AM8340 JP-6 VCO

Overview  This is replica of the VCO in the Roland Jupiter 6 analog synthesizer introduced in January 1983, as a lower cost flagship analog polysynth. Roland moved to using the popular CEM3340 for the VCO’s, prior to developing their own VCO chip the IR3R03. The Jupiter 6 VCO can combine waveforms; the triangle, sawtooth and either square or pulse waveforms can be  mixed simultaneously. The  Jupiter 6 provides additional features over the Jupiter 8, but at the cost of no sine waveform on VCO-2.

  • VCO waveforms can be mixed
  • Cross Modulation is bidirectional
  • Cross modulation is voltage controlled
  • Sync is bi-directional

Inside S226W83S

The new VCO followed the CEM3340 datasheet for the input side of the oscillator but Roland came up with their own sync circuit and wave converters. Both these circuits were built on a ceramic substrate with SMD components (S226W83S). The wave converters ensure each waveform is at a consistent level centered around 0V. The sync circuit enables digital control of the ON/OFF status without using a VCA or analog switch. A CEM3360 serves as the cross modulation VCA.

EuroRack Module:   16HP wide
User Manual:             Download PDF

Description  The AMSynths circuit is exactly the same as the original, including the transistor based sync circuit and wave converters. The cross-modulation, pulse width modulation and sync capabilities of the original have  been retained. Two AM8340 VCO modules can be internally patched together using a 6-pin IDC cable. This interconnects the sync, cross modulation and MOD and ENV signals, and is the recommended way to use these modules as there are no external connections for sync.

The cross modulation source is what ever waveform(s) have been selected on the source VCO. This can create a harsh sound unless the triangle wave is used. In the Jupiter 8 the cross modulation was only unidirectional (VCO-2 to VCO-1) and the source waveform was always the sine wave which is not available on the Jupiter 6. The AMSynths module includes sine wave as a cross modulation source using a front panel switch to select either the sine wave or the mixed waveforms.

The panel design keeps close to the original with a quantized four octave range selection from 16′ to 2′, a choice of four waveforms (triangle, sawtooth, square, pulse) that are selected by push buttons with LED indication. There are six slide potentiometers for:

  • Frequency modulation (MOD, ENV)
  • Cross modulation (XMOD)
  • Cross modulation by ENV
  • Pulse width
  • Pulse width modulation

There is also a small rotary potentiometer for Fine Tune (+/- 1 semitone). There are three slide switches on the module:

  • VCO Mode (NORMAL or LOW)
  • Sync (ON, OFF)

The use of a microprocessor in the Jupiter 6 enabled Roland to add a neat feature on the second VCO. Rather than a rotary switch selecting the VCO range (as in VCO-1), a potentiometer was used to move across 4 octaves of semitones. The AM8340 replicates this functionality using a PIC microcontroller and external 16-bit DAC chip.

The VCO derives its pitch from a panel mounted KYBD jack socket which is calibrated at 1V/octave, there is no option to use the pitch CV from the Doepfer keyboard bus, as we have found this can be implemented with offset CV errors which impact VCO tuning.

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

  • Pitch CV (KYBD)
  • Frequency Modulation (MOD, ENV)
  • Cross Modulation
  • Pulse Width Modulation
  • Audio Output

Outcome & Availability  The AM8340 module was developed in the spring of 2022 and a prototype tested in May.

Copyright AMSynths 2022