PROMARS Analog Synth

JP-4 Voice Card

Overview Back in 2018 I bought an original Jupiter 4 Voice Card with the plan to turn it into a complete single voice synthesizer in a Moog 60 HP case. I added a replica Controller PCB and a new Pot PCB that holds the switches, LED’s and potentiometers. In 2020 I got as far as a prototype but with some problems:

  • Voice Card too close to Controller PCB, causing shorts
  • The 4053 in the LFO was wired to +/-15V
  • There were errors in my schematics
  • It sounded rather thin and weak, with only one VCO
  • BA6110 signals are too hot (+3dB over BA662)
  • The Controller board is too complex for a synth with no presets
  • I could not find suitable 4-way slide switches

Pro Mars Tune

Revised Plan I decided to simplify the design and strip back the Controller PCB, remove the Bend Modulation section, and align the design to be used with an Arturia Key Step which has a mod output (and arpeggiator). The 4-way slide switches were replaced with a rotary switch (LFO waveform), or slide potentiometers (Key Follow and PW).

To solve the thin sound I took a look at the Roland Pro Mars and realised it uses a second VCO card and not a full voice card. I could do the same, add a second VCO, especially as I had already proven my VCO deign in the AM8400. It requires a small modification to the Roland voice card to enable the second VCO to be patched in before the VCF. I have added the TUNE-A and TUNE-B  potentiometers for VCO2, which are selected with a front panel slide switch.

First Prototype PCBs

I reworked both the Panel and Controller PCB’s in the Spring of 2021 and 2022, and added a VCO card. Here are the changes:

  • The Voice Card mounting has been reversed with component side facing out
  • Implemented the JP-8 LFO with CMOS switch waveform selection, and a S&H output
  • Implemented the JP-8 LFO Delay circuit (VCA based)
  • S&H modulates the VCF via a dedicated slider
  • PW is a slider rather than 4-way switch
  • LFO waveform selection is on a rotary switch
  • Key Follow is a slider rather than a switch
  • Key Follow circuit removed
  • VCF Envelope output patched to KF0 and then onto PWM
  • Removed the voltage control from the VCO, VCF and VCA modulation
  • Implemented SMD AS662 VCA’s to eliminate the +3dB boost of the BA6110
  • Moved to PTL30 slide potentiometers with LED’s
  • Added a Gate Boost circuit
  • Added VCO-2 controls to the front panel
  • Added front panel audio output (as well as rear)

This involved quite a lot of work and the release of the AS662 chip inspired me to get the job done. The synth is really now a Pro-Mars without presets, or at least a Jupiter Two!

Outcomes I ordered the VCO-2 PCB in March 2021 and the revised PANEL and CONTROLLER PCB’s in May 2022. I build and tested the PCB’s in June 2022. The prototype VCO-2 PCB was populated, corrected and worked perfectly. There is a typo in the original schematics, R39 (the pull up resistor for the VCO-2 off/on JFET) looks like 1k but its actually 47k. The 1k resistor got very hot and pulled lots of current! The CONTROLLER PCB was fully populated and the PANEL PCB gradually built and each section tested in turn.

By the end of June most of the circuits were working well except for:

  • Some minor trace errors needed correcting
  • Attack Clock interfering with the LFO speed, needs a PCB trace rerouting
  • LFO bleed into VCO via power supply, improved regulation needed
  • Envelope clocks bleeding into power supply (not audible), improved regulation needed

A third PCB run is needed to correct the LFO bleed problem and to add LDO’s for each of the main sections (LFO, VCO-1, VCO-2 and ENV GEN clocks).

I also took the opportunity to implement a fix to the LFO bleeding into the voice card output(s) via the LPF modulation signal. These two signals are right next to each other and in the JP-4 the traces run in parallel. I am unlikely to encounter the same amount of problem, but its worth doing. The post VCA 47k resistor at R97 on the voice card is shorted and moved to the input of the audio output summing Op Amp (IC30B). In my PROMARS this is an easy change with a 0R resistor at R97 and a new 47k resistor on the PANEL PCB.

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