AM8120 JP-8 VCO

Overview  This is replica of the VCO in the Roland Jupiter 8 analog synthesizer introduced in 1981. This was Roland second attempt at a stable oscillator for use in an analog polyphonic synthesizer. Prior to developing their own oscillator on a chip (the IR3R03 in 1984) or using the CEM3340 in the Jupiter-6 Roland re-purposed its latest mono synth VCO from the SH-2 and 100M, and replaced the ua726 with a CA3046 matched transistor chip. The new VCO was considerably different to the Jupiter 4 VCO which used more components.

EuroRack Module:   16HP wide
User Manual:             Download PDF

Description  The AMSynths circuit is exactly the same as the original, except for an improved NPN matched pair in the expo generator (SSM2212 rather than CA3046) and a change in the tempco resistor used. The same care in using precision voltage rails has been used with modern reference voltage chips. The availability of modern JFET OP Amps enabled Roland to ditch the CA3130 used in the 1970’s and use TL080 chips.

The cross-modulation, pulse width modulation and sync capabilities of the original have  been retained. Two AM8120 VCO’s can be patched together for cross modulation. The second output of VCO -2 (which is always a sine waveform) is patched into the XMOD input of VCO-1. A jumper setting on the POT PCB enables the XMOD source to be switched from being always a sine waveform to which ever waveform has been selected on VCO-2. IF the two VCO’s are alongside each other an internal 6-way IDC cable can be used to interconnect the VCO’s. This cable carries the SYNC and XMOD outputs from one oscillator to the other.

The panel design keeps close to the original with a four octave range selection from 16′ to 2′, a choice of five waveforms (sine, triangle, sawtooth, square, pulse) and slide potentiometers for:

  • Fine Tune (+/- 1 semitone)
  • Frequency modulation (x2)
  • Cross modulation (XMOD)
  • Pulse width modulation

There are three slide switches on the module:

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

The use of a microprocessor in the Jupiter 8 for control panel reading, CV driving and the storage of patches; also 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 AM8120 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 eight 3.5 mm jack sockets on the base of the front panel:

  • Pitch CV (KYBD)
  • Frequency Modulation x2
  • Pulse Width
  • Sync In and Out
  • Audio Output (x2)

Outcome & Availability  The AM8120 module was developed in the spring of 2022 and a prototype tested in May.


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