A note to document the modifications made to my Korg Sigma. I will be making a YouTube video about my Sigma shortly. It is quite a famous Sigma already; it is the very same Korg Sigma that features in the following YouTube video:
Originally I bought it cheap because nine of the eleven Instrument tabs weren't working properly. I traced this fault to a broken 4013 dual D-type flip flop and replaced the chip.
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.
Rather, Sigma contains a separate signal path for each of the 19 sounds, and these may be enabled simultaneously with large rocker switches. The sounds are divided into two sections, Instrument and Synthe, corresponding to the type of sounds intended to be available in each section. Imitations of acoustic instruments on the Instrument side and synthesizer sounds on the Synthe side. Be in no doubt, though, this is a 1979 analogue synthesizer. Both sections make sounds that befit such a machine.
Korg provided two separate VCOs, one for the Instrument and one for the Synthe sections, so that two oscillator sounds may be made with Sigma. Korg's VCOs generate four sawtooth footages simultaneously (32', 16', 8' and 4') and different footages proceed to the preset voice circuits for waveshaping, filtering and envelope generation specific to each preset sound.
Page 5 of the Korg Sigma service manual shows the circuits for the Instrument presets, and page 6 the circuits for the Synthe presets. The two VCOs are on page 4 along with various performance features such as the pitch joystick, aftertouch sensor, transpose rocker switches and the unusual Quarter Tone mode.
Each preset sound is equipped with a single potentiometer control for adjusting some aspect of that sound. On the Synthe side they are mostly envelope controls and on the Instrument side a mixture of tone (filtering) controls, envelope controls and one pulse width control. Every preset sound has its own single transistor VCA, an envelope generator and some form of waveshaping and filtering. Here is where Sigma gets very interesting.
The Instrument section has four preset sounds, Electric Bass, Tuba, Horn and Trumpet, which have their own KORG35 voltage controlled filter circuits for shaping the sounds. Electric Bass uses a pulse waveform with a fairly narrow pulse width and Tuba, Horn and Trumpet all use sawtooth waveforms. All four presets have simple envelope generators with very short attack times and long decay times, these are used to control each preset's VCA (a single NPN transistor) and to provide an amount of VCF cutoff frequency envelope modulation. The Electric Bass envelope generator decays to zero so this preset may be considered a percussive sound, whereas Tuba, Horn and Trumpet all feature envelope generators which decay to non-zero sustain levels, and so may be considered sustained sounds.
The other seven preset sounds in the Instrument section have organ-style fixed filters for shaping the sound, and simple envelope generators controlling their single transistor VCAs. Thus there is no dynamic timbre variation resulting from filter cutoff frequency changes in these presets. That's not to say they aren't interesting sounds, however. Fuzz Guitar uses a pulse waveform where the envelope generator modulates the pulse width before proceeding to fixed filtering and the single transistor VCA. Clavi uses a pulse waveform where the potentiometer control adjusts the pulse width.
Clarinet is a square wave with a tone control for crossfading between the outputs of one fixed filter and another fixed filter. Double Reed is a pulse wave with a similar tone control, String is a sawtooth wave with a control for the envelope generator's attack, Flute is a sawtooth wave with a tone control and Hammered Percussion is a sawtooth wave with a control for the envelope generator's decay.
On the Synthe side, each preset sound is provided with a KORG35 VCF, an envelope generator, a single transistor VCA and some waveshaping. A joystick control adjusts an offset voltage applied to all eight VCFs, and some of the presets feature envelope modulation of the VCF cutoff frequency as well. In fact only the 16' PWM and 4' sawtooth presets have no VCF modulation at all; the others either have envelope modulation of their VCF or in the case of the 16' sawtooth S/H preset, the VCF cutoff frequency is modulated by a sample and hold circuit which gives the preset its name.
Therefore, the entire synthesizer has twelve KORG35 voltage controlled low pass filter circuits. This is a lot more than a Korg MS-10 (one) or a Korg MS-20 (one low pass and one high pass), and yet those synthesizers fetch a lot more on the open market. No doubt that is because those machines offer a lot more apparent control over the sound with their variable controls and patch panels. Yet Sigma features 31 potentiometers and two X-Y joysticks over its entire control surface - it is not a synthesizer without control possibilities! Furthermore, Sigma can generate sounds that feature 32', 16', 8' and 4' footages simultaneously. No MS series instrument can do that.
A few things frustrated me about Sigma from the outset. One, the relatively simple VCF envelope modulation possible with A/R or D envelope generators. Two, the lack of VCF aftertouch modulation.
Further frustrations were the lack of VCF LFO modulation and the rather curious Delay Vibrato rocker switch which provides delayed vibrato for the Instrument VCO, and has three associated controls for the purpose (depth, speed and delay), but with no equivalent for the Synthe VCO. Finally, the reduced range of the pitch bend joystick on the Instrument VCO relative to the Synthe VCO.
So I decided, having already 'modified' my Korg Sigma by replacing its original broken 4013 chip with a new one in an IC socket, to go further and do something about the above limitations.
The first modifications I did were to increase the range of the pitch bend joystick on the Instrument VCO, and make the Delay Vibrato rocker switch apply to the Synthe VCO as well. For some reason Korg decided to make the range of the Instrument VCO pitch bend approximately half that of the Synthe VCO. This can be changed by replacing R197 (470K) on page 4 of the Sigma service manual with a 240K resistor, to equal the value of R163. Delay Vibrato can be applied to the Synthe VCO as well as the Instrument VCO by adding a 470K resistor between the output of the delay vibrato pass transistor Q13 and the inverting input of the Synthe VCO pitch modulation amplifier IC24 (pin 6).
To add VCF LFO modulation to the presets that feature KORG35 VCFs, I observed that the Instrument and Synthe circuit boards both have a VCF offset adjustment, which are provided as trimmers on the back surface of the machine. The outputs of these are buffered by inverting amplifiers and applied to pin 2 of the KORG35 VCFs.
The above picture shows the VCF offset amplifier for the four KORG35 VCFs in the Instrument section. By adding in a signal from the delay vibrato LFO here, one can add VCF LFO modulation. I did this by attaching a 10K pot to the output of the LFO (pin 7 of IC10 on page 4) and connecting a 100K resistor between the output of this pot and the inverting input of the VCF offset amplifier above (pin 2 of IC10 above).
The next picture shows the Synthe VCF joystick amplifier and VCF offset amplifier for the eight VCFs in the Synthe section. VCF LFO modulation can be added to the Synthe section in the same way as for the Instrument section, by connecting a 100K resistor between the output of the VCF LFO modulation pot and the inverting input of the Synthe VCF offset amplifier (pin 6 of IC1 above).
VCF aftertouch modulation can be added to the Synthe section VCFs by connecting a 10K pot to the output of the keyboard sensor amplifier (pin 7 of IC12 on page 4 - or after diode D26) and a 100K resistor between the output of the pot and the inverting input of the Synthe VCF FcM IN amplifier (pin 2 of IC1 above). The reason for adding it in here is that the FcM IN amplifier provides an extra stage of inversion. If it were added to the offset amplifiers already discussed, the result would be a decrease in VCF cutoff frequency when the keys are pressed harder. More useful musically speaking is an increase in filter cutoff frequency, hence it is added in here. Applying aftertouch modulation to the Instrument VCFs in a similar manner will require an extra inverting amplifier.
The above modifications have been implemented on my Korg Sigma and I am very happy with the result. Gordon Reid, author of many Sound on Sound reviews of analogue instruments, has seen the mods and given them the thumbs up. There are two remaining improvements I would like to perform on my Sigma.
First, the VCF LFO modulation pot provides more movement in the sustain phase of a note, but it cannot completely hide the relatively simple A/R and D envelopes applied to the VCFs. What is needed is a full ADSR envelope generator, and this can be applied to the Synthe VCF FcM IN input without any further internal modifications of the Sigma.
Second, and in common with other early Korg instruments including the 700, 700S, 800DV, 770, MS-10 and MS-20, Sigma has no VCF keyboard tracking. This is partly because Sigma (and Korg's other early instruments) features linear oscillator control voltages and exponential filter control voltages. To provide a VCF keyboard tracking control, a logarithmic amplifier is needed, such as the one in the Korg MS-02 interface. This interface converts linear (Hz/V) control voltages to exponential (V/Oct) control voltages. The linear keyboard CV output can be converted to exponential using such a log amp and applied to the Synthe VCF FcM IN input as well.
A word about resonance - one comment in the original Sound on Sound retroview of Sigma expressed the author's frustrations at the lack of a resonance control. In fact, each of the twelve KORG35 VCF circuits has a preset amount of resonance designed into the Sallen & Key filter circuit by the component choices made. To provide a resonance control for each of the VCFs would require twelve resonance controls, and a much more complicated VCF circuit for each. Sigma manages to pack in twelve VCFs into its highly unusual synthesizer architecture by keeping each circuit as simple as possible, and therefore I can forgive Korg for the lack of resonance controls; indeed I'm very glad they designed the synthesizer this way because it made Sigma more commercially viable in the first place. Already Sigma cost almost twice as much as the MS-20 when first released (185,000 yen versus 98,000 yen). This may explain its relative scarcity on the modern day second hand market.
To compensate for the lack of resonance controls, one can become familiar with the preset resonance amounts applied to each of the VCF-filtered preset sounds. This can be done either by listening to the sounds or by studying the component choices (capacitor values) for each of the KORG35 filter circuits. The presets having the highest resonance are the 8' sawtooth, 8' square, 16' S/H and Noise waveforms on the Synthe side. The presets having the lowest resonance are the Tuba and Horn sounds on the Instrument side.