Korg Sigma

Name: Korg Sigma
Brand: Korg

The Korg Sigma is a strange one. It's an analog monophonic synthesizer that looks more like a home-organ add-on than a serious instrument, and it was in fact the keyboard version of Korg's X-911 guitar synthesizer. Players who wanted something simpler than the knob-laden analogues of the period liked it. It also has something of an identity crisis, as Sound on Sound (June 2001, p.258) put it when they revisited it decades later.

The control layout is what sets the Sigma apart. There's no conventional panel of knobs. The left-hand section is a keypad split into two rows of nine push-button switches, 18 in total, used for playing bass lines while the right hand plays melodies on the 37 narrow keys (slightly wider than miniature keys). There are left-hand touch controls as well, and the whole circuit was designed to imitate guitar effect pedals. You hear that in the sound. It isn't the thick, warm tone of other late-70s synths. It's funky effects in the style of guitar processing. Even with the filter wide open, the sawtooth VCO has an organic quality, and the filter can produce nice phaser/flanger-like effects. It won't give you thundering bass or ripping anthemic leads, but it does come up with strangely appealing ear candy, as Sound on Sound noted in the same retrospective.

The Sigma is monophonic, with a single oscillator per key plus a sub-oscillator switch for -1 octave. The filter has two modes with Frequency, Peak and Amount controls, and there's a noise generator and VCA on board. Output comes from a built-in guitar preamp with Volume and Tone controls. There are pedal inputs for Expression, Volume and Sustain, plus a mono audio out and headphone socket. It runs on a 9V AC adapter. No batteries, no CV/Gate, no MIDI. The background hiss gets audible at maximum master volume. The Sigma does allow single or multiple envelope triggering.

Despite its oddities, when played across the full keyboard range it works quite well as a competent lead synth, and it can produce a nice soft pad-like sound.

Known specs:

- 37 narrow keys, monophonic

- 18-switch keypad for left-hand bass playing

- VCO (sawtooth), sub-oscillator (-1 octave), noise generator

- VCF with two modes, resonance; VCA

- Guitar preamp output with Volume/Tone

- Pedal inputs: Expression/Volume/Sustain

- 9V AC power supply only

- Dimensions: 83 × 44 × 10 cm; 6 kg