Korg Sigma Modifications

"Sigma is an extremely interesting synthesizer. Essentially a preset monophonic synth, it offers many of the same features as other preset monosynths - a 3 octave keyboard with aftertouch sensitivity, a range of preset sounds (19 in Sigma's case) and various performance controls.

Unlike other preset monosynths, however, it does not feature a single VCO-VCF-VCA signal path with a diode matrix or diode and resistor matrix to select the preset sounds."


via Richard Atkinson's blog here where you'll find the full details.
via Richard Atkinson in the comments of this post.