AM8400 Jupiter 4 VCO

Overview  This is replica of the VCO in the Roland Jupiter 4, their first polyphonic synthesizer introduced in 1978. The original  VCO is a unique design based around a 555 timer and CMOS divider chips, which was not seen in any other synthesizer. The Jupiter 4 is famous for its powerful sounds and efficient controls. The constraints of microprocessors in 19878 mean there were only 16 sliders that could be memorised, which limited the functionality of the VCO to 4 octave range and 3 wave forms.

This VCO is the sound of early Human League and Depeche Mode albums.

EuroRack Module:   16HP wide
User Manual:          Download PDF

Detailed Description  The AMSynths circuit is very close to the original except for a new Tempco based matched transistor pair rather than the obsolete heated uA726 chip, as the exponential generator. Original components have been used, along with a few careful upgrades:

  • DN819 CMOS divider chip
  • 1% Polystyrene Capacitors
  • LS312 Matched Transistor Pair
  • Precision 10V reference

The panel design keeps close to the original with a rather sparse set of controls; Range from 2′ to 32′, a choice of three wave forms (off, triangle, square, pulse), a modulation depth slide, a pulse width selection switch from manual to active modulation, a four way switch for modulation amount and a sub oscillator on/off. The VCO can be used standalone, or in pairs like the ProMars using the Doepfer bus, or four VCO’s can be used with a MIDI to polyphonic converter for the full Jupiter 4 sound.

There are just three 3.5 mm jack sockets on the base of the front panel:

  • Keyboard Input (defaults to Doepfer bus)
  • Modulation input
  • Audio output
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