Korg MS-20 (1978–1983)

Name: Korg MS-20
Brand: Korg

Two voltage-controlled filters, a patch panel that rewires the whole machine, and a tone that never learned to behave.

Overview

Thirty-seven keys, three octaves, and a control panel that looks more like a laboratory breadboard than an instrument. The MS-20 is the monosynth Korg built to let the player take it apart while playing it. Korg's own advertising for the period put the pitch plainly: "Above and Beyond," with the headline feature being two voltage-controlled filters - low-pass and high-pass - for "extended control of the entire frequency spectrum" (Polyphony, May 1979, p.43). Most monosynths of the late '70s gave you one filter and called it a day. The MS-20 gave you two, in series, and then handed you a patchbay so you could decide what happened next.

The architecture is semi-modular. Out of the box it behaves like a conventional hardwired synth - the internal normalled connections carry signal from oscillators to filters to output without a single cable. Plug something into one of the patch points and you break that internal path, rerouting the signal wherever you send it. So someone who has never patched anything can play the MS-20 right away, and six months later find routing tricks they never suspected. The owner's manual is candid about this being the point: it describes the patch panel diagram as "the point of departure for the adventure that will reveal the countless hidden possibilities" of the instrument.

Design

The oscillator section is a two-VCO affair. VCO 1 covers triangle, sawtooth, pulse with variable width, and square, plus white noise. VCO 2 handles sawtooth, pulse, and square. The manual's scale markings run 32', 16', 8', and 4', and the frequency range is quoted at 4 Hz to 16 kHz. That reaches well below audible bass and far enough up that the top end is mostly useful for sound design rather than notes. VCO 2 has a dedicated pitch control that shifts it over a range of plus or minus one octave, so it can be tuned in unison with VCO 1 or set to any interval - a third, a fifth, whatever the patch calls for. The manual says pitch levels remain stable across the playing range "thanks to Korg technology," which is the kind of claim every manufacturer made in 1978 and which owners have spent forty-odd years testing.

The modulation generator - Korg's term for the LFO - produces triangle and rectangle waveforms simultaneously, with continuously variable shape via the Waveform control. Its frequency is set by a single knob and confirmed visually by a flashing red LED, which is useful on a dark stage. The triangle wave is internally patched to the VCOs and VCFs through their respective MG intensity controls, and both waveforms are available at the patch panel. The manual also notes that a different modulation source - sample and hold, delayed vibrato, wheel-controlled vibrato - can be substituted at any time and routed to all three intensity controls via the T.EXT jack.

Two envelope generators, both of the AR (attack-release) type. Envelope Generator 1 has a variable delay control. This is the part of the MS-20 that generates the most complaints, and the Sound on Sound review from November 1996 (p.256) described the frustration precisely: turn the envelope depth control up and nothing happens until you also turn the sustain level down, and the further down you go, the more of the envelope contour you hear. The reviewer's exasperated "what were Korg thinking of?" sums it up. It's not a broken design, but it is a counterintuitive one, and it takes a while before the interaction between depth and sustain stops feeling like a puzzle.

The two filters are the heart of the instrument. Both are voltage-controlled, both are 12 dB/octave, and both can self-oscillate. The high-pass filter's cutoff knob runs 0 to 10, but the "0" position is fully open - the filter does nothing and the waveform passes through unchanged. Turning the knob up progressively strips harmonic content. The low-pass filter inverts that logic: "10" is fully open, and turning counterclockwise rounds the sound off until, at the lowest setting, the manual admits it becomes "barely recognizable as a sound." Each filter has a Peak control that emphasizes the region just before the cutoff point, and at high Peak settings the filter tips into self-oscillation and becomes a sound source in its own right. That's the MS-20's signature trick, and it's why the synth has spent decades being used as a signal processor as much as an instrument. Feed a drum machine or a guitar through those filters and it comes out as something else.

The patch panel is where the routing gets serious. There's a VCO 1+2 CV input that lets an external controller take over pitch for both oscillators, a VCO 2 CV input for the second oscillator alone, a trigger input for firing the envelope generators from an external source, and a separate EG 1 trigger input. The keyboard's control voltage goes out via KBD CV OUT, so the MS-20 can drive another synth. The manual describes using it that way, patching the MS-20's KBD CV OUT and TRIG OUT into an MS-10's VCO CV IN and TRIG IN so both instruments play together from the MS-20's keyboard. The catch is obvious, and the manual says so: once those cables are in, the MS-10's own keyboard no longer does anything, because the patch jacks have disconnected it. Decide which keyboard you want to use before you set up the patch, the manual advises.

There's also an External Signal Processor, covered in a separate owner's manual supplement, which is the MS-20's pitch-to-voltage and envelope-follower section. It takes an external instrument - the supplement uses guitar as its worked example - and derives control voltage and trigger information from the incoming audio, so the synth tracks what you play. The supplement is honest about the difficulty: all instruments have different ranges in pitch, tone, dynamics, and harmonic content, so the ESP's signal level, low and high cut frequency, CV adjust, and threshold controls all have to be set for the specific instrument. The high cut frequency has to be dialed carefully, because if it's set too high it lets unwanted harmonics through and the pitch-to-voltage converter triggers on the wrong note. Get it right and the MS-20 tracks. Get it wrong and it stutters and misfires. The supplement recommends starting with low cut at 0 and high cut at 10, then working inward - approximate settings for guitar land around low cut 0, high cut 5 to 7.

Context

The MS-20 was released in 1978 and discontinued in 1983, right when analog monosynths were being pushed to the margins by polyphonic and digital instruments. It shared a family with the smaller MS-10 - the manual's expansion section treats the two as a pair, with the MS-20 able to drive the MS-10 via patch cables - and it could be driven by the SQ-10 analog sequencer, either running through programmed steps from the SQ-10's internal clock or advancing one step per key press on the MS-20. The manual describes both approaches as standard practice rather than exotic techniques.

One warning in the manual is the single most common way these synths get damaged. There are eight removable covers on the MS-20, and underneath them are semi-fixed volume controls that were set at the factory. The manual is emphatic: never touch them. Turning them will degrade performance and may damage the synthesizer, because the VCO - "the heart of the synthesizer," in Korg's words - has its balance set at those trimmers, and once that balance is upset, everything downstream goes with it. Anyone buying an MS-20 that has been "adjusted" by a previous owner should budget for a proper recalibration.

Collectibility

The Sound on Sound piece from 1996 treats the MS-20 as a known quantity with known quirks rather than a curiosity, and by the 2000s Korg was reissuing the design in software. An EQ Magazine piece from July 2004 (p.33) describes Korg building softsynth versions of the MS-20 and Polysix, noting that the company held its own high standards because of the originals' warm, fat sound. The same publication, in December 2005 (p.86), covered a Korg Legacy Collection plug-in that let users run the MS-20's distinctive filter, envelope follower, pitch follower, and trigger section as a VST-compatible signal processor inside a DAW.

Condition matters a lot on the used market. The patch jacks are mechanical contacts that wear with use, and a unit with intermittent jacks will produce dropouts and false triggers that are maddening to diagnose. The trimmers under those eight covers are a common source of trouble on units that have passed through careless hands. The keyboard is not velocity-sensitive, so the key press affects the gate and that's all. And the ESP section, when it works, is one of the more unusual features on any monosynth of the era. When it doesn't, most owners simply ignore it.

The 12 dB/octave filters are gentler in slope than the 24 dB designs found on many contemporaries, but the resonance character, the ability to run both filters in series, the self-oscillation and the patch panel mean the instrument covers ground that more conventional monosynths can't reach. It's not a synth for someone who wants a reliable bass patch and nothing more. It's a synth for someone who wants to find out what happens when the high-pass filter is modulated by the LFO while the low-pass is self-oscillating and the external input is carrying a drum loop.

Market Value

┌──────────────────────────────────────────────────────────────────────────┐
│ KORG MS-20                                                      Nov 2026 │
├──────────────────────────────────────────────────────────────────────────┤
│ High      $1,929                                  Latest            $750 │
│ Median    $412                                    Trend                — │
│ Low       $120                                    Volume           ~4/mo │
├──────────────────────────────────────────────────────────────────────────┤
│ $2,000 ┤                                                                 │
│        │                                        .   .                    │
│        │                                                                 │
│ $1,186 ┤                                                .        .       │
│        │                                                                 │
│        │.                                                               .│
│        │                                ...   . .  ..       .. .. .  ..  │
│   $100 ┤                           .                 .  .   .            │
│        ├─────────────────────────────────────────────────────────────────│
│    vol │▂                          ▂    ▂▂▂   ▂ ▅  ▂▅▂  █   ▅▂ ▅▂▂▂  ▂▂ ▂│
│        └─────────────────────────────────────────────────────────────────│
│         2026-01                      2026-05                      2026-08│
└──────────────────────────────────────────────────────────────────────────┘

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