MKS-40 Voice Card


Overview The Jupiter 4 uses a vertical card cage to house four voice cards and a module controller. In the MKS-40 the voice PCB is the same size as the original but with some minor changed which make it incompatible with a Jupiter 4. These changes are to improve and simplify the design.

Oscillator The UA726 heated transistor pair has been replaced with a modern matched transistor pair (SSM2212 in SMD) and the expo generator changed to use a 1k Tempco and HF trimmer. The “front end” of the VCO uses precision +/-10V power regulators. This “modern” design was proven by the AMSynths AM8400 VCO and works really well, providing a more accurate and stable pitch generation between octaves.

To reduce the component count on the Processor PCB the switch data decoding has been simplified in the MKS-40. This requires some changes to the Voice Card;

  • The VCO waveform selection now uses a 4052 analog switch
  • The 4066 Sub Oscillator analog switch has been replaced with a SPST DG418

Filters The MKS-40 uses the early version of the 24dB low pass filter, with modern AS662D chips replacing the old BA662’s. The later version of Resonance control (constant voltage) has been used, as it provides independent resonance trimming across the four Voice Cards. This change was introduced at serial number 800800 in the Jupiter 4 and is an important improvement.

The filter have been upgraded with high quality, low tolerance, Wima polypropylene capacitors in the cores and Nichicon audio grade electrolytic capacitors. The BJT switches in the Key Follow circuit have been retained.

Envelope Inversion

VCA/ADSRs Very few changes were needed in these circuits, with an AS662D in the VCA. The low offset version of the AS662 (the D) should mean less need for offset trimming.

The BJT for selecting the inverted envelope has been replaced with a SPDT DG419 analog switch. The envelope inversion is still based on connecting the signal to either the + or – inputs of the envelope level OTA at IC18. However the trimmer for VCF envelope inversion  (VCF ENV) has been removed, as we should have no offset issues by using the DG419.

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