SOUNDWORLDS 3: Korg MS-20 Retro Expansion

KORG MS-20 RETRO EXPANSION

By the time I bought my Korg MS-20 Mini, various enterprising musicians (take a bow Auxren and Remörk) had already added extra features to both the orginal MS-20 and the Mini version (both circuits are almost identical).

However, as well as these mods I'd already discovered, I wanted also to have a stab at some additional features that I hadn't seen anyone attempt before, such as audio frequency modulation of OSC2 by OSC1, a more retro-sounding transformer/diode ring modulator, plus a mixer expansion to incorporate all my new sounds into the final output signal.

A RICHER SOURCE

MULTI-OUTPUTS: To supplement the existing choice of one of four OSC1 waveforms I added separate TRI, SAW and PULSE output sockets, all available simultaneously.

Meanwhile, for OSC2 I added an extra individual waveform output (the circuitry restricts this oscillator to one waveform at a time, but having this separate output became very useful later on).

PWM: Next up was an additional input for OSC1 variable pulse width modulation, using the existing pulse width knob to control PWM level. You can of course modulate pulse width with the Modulation Generator (LFO to everyone else but Korg) for those wonderful swirling effects, but also with more exotic options such as an Envelope Generator for sweeping tonal effects during each note.

LINKING & COMBINING

SYNC & FM: A Sync toggle switch was added to OSC2, to lock it with OSC1 for those screaming lead sync FX (these are the blue and yellow wires respectively visible on the circuit board photograph), and an FM input with its own rotary level control (soldered to the red wire near the VCO2 pitch control). This was usually fed from one of the OSC1 multi-outputs for some serious audio frequency mayhem, but the FM input also proved very useful with the sync activated when fed by an envelope generator.

VCO MIXER: I also expanded the existing mixer section, adding two extra input sockets (IN3 & IN4, attached to the existing MS-20 virtual earth mixing point via the purple wire shown on the circuit board), each with its own level control knob. These can be used to mix together several oscillator waveforms, or to mix in other patchbay or external audio signals. Favourites combinations include triangle with a dash of narrow pulse for really phat bass sounds with an edge, and triangle plus noise for those 'chuffy' flute-like tones. With the pre-wired RING MOD output (IN5 - see next section) and the existing EXT SIGNAL IN, these mixer additions let me mix a grand total of six pre-filter input signals, for some wonderfully rich sound layering.

RINGING THE CHANGES

DIODE RING MOD: To supplement the rather limited and dirty-sounding RING option of OSC2 (which is actually XOR, aka cross modulation, and only sounds like ring modulation in the special case that VCO1 and VCO2 are both square or pulse waves), I added a more traditional transformer/schottky diode ring modulator circuit that also provides extra harmonic 'warmth', preceded by active input/carrier buffers that also boosted the post ring mod output level.

This let me achieve far more mellow ring mod options using (for instance) triangle and sawtooth input signals (hopefully the reason for my separate OSC2 output now becomes clear). By default, my RING MOD output was pre-wired via a further dedicated rotary level control to yet another mixer input, or by plugging in a patch cable you can route its output elsewhere on the MS-20.

A DOSE OF RETRO STYLE

CASEWORK: Matthew at Dolmage Designs hand crafted for me a beautiful pair of wooden side panel upgrades in cherry hardwood, and their gorgeous grain detailing gave my synth its far more expensive 'Retro' look.

CONTROL PANEL: My Retro Expansion control panel was designed using Photoshop (it took ages to get the markings and lettering in keeping with the Korg house style!). Once finalised, I printed the panel onto photo paper, then heat sealed it into a matt laminated pouch for protection, before punching the sixteen holes for sockets and pots.

KNOBS & SOCKETS: I ordered some extra rotary control knobs from Korg's own Spares Department to perfectly match the existing ones, while the jack sockets I used (Thonk PJ301BM with hex nuts) were mounted via 12.5mm diameter M6 Form B washers to more closely match the look of the Korg 'chrome' versions. Finally, on the MS-20 rear panel I added a full-sized 1/4-inch output jack socket for more robust connection to other gear in the studio, soldering its cable onto the connections of Korg's existing but somewhat more flimsy 3.5mm front panel output jack.

By the way, I built the MS-20 Retro Expansion as a one-off for my own sound design purposes, although there should be enough information here for a keen DIYer to recreate all my mods. They should also work not only on the MS-20 Mini, but also on the original MS-20 Mk1 and 2 versions, as well as the more recent MS-20 Kit and MS-20M Kit, since the circuitry is almost identical where it matters - you'll just have to trace the equivalent circuit points to solder on each mod.

Further Information on Martin's Korg MS-20 Retro Expansion