fuzz Guitar is nicely quirky, but I doubt anyone would use it often. You'd have to be a real optimist to expect these imitations to sound very close to the real thing, but they all have a naive charm that is hard to actually dislike — although the Trumpet probably comes close to being the worst preset sound in my studio!

The Synthe tabs are a strange collection, ranging from a meaty 32' sawtooth wave to a depressingly 'plinky' 4' sawtooth. In between are pulse and square waves, and a nice 16' pulse wave with rich Pulse Width Modulation built in. A noise source is also included, with variable attack/release time.

Above each voice tab is a knob, allowing one parameter of the corresponding voice to be controlled in real time. The Flute, for instance, has a tone control above it, while Electric Bass has a filter cut‑off control. Similar knobs accompany the Synthe tabs, allowing, for example, the attack and release times (but via a single control, remember) of some waveforms to be adjusted. At first this seems quite limiting, and it can be frustrating for anyone brought up on full, hands‑on patch creation. Some of the controls are perhaps strangely chosen, and others are just plain misleading — the 'Decay' control actually governs both Decay and Release. The most frustrating omission for a programmer is likely to be that of any resonance control.

One of the Sigma's real strengths lies in the fact that any number of tabs can be selected simultaneously. Presets that sound a little anaemic on their own suddenly gain a new lease of life when combined with others, and some of the most unusual combinations have a strong voicing that the individual tabs don't exhibit in isolation. The Tuba is a good example of this, giving a strong range of low‑frequency energy to the 16' pulse waveform, for instance. If detuning between different Instrument and Synthe tabs were possible, the Sigma might be capable of some truly meaty patches, but there's obviously some sharing going on inside that keeps them all perfectly locked together.

Sticking With It

The useful dual joystick controllers.The useful dual joystick controllers.

Rather at odds with this slightly weedy preset‑ness are some of the most inspiring performance features available at the time. The Sigma has not one but two joysticks, the left being centre‑sprung and controlling LFO modulation (vibrato) in the forward direction, noise modulation in the backwards direction, and pitch‑bend from left to right. This joystick acts on the machine as a whole, although the amount of modulation is separately adjustable for the Instrument and Synthe sections, allowing for some alluring unison‑to‑interval pitch sweeps. The other, non‑sprung, joystick controls the cut‑off frequency of the Sigma's 12dB/octave high‑ and low‑pass filter — low‑pass left to right, high‑pass front to back. Try to imagine how much fun this is, and also how much control it gives over the Sigma's sound. The quality of the filter is actually pretty good too — silky rather than aggressive. Unfortunately, it's not possible to route the Instrument voices through the same filter. Neither is there any capability to route external sounds through the filter, which alone would almost make the purchase of a Sigma worthwhile.

The fun doesn't end with joystick modulation, because the keyboard is aftertouch‑sensitive, too. Other performance synths of the time, such as ARP's Pro‑Soloist, also had aftertouch, but personally I prefer the Sigma's 'playability'. One man's meat is another man's veggie burger, but if you long to 'feel' aftertouch under your fingers, you'd probably find the Sigma a treat. There's quite a lot of travel over which the modulation naturally feeds in, and you can feel the spring working under the pressure. It's a delightfully tactile, if slightly clunky, experience.

So we have aftertouch: what can we do with it? Use it to control pitch‑bend and vibrato. (What a shame filter cut‑off isn't an option.) It's possible to set the amount of aftertouch sensitivity, the direction of bend (up or down) and whether it is applied to the Instrument sounds, Synthe sounds, or both. Irrespective of aftertouch, vibrato may also be added to the Instrument section, with variable rate and delay time.

Usefully, the Sigma allows for single or multiple envelope triggering. Single triggering is useful for playing a monophonic instrument, such as a flute, where you would not want the attack of the sound to be repeated during deliberate slurring of notes. Bass sounds, on the other hand, might be better to forgivingly re‑trigger during slurred playing, to keep the rhythmic pulse going.

Hold On!

The Key Hold tab may seem like a fairly mundane control, but I'd love to see it on every synth. Flip it on and the Sigma just opens its envelopes and drones. Use it for tuning, use it to play a 'pedal' note while you play another keyboard, use it to apply edits without having to keep one hand on a key. Exciting it isn't, but it is handy to have.

You can click the keyboard up or down an octave from its standard pitch. There's also the ability to switch the keyboard into quarter‑tone tuning. I'm still struggling to imagine the design team meeting that gave us a synth with no filter resonance control, yet quarter‑tone tuning!

The ring modulator is predictably riotous, actually producing a cross‑modulation between the Instrument and Synthe sections, with a bias pitch of its own. Flip all the tabs and add the Ring Modulator to hear what an hour's random fiddling with a modular synth system might conceivably spawn! This is a prime source of sonic unpleasantness that can give your neighbours nightmares for months.

Rear Window

For a stand‑alone performance keyboard, the Sigma has a back panel that seems to be very busy. You're able to take the Synthe and Instrument sounds out separately, or as a mix, and there's a headphone jack. Perhaps surprisingly, the rest of the jacks provide a decent amount of external voltage control for linking up to other analogue synths, sequencers, or a MIDI‑CV converter. (Be aware, though, that the Sigma uses the Hz/Volts system, not the more common 1V‑per‑octave system.) Not only does the Sigma offer the usual key‑trigger and VCO control voltage in/out jacks, but also a VCO frequency modulation and filter cut‑off voltage input for the Synthe section — ostensibly for footpedal control. It's a shame that there are no output voltages for the Sigma's performance controls, which would make two coupled Sigmas react as one. I know of one person who has tried to bully a pair of Sigmas into doing just that, but finding the right points from which to tap the voltages proved extremely difficult, and the idea was eventually abandoned.

Sonic Strengths?

A Sigma needs effects like a dog needs legs —adding a long delay lets the sound breathe, as does a decent pre‑delayed reverb. I have tried adding chorus to bring some movement to the very 'static'‑sounding solo tabs, but this never seems to have the desired effect to my ears, maybe due to the lack of complex harmonics in the sounds. Adding a distortion or fuzz pedal can yield some good results, though.

A Sigma is unlikely to give you that thundering bass, burbling sequence line, or ripping anthemic lead that you may dream of, but it will come up with a world of strangely appealing ear candy that will more than pay for its upkeep. Flipping a fistful of tabs down can produce meaty cacophonies, further enhanced by the ring modulator.

Despite all this, if there was any such thing as a sound‑to‑weight/size ratio, the Sigma would seem a very poor contender, being more 'brash' than 'fat'. None of its voices are 'big' in the way of a Minimoog or Sequential Pro One. Neither do they have the squelchy charm of old Roland machines. And anyone looking to brew up some chillin' home‑made sounds will initially be disappointed at the instrument's lack of tweakability.

Why Buy?

The Sigma is a difficult machine to fairly categorise. It was primarily aimed at being a performance synth (clearly indicated by the aftertouch‑sensitive keyboard and joysticks), and was certainly at home serving up a home–organist's fare of predictably 'safe' lead and bass sounds. However, the Synthe section took a sideways step into the dangerous world of 'real' synthesis. It's certainly closer to being a preset 'instant instrument' than it is a fully‑fledged synthesizer, yet it sports enough control options to take it quite a distance into the kind of territory held by the likes of ARPs, Pro Ones and SH101s.

It's not easy to come up with a long list of famous Sigma users. Jean‑Michel Jarre, Vangelis, Rick Wakeman and Mike Oldfield all dabbled with a Sigma at some time, but I think you'd be hard pushed to have noticed. Most players overlooked the Sigma, and the ones who didn't probably wouldn't have wanted to shout about it much — with all its home‑organ connotations, this was a seriously unfashionable synth. The top edge of the front panel pulls out to provide a music stand, and that's a danger sign by anyone's standards!

I've heard enthusiastic waxings about the sound of the Sigma from time to time over the years, and I've had to ask myself whether they were about the same instrument, because I'd still describe it as generally 'nasal'. To be honest, I've never been a great fan — so it would be reasonable to ask why I bought one! Well, I was constantly exposed to the Sigma for many years while working with synth duo Wavestar and solo artist John Dyson (one of the Sigma's most staunch supporters). After hearing those delicate flutes and reeds, the cutting strings and eerie filter‑swept basses for so long, I realised the Sigma had its strengths, and I knew I had to pick one up if the opportunity arose. At the price I paid, I haven't regretted it one bit, but I'd be hard pushed to recommend it over more capable analogues, or even more capable virtual analogues, for that matter. So I seem to be just as confused as the Sigma's design team!

Look What They've Done To My Sig‑Ma

The story of my own Korg Sigma reads rather like a story from Pet Rescue. I answered an advert and was told that the machine was in complete working order, and that the owner just wanted to get rid of it to save space. I have never seen a synth in such a pitiful state. Finger grease and dirt had accumulated on the controls and keys to the point where a hardened crust decorated each one. You could actually feel the ridges of fingerprints in the grime! The voicing tabs were completely illegible; in fact they all looked the same colour — black! The small control knobs were so coated with dirt that neither their colour nor position indicator were visible. Both joysticks were broken off (although, admittedly, this is common on even the best‑kept Sigmas). Several keys did not work; the mains cable and plug were a serious threat to health; the wooden end‑cheeks were both loosely hanging away from the frame... In short, it was a tragic mess.

Playing the synth revealed some serious electronic faults. Pressing any knob on the right–hand side of the front panel sent the oscillators into orbit, screeching and wailing like an analogue banshee. Some tabs just didn't work at all. I suspected that an impact had jarred the front panel; there was some movement in the right‑hand controls, allowing them to be pushed back into the panel.

Like a child in a pet shop, I just couldn't bring myself to leave the poor thing there (you do not even want to think about what this guy had done to the two Roland SH101s in his possession!). I negotiated an absurdly low price, but left feeling as if I was somehow performing an act of mercy!

Once I had the machine back home, the first job was to strip it down and begin the distasteful process of cleaning away all those years of accumulated grime. To my joy and amazement, I found that the sheer depth of encrusted dirt had actually protected much of the synth! A couple of hours of careful cleaning revealed the controls and legending in all their glory. It was rather like cleaning an oil painting to reveal a masterpiece buried underneath. One of the end cheeks merely needed tightening up, whilst the other, although still a little wobbly, went back into place sufficiently well to wait for a proper fix on another day.

Turning my attentions to the machine's more serious, functional problems, I spent a short time with multi‑meter and crocodile‑clips, determining whether the trouble was a physical, rather than a component, problem. Flexing the main board reproduced the oscillator screech, but the symptom wasn't consistent. After a short time, I had tracked it down to one area of the main voicing circuit board and I began to take a closer look at the tracks. One PCB track was clearly lifted away from the board, whilst another reacted to my pressing it with the multi‑meter's probe. And then I saw the problem. Some heavy impact on the front panel had obviously pushed the control knobs back through the panel, cracking the circuit board en route. It could have been this way for years, gradually worsening with use. I fixed the problem by soldering link wires (cut from the legs of a couple of resistors) to bridge across the tracks, and then applied some super‑glue to the board itself, to try to prevent the crack in the board worsening in future.

The result? A beautiful, perfectly working synth. The end cheeks are tatty and the joysticks are still missing — the threads of the old joysticks are snapped off in place, so replacements would be hard to fit, even if they could be found, which they can't. Unless, of course, any readers out there know better?

Second (Hand) Thoughts

Reasonable second‑hand prices for a Sigma are likely to start at around £100 and peak at £150. Any more than that and I'd suggest that you need to be looking at a very tidy machine, complete with its original manuals. Any less and you'll need to accept a few nips and tucks, though you should check out all the sound circuitry. Although relatively simple in many respects, some of the Sigma's internals are nearly impossible to find replacements for.

Those joysticks are a weak spot, and examples with both still intact seem to be a rare sight these days. Crackly pots are common, but key contacts seem fairly robust.

Don't forget that the Sigma works to the Hz/Volts control voltage standard, not the more common 1V‑per‑octave standard (as mentioned in the main text). Bear this in mind if you're hoping to hook it up to a MIDI‑CV converter.

Published June 2001