The Korg MS-20 is a patchable semi-modular monophonic synthesizer which Korg released in 1978 and was in production until 1983. It was part of Korg's MS series of instruments, which also included the single oscillator MS-10, the keyboardless MS-50 module, and the SQ-10 sequencer. Additional devices included the MS-01 Foot Controller, MS-02 Interface, MS-03 Signal Processor, MS-04 Modulation Pedal and VC-10 Vocoder.
Although the MS-20 follows a conventional subtractive synthesis architecture of oscillators/filter/vca, its patch panel allows some rerouting of both audio and modulation, and it includes an external signal processor. This flexibility led to its resurgence during the analog revival of the late 90's.
The MS-20 has two VCOs. The output of VCO1 can be set to triangle, sawtooth or adjustable-width pulse. The output of VCO2 can be set to sawtooth, square, rectangle or "ring mod" which is in fact an exclusive-OR between the pulse output of VCO1 and the square wave output of VCO2. Although the pulse width of VCO1 is adjustable, there is no voltage control for this parameter and as a result pulse width modulation is not possible. The VCOs have Hz/V voltage control inputs.
Filters
There are two VCFs in series. The first is a 6 dB/oct [1] [2] high-pass, the second a 12 dB/oct low-pass. Resonance ("peak" in Korg parlance) is adjustable but not voltage-controlled.
Originally, the filters used Korg's proprietary KORG-35 chip. Later, the filters were redesigned to use off-the-shelf LM13600 OTAs. MS-20 fitted with LM13600 filters have a small daughter board.
Output
The final component in the audio path is a VCA. It is a single-transistor design, based on a selected 2SC945.
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