AM8400 Jupiter 4 VCO

AM8400 JP-4 VCO

Overview  This is replica of the VCO in the Roland Jupiter 4, their first polyphonic synthesizer introduced in 1978. The VCO was a unique design, based around a 555 timer and a CMOS divider chip, 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 1978 mean there were only 16 sliders that could be memorised, which limited the functionality of the VCO to 4 octave range and 3 waveforms.

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

EuroRack Module:   14HP wide
User Manual:              Download PDF

Pulse Wave

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 and variable slider controlled Pulse Width rather than a 4-way switch.

The waveform shapes are created from the 555 timer which generates square waves that are divided down for the lower ranges. These square waves are used for the square wave output and are perfect and stable 50/50%. They are also approximated into a sawtooth which has as many steps as the range (16′ has 16 steps, 2′ has just 2 steps). The sawtooth wave is therefore not perfect and is made up of steps, this creates a different sonic shape as you change octave.

The pulse width wave is created from the sawtooth rather than the square wave, and therefore has some variability in width, rather than being totally stable. Adjusting pulse width is effectively quantised by the sawtooth steps and jumps from one setting to the next rather than being smooth.

Jupiter 4 Sawtooth

AM8400 Saw Wave

The sawtooth and pulse width waves are quite different to any other synthesizer and contribute a lot to the unique Jupiter 4 sounds, and contain more harmonics and movement. It also appears to be a stable design which used with a modern expo generator.

Components Original components have been used, along with a few careful upgrades:

  • Original DN819 CMOS divider chip
  • Modern 1% polystyrene capacitors
  • Metal film 1% resistors
  • Precision matched transistor pair
  • Precision 10V reference voltages

AM8400 Square Wave

Panel The panel design keeps close to the original with a rather sparse set of controls. Frequency range on a rotary switch from 2′ to 16′, a choice of three waveforms (off, sawtooth, square, pulse), a frequency modulation slider, a Pitch rotary potentiometer, a pulse width selection switch (manual/external), a slider for modulation amount and a sub oscillator on/off switch.

The VCO can be used standalone, or in pairs like the ProMars using the Doepfer bus as the controlled keyboard CV, 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:

  • Frequency Modulation Input
  • Pulse Width Modulation input
  • Audio Output

Module Availability  The AM8400 module was developed in 2018 – 2019 and a prototype was successfully tested and the circuit adjusted for 12V EuroRack operation. The VCO had an initial manufacturing run in late 2021.


Copyright AMSynths 2019