Korg MS-20  The Korg MS-20 is a patchable semi-modular monophonic analog synthesizer which Korg released in 1978 and which 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, the SQ-10 sequencer, and the VC-10 Vocoder. Additional devices included the MS-01 Foot Controller, MS-02 Interface, MS-03 Signal Processor, and MS-04 Modulation Pedal. 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 1990s. In 2013, Korg responded to a revived interest in monophonic analog synthesizers by reintroducing the MS-20 in a reduced-sized form, as the MS-20 Mini. Apart from being 86% of the size of the original, the use of 1/8" jacks on the patch area, and the addition of MIDI capabilities, the MS-20 Mini is otherwise electronically identical to the original. The MS-20 has two VCOs. The output of VCO1 can be set to triangle, sawtooth, adjustable-width pulse or white noise. 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 although mods have successfully achieved this. The VCOs have Hz/V voltage control inputs but note that oct/V scaling is available via judicious use of the "total" CV input socket. There are two VCFs in series. The first is a 6 dB/oct  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 thick film IC. Later, the filters were redesigned to use off-the-shelf LM13600 OTAs. MS-20 units fitted with LM13600 filters have a small daughter board; the front panel on such units "often" has a tell-tale mounting screw whose head is visible in the vicinity of the VCA block diagram.