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Korg Opsix

FM Synthesizer By Gordon Reid
Published February 2021

Korg Opsix

Korg’s Opsix takes FM synthesis to places it’s never been before.

When Korg showed a couple of concept synths at the 2020 NAMM Convention many eyes turned to something called the Opsix, which looked much like a 21st‑century DX7; solid, and complete with a full‑size five‑octave keyboard. But when the pre‑production Opsix was deposited on my doorstep a few months ago it was a very different beastie — small, light, and with a 37‑note keyboard. So, how well has Korg’s enhanced FM sound engine been squeezed into another of its baby synthesizers?

The Voice Generators

Instead of conventional analogue or digital oscillators, FM synths have Operators that each comprises a waveform generator and a dedicated envelope generator that controls its pitch, as well as various inputs that allow other mathematical functions to affect its level and pitch. These Operators can then be combined in various ways in a given Program using an ‘algorithm’. In the simplest case, two Operators can sit side‑by‑side to be mixed like two conventional oscillators or the output of one (the ‘modulator’) can feed the frequency control input of the other (the ‘carrier’) to create new tones that can then be made to evolve in interesting ways. You might think that this architecture would be insufficient to create useful sounds, but synth history proves otherwise: in the early ’80s Yamaha launched four important preset synths based upon 2‑op FM — the GS1, its sibling the GS2, plus the CE20 and CE25. But it was Yamaha’s programmable 6‑op architecture with its 32 preset algorithms that, for most people, has come to define FM. That’s not surprising; I’ve just described the DX7.

As its name implies, the Opsix is also a 6‑op synth, but it isn’t constrained to conventional FM synthesis because its Operators offer four additional modes of sound generation. Furthermore, each Operator generates no fewer than 21 initial waveforms. The first three of these are sine waves of various resolutions, and these are joined by a triangle wave and two approximations to sawtooth and square waves. There are then 11 ‘additive’ waves and two noise generators. The Opsix also offers 40 preset algorithms and you can add a 41st that you design yourself using the User Algorithm programmer (see box).

The first synthesis mode is DX‑style FM with an additional Width parameter that determines the duration of the selected waveform as a percentage of the wavelength at any given frequency. If this sounds a bit exotic, don’t worry, it’s obvious when you use the on‑board oscilloscope to show you what’s going on, and it allows you to obtain spectra that reach far beyond the 21 starting points.

The second is ring modulation. Here, the signal produced by the modulator can be rectified by a user‑determined amount and then, when directed to the carrier’s modulation input, it generates amplitude modulation rather than frequency modulation. A mixer then allows you to determine the proportions of the original carrier and the AM‑derived signals sent to the next stage in the signal path.

The third mixes the modulator and carrier signals and then passes the result through a dedicated resonant filter that tracks the carrier pitch. Twelve filter types are available including emulations of the MS20’s low‑pass and high‑pass filters.

The fourth passes the carrier signal through a filter that again tracks the carrier pitch, but uses the modulator to modulate that filter’s cutoff frequency. I like this; there are things that you can do with filter FM that are hard to emulate by other methods.

The final mode is a waveshaper. This allows you to control the level of the Operator’s output, and folds back any waveform that protrudes above a fixed threshold. At low gains this makes the signal brighter; as you increase the gain the sound can become extreme and harsh, especially when you fold the waveform multiple times. You can also add a DC offset, which changes the harmonic structure of the output in yet more ways. (For example, you can bias the wave so that only one ‘side’ is folded, which sounds quite different from symmetrical wave folding.) If this were the only mode offered by the Opsix, you could still spend a good chunk of your life experimenting with it.

Operator controls and data entry knobs.Operator controls and data entry knobs.

In all five modes, you can decide whether an Operator tracks the pitch that you play or has a fixed frequency. If the former, you can tune it to ratios of the fundamental (octaves, fifths and so on) and detune it. But whereas the DX7’s lowest ratio was 1/2 (an octave below the fundamental) the lowest on the Opsix is 1/128, which can send the Operator deep into LFO territory.

The Operators also have three phase modes: all Operators key‑sync’ed, all Operators key‑sync’ed with random phase at the onset of each note, and Random. The last of these is a revelation — it’s FM with subtle differences on each note and it sounds far more organic than fixed phase FM. Furthermore, there’s a LoFi mode that decreases the resolution of the waveforms and control signals, although the effect of this is perhaps too subtle to be important. Finally in this area, there’s a parameter to set the maximum feedback amount within the algorithm, thus allowing you to programme an Operator’s feedback and audio output levels separately.

In addition to using each Operator’s dedicated envelope generator, you can affect its level using velocity, key tracking (defined in the standard DX fashion), and a clock sync’able and key‑sync’able LFO. Called LFO1, this generates no fewer than 23 waveforms including various stepped, random and guitar‑like (positive only) waves, and can itself be affected by 13 control sources. The pitch of each Operator can also be modulated using an external envelope (EG1) and the aforementioned LFO1.

If there’s an unexpected limitation here, it’s in the shapes that you can obtain from these envelope generators. Whereas those in the DX7 had five‑stages with assignable levels, those in the Opsix are ADSRs, so there’s one fewer stage and much less flexibility in the shapes that you can obtain. Sure, you can modify the curve of the envelopes’ Decay and Release stages from linear to exponential, but this doesn’t come close to making up for the shortfall. Korg’s engineers must have understood the limitations that this would impose... which makes me wonder why they did it.

Once the Operators have generated the initial signal for each voice, this passes through a resonant filter section that offers various low‑pass, high‑pass, band‑pass and band reject responses, and includes models of the self‑oscillating filters from the MS20 and the Polysix. By default, you can modulate the filter cutoff frequency using LFO2 and EG2 (each of which allows you to select a source to control the amount of modulation dynamically) and Key tracking is again provided.

The audio now passes to three effects units in series. You can select the effect inserted into each slot from an extensive list — modulation effects, amp models, synchronised delays, reverbs and more — and, while they’re not the most sophisticated in the world, they make a huge difference to your sounds. (In a good way.) Finally, the signal reaches the master volume control and the outputs.

The Opsix is compact and lightweight, measuring 565 x 338 x 90 mm and weighing in at just shy of 3kg.The Opsix is compact and lightweight, measuring 565 x 338 x 90 mm and weighing in at just shy of 3kg.

Modulation, Arpeggios & Sequencing

There wasn’t much in the way of conventional modulation on early FM synths, but that’s not the case here. In addition to EG1, LFO1, EG2 and LFO2, there’s a third pair, EG3 and LFO3, that are not pre‑connected to anything, and all six are connected to a thing called a Virtual Patch, which we would more usually call a 12‑slot modulation matrix. You can select one of 16 modulation sources for each slot (including these envelopes and LFO as well as your choice of a MIDI CC, aftertouch and poly‑aftertouch) and the amount by which this is applied can be controlled using your selection from 13 controllers. You can then direct the resulting signal to one of 77 possible destinations within the Program.

Driven by the same clock as the LFOs as well as any appropriate effects and the sequencer, the latching arpeggiator plays across one to four octaves with seven patterns: as played, up, down, alternating with top and bottom notes repeated or not, random, and ‘trigger’ which replays all notes simultaneously. The confusingly named Reso (resolution) controls the arpeggio speed, while the Gate time ranges from zero to 100‑percent of the step length. Having defined an arpeggio, you can then use the mod matrix to modulate the resolution, gate time and octave, which suggests all manner of interesting possibilities. However... despite what it says in the manual (which I understand is now being corrected) the output from the arpeggiator is not transmitted over MIDI. Instead, the notes that define the arpeggio are transmitted.

Alongside this, the six‑note polyphonic sequencer is much deeper than it appears. Recording in step time, you can program notes, velocities, gate times and swing, and you can offset the start of any given note to create grooves and strums. Six playback modes are provided: forward, backward, alternate, Center (which plays from the outermost left‑hand and right‑hand steps inward), Even/Odd, and random. Once recorded, you can erase, copy, mute and jump steps, and more detailed editing is available via the menus.

Alternatively, you can record in real time from the keyboard or over MIDI. Once you do, it’s apparent that the internal resolution is far greater than the visible 16 steps. I don’t know what the maximum is but, if you don’t invoke quantisation, you appear to be able to record up to six notes freely per step. Things then become even more interesting when you realise that the sequencer can drive the arpeggiator. If you set the sequencer to replay at a low speed and the arpeggiator to a high one you can obtain scores of ‘virtual steps’ per step. What’s the maximum within a single cycle of 16 steps? I haven’t the faintest idea, but it’s a lot. Upon playback, you can also make any note that you play on the keyboard the root note of the sequence, which means that you can transpose it in real time.

Now remember that one of the arpeggio modes in ‘as played’... do you see where I’m going with this? But this isn’t the full extent of the sequencer’s capabilities because, in addition to the six lanes for notes, another six provide motion sequencing: real‑time recording and quantised playback of changes made to the knobs, sliders and controllers. Having recorded a motion sequence in a given lane, you can determine the transitions from one step to the next — instant, linear, exponential or logarithmic — and edit it further before directing it to an extensive list of destinations that include the on/off status, pattern, speed, number of octaves and gate time of the arpeggiator! But again, the output from the arpeggiator isn’t transmitted over MIDI. The sequence is, as are the notes that you play on the keyboard, so there are some interesting possibilities here, but it would have been great if the whole creation could be transmitted.

In a world of reissues, the Opsix is a rarity — a synth that’s capable of generating something new.

In Use

Korg Opsix illuminated FM controls and sliders.Korg Opsix illuminated FM controls and sliders.

Programming the Opsix is nicer than I had expected. The knobs feel solid, the small monochrome screen is clear, and there’s not too much menu diving. Given how narrow it is, there’s not much space for controls so I’m impressed that its designers managed to make it as accessible as they have. For example, the six knobs for the Operator ratios and the six sliders for the Operator levels make it simple to obtain new timbres even if you have no idea of what FM does or how it does it. Korg has even colour‑coded these, with carriers illuminated in red and modulators in blue. And, if you program an algorithm in which an Operator acts as both a carrier and a modulator, it turns purple. With the Shift button depressed, all 12 controls turn green whereupon the knobs choose the Operators’ modes and the sliders change their waveforms. This is streets ahead of traditional FM programming.

On the other side of the screen there are six knobs named A to F that provide the majority of control over the sound that you’re creating. It works like this. First, you select the programming section that you want to adjust. Having done so, the first of its pages will appear on the screen. You can then step through further pages using the appropriate buttons. When you reach the page you want, you’ll find that it offers six parameters mapped to those knobs.

Of course, not everyone wants to delve this deeply and, for them, there’s a Ramdomise function. You can select which elements of the sound you want to randomise and by how much, but I rarely obtained anything useful from this. Fortunately, Shift‑Randomise reinitialises any sound to a sensible set of Init parameters.

The final major group of controls comprises 22 buttons that control the arpeggiator and sequencer, as well as storing and selecting up to 16 sounds within four banks of favourites. These are always welcome, whether you use them for set lists, studio projects, or whatever. There are many other nice operational features that there’s no room to discuss here, but one deserves special mention. This is the ability to modify the Attacks and Decays/Releases for all of the operators simultaneously, similar to controlling the VCF and VCA contours on an analogue synth.

Korg Opsix sequencer and favourite controls.Korg Opsix sequencer and favourite controls.

Moving on, I was delighted to find that the Opsix’s CCs conform to the MIDI v2 implementation and, where parameters lie outside the specification, otherwise unused CCs have been chosen so there’s no risk of something untoward happening. Furthermore, all of the knobs and faders on the top panel send the appropriate CCs, so automation is going to be simple.

The Sound

The factory Programs comprise 225 finished sounds and 25 templates within the synth’s 500 memories. Many of these are impressive, especially if you invoke the sequencer and experiment with modulation. One of my favourites is Program 53: Strum Down, which demonstrates how far the sequencer can exceed your expectations. Another is 217: Electric Drum Kit. Although the Opsix is mono‑timbral, this is a sequenced percussion kit containing six instruments (kick, snare, hi‑hats, and so on) created using individual Operators distributed across the keyboard and separated using brick‑wall key‑tracking. There are many other Programs that use this trick — for example, four‑on‑the‑floor kicks combined with stabs, and much else. Kudos to the programmers; this is clever stuff.

But it’s when you push the Opsix in other ways that things get really interesting. One of my earliest patches used all six un‑sync’ed Operators as carriers, each generating a sawtooth and each detuned a tad from the next. The result was a six‑oscillator per note polysynth that would grace any recording. I developed this to obtain all manner of thick pads, quasi‑orchestral ensembles, and other sounds that I suspect are unique to the Opsix.

I then set up sounds that used Operators in different modes within a given Program to obtain things like tinkling e‑pianos over ensembles. It wasn’t layering in the workstation sense, but the results were nonetheless impressive. Moving on, I created sounds using the waveshaping, filter and filter FM modes, and these took me into altogether different realms. For example, I selected noise as all six Operators’ waveform and programmed self‑oscillating filters in Filter mode to create ghostly voices. Other experiments ranged from extra‑sparkly fairy dust to the street noise in the City of Dis. These Programs rarely fooled me into thinking that I was playing an analogue polysynth, a conventional VA, let alone a PCM‑based workstation, but that’s not a criticism. If you’re programming and playing an FM‑based synth, even one with extra modes and filters, you should expect to get sounds with an extended FM‑y character. If there’s one obvious limitation (and there is) it’s the Opsix’s inability to create smooth, creamy monosynth sounds, but that’s a bit like criticising the PS3300 for being unable to produce a convincing grand piano.

Before leaving the topic of sounds, I must compliment Korg on its Smooth Sound Transition feature, which allows you to hold existing notes complete with their effects for up to 60 seconds after selecting and starting to play a new Program. Various implementations of this idea have featured on workstations for a few years, and it’s nice to discover it so well implemented here.

Second Thoughts?

But despite all the good stuff, I’m always disappointed when a powerful synth has a keyboard that’s too narrow to play conventionally, lacks sufficient control pedal inputs, and is unable to generate aftertouch or poly‑aftertouch even though the sound engine is capable of recognising it. Sure, velocity and release velocity sensitivity are welcome, but even the DX7 could control a sound’s pitch, loudness and tone using aftertouch, and that was nearly 40 years ago. Furthermore, the Opsix’s keyboard is light and springy, and unsuited to the kind of playing that the sound engine deserves. To complete my moan, the pitch‑bend and modulation wheels lie behind the keyboard rather than alongside it. In short, the Opsix wasn’t designed for the likes of me, so I played it from a MIDI controller and used it as an expander. What a difference it made!

That aside, I only encountered one problem with the Opsix — an occasional hanging note. Given that I was trying to make a pre‑production unit jump through hoops that Korg had probably never planned for it, I was impressed by its reliability.

Final Thoughts

In a world of reissues, the Opsix is a rarity — a synth that’s capable of generating something new. Some people will love it; others will moan that it doesn’t recreate their favourite DX7 Program to the 10th decimal place. But don’t listen to the haters. The Opsix offers an impressive marriage of depth and programmability, and it will take you a long time to stop discovering new things that you can do with it. Nonetheless, its keyboard and lack of performance inputs are (at best) controversial. Why did Korg hobble it in this way? To me, the answer seems obvious. The bedroom is the hottest market in electronic music at the moment and in this environment small is good, aftertouch is rarely an issue, and few want a keyboard the width of a Bösendorfer.

Finally, there’s the question of price. If you equate the size of a keyboard to its value, you might view the Opsix as expensive. But if it were the £700$1000 module of a £1800$2000 flagship synth, I don’t think that anyone would bat an eyelid. However you view it, there’s a heck of a lot of synthesizer inside it.

The Rear Panel

Korg Opsix rear panel.

The audio I/O on the rear panel is limited to just quarter‑inch L/R audio outputs and a quarter‑inch stereo headphones output. (It would be nice if this were at the front, but it isn’t.) There’s only one controller input, and this is for a sustain pedal. MIDI is catered for by five‑pin DIN In and Out sockets as well as class‑compliant USB B, which supports the librarian as well as MIDI, but not audio. The final hole is a barrel socket for a 12V DC external power supply, and a small cutout in the case acts as a cable hook.

Note, however, that the sockets are recessed and it’s impossible to find them unless you walk around the back or pick the Opsix up and flip it over. It would have been a good idea to copy their legends to the top surface of the synth.

User Algorithms

The Opsix doesn’t have the luxury of a large display, but its two User Algorithm screens nonetheless allow brave souls to create new algorithms and try to interpret what’s going on. To use them, you select the User Algorithm option for the current sound and the first User Algorithm screen then allows you to select each Operator and determine whether it feeds back upon itself and how it’s affected by any or all of the others, and by how much. You can step through all six to create up to 36 pathways but, if you go wild, you’ll never work out what’s happening, especially if you use modulation to affect the nature of the algorithm dynamically. The second page allows you to determine which Operators are acting as carriers. This means that all six can act simultaneously as both carriers and modulators. Abandon hope all ye who enter here!

Emulating The DX7

The Opsix can receive SysEx files containing sound banks (32 Programs) conforming to the format used by Yamaha’s first‑generation 6‑op FM synthesizers. Nonetheless, the architecture of the Opsix is different from that of the DX/TX series, so the Programs must pass through a conversion process before being stored. I dumped a selection of Yamaha’s factory banks into both the Opsix and a TX7.

Comparing the two was fascinating; some Programs sounded similar, while others were miles apart. This is not surprising; the Opsix’s ADSRs will rarely recreate the DX’s five‑stage envelopes, and this will have a significant effect on many sounds. As for the underlying characters of the two synths, there’s something ineffably DX‑y about early DXs and TXs — a sort of globular, bell‑like, glassy character. The Opsix is more neutral, so it’s not a drop‑in replacement.

Voice modes, Polyphony & Unison

The Opsix offers three voice modes — polyphonic, monophonic and legato — plus two modes of portamento with a maximum transition time of 50 seconds. There’s also unison of between two and eight voices with programmable detune and stereo spread, all of which makes it possible to create everything from gentle chorusing to thick ensembles. However, there’s a cost in terms of polyphony. Most of the time the Opsix is 32‑voice polyphonic but, if you use its HD waves, the maximum drops to 24 voices. This means that the maximum number of simultaneous notes can drop to four or three, respectively. Thirty‑two voices are adequate for playing your favourite DX7 e‑pianos, but there are other times when you’ll want to think more carefully about how you allocate them.

The Librarian

Korg Opsix librarian software.

Buying an Opsix entitles you to download a huge swath of bundled software including Korg’s Gadget 2, Module and M1 Le, as well as software from the likes of AAS, Reason Studios and UVI. Unfortunately, the thing that’s missing from the bundle is an Opsix editor (which would make it easier to understand and program). Nonetheless, I was able to test the beta version of its librarian.

To use this, I first had to upgrade the synth to v1.0.1 firmware, but this proved to be trivial and took just moments. I was then able to launch the librarian on my MacBook Pro and the contents of the Opsix appeared a few seconds later. As expected, you can move Programs around within libraries, edit their names (which is much easier than using the parameter‑access system on the synth itself), change their categories, edit the author’s name, and add notes to each. You can also create banks of Favourites quickly and easily, which will be most bodacious if your band is forever changing its set‑list.


  • It offers immense flexibility.
  • It produces that rarity — new sounds!
  • It’s much quicker and easier to program than you might imagine.
  • At less than 3kg, it’s extremely transportable.
  • It comes with a valuable software bundle.


  • The keyboard is too narrow and of insufficient quality to take full advantage of the sound engine.
  • It was a poor decision to reduce the FM envelopes to just four stages.
  • It has just one pedal input and responds to aftertouch only over MIDI.
  • The manual has errors and omissions (although these are being addressed).
  • There’s no audio over USB.
  • It uses an external power supply.


Another superb sound engine from Korg encapsulated in a package that won’t appeal to everyone.


£649 including VAT.