Jupiter One
- At June 08, 2018
- By amsynths
- In Uncategorized
18

Introduction I picked up a Roland Jupiter 4 voice card on eBay in late 2017 in good condition. In June 2018 I started a project to build a complete single Jupiter voice based around this card. The synth will fit into a Moog 60 HP case (another eBay find) with an aluminium front panel that replicates (most of) the controls of the Jupiter 4.
To create a fully working single synthesizer voice means a lot more electronics are required than just the voice card! The complex module controller card has to be replicated along with a LFO. The arpeggiator has been left off the synth as it requires the 8048 micro-controller, which means no programmabilty or presets either. The JP4 modulation capabilities have been retained and enhanced, with a 5-axis thumb joystick and a revised S&H circuit for the VCF modulation.
Voice Card This is an early 1979 D version of the card with a BA662 based low pass filter. The first job was to clean and dry the card and remove the metal guides. This reveals 4 mounting holes for the top of the PCB. The 4 green connectors are replaced with 9 pin SIP headers, so the card can be mounted to a new motherboard PCB. Some of the ceramic capacitors needed replacing and the trimmers will be replaced with high quality cermet trimmers.
A hack to the card to add a potentiometer to adjust the VCA Gain trim has been removed and a trimmer put back on the PCB. The card is spaced 12 mm from the motherboard. I am hoping the voice works, but I am sure there will be some repairs needed. The card has additional power supply caps mounted on the rear, which I have retained.
The card is a Revision D but with Revision E kludges (power conditioning capacitors, 100 and 2200 pF capacitors around the uA726). The FET Op Amps in the card are LF353 rather than TL082, but they exhibit the same low current at 3.6 mA and 13 V/us slew speed, so equivalent performance.
Module Controller The Jupiter 4 has a dedicated 5th card in the card frame to hold all the circuits that translate and mix the programmable CV voltages for the Voice Card. It also contains the CMOS based envelope clock generators. All these circuits have been put on the Motherboard PCB of the Jupiter One which measure 295 mm x 105 mm.
The module control circuits and Noise circuit has been retained, augmented with the S&H from the Jupiter 8 and revised LFO. Op amps have been upgraded to TL072’s and BA6110 OTA chips replace the BA662’s. This PCB is spaced 11 mm from the panel PCB and it contains the +/- 15V power rails, derived from a switched and precision LDO power supply.
Jupiter LFO The LFO in the Jupiter 4 is on the main board and is almost identical to the one used in the Jupiter 8. The JP4 version uses discrete FET switching and a two chip CMOS 2 to 4 address decoder, this is rather component intensive and a simpler approach is needed. I have taken the same approach as Roland did on the Jupiter 8 LFO and used a 4052 analog switch.
4-Way Switch The Jupiter 4 relies on 4-way slide switches which are difficult to source, but we think we have the right one. It may mean making sub PCB’s to raise them up to the right level, but its the simplest electronic solution. The JP4 micro-controller decodes them the 4-way into 2-bit binary signals, using pull up resistors, for use in the Controller and Voice cards, which we can copy for the Pulse Width Modulation, LFO Wave Form and VCF Keyboard Follow.
Panel PCB The front panel PCB is 295 mm x 105 mm and contains 19 slide potentiometers, 4 rotary potentiometers, 2 rotary switches, 11 slide switches, the joystick and 4x jack sockets. The PCB mounts to the Motherboard via a set of 9 pin SIP sockets and headers. It is spaced 11 mm from the front panel.
Joy Stick The Jupiter One has a small thumb sized 5-axis Joy Stick mounted on the front panel. This controls the Bend (X axis) and LFO Depth (Y axis) with the integrated push switch acting as Gate On. This provides excellent control over 3 of the synths key parameters. The Jupiter 4 Modulation controls (next to the keyboard) have been retained so that the Bend Amount and LFO can be patched into the VCO, VCF and VCA.
Inputs & Outputs The external connections have been kept to a practical minimum to conserve panel space, so this is not a semi modular. There are inputs for CV and Gate, as well as the X and Y CV’s that over ride the joystick if plugged in (Bend and LFO Depth). The 6.35 mm analog output jack is mounted on the rear of the 60 HP case, along with the 12V AC power supply input.
Missing Controls The lack of micro controller means there is no arpeggiator or presets or portamento. The Trigger Section of the synth therefore has not been included (like the Pro Mars). The transpose switch has been replaced by having a 4 octave range switch.
Commercial Option I may release the Jupiter One as a commercial project using new Jupiter 4 voice cards based on the BA6110 and a BJT expo generator to replace the ua726. DIY and completed synths may also be available. A two voice version is planned (same 60 HP case) with programmable presets and OLED display.
autoy
Yes for a commercial project of this! Do you think it would be feasible to make a voice expansion project using clone JP4 cards to upgrade existing machines? Cheers.
amsynths
Hi, unfortunately its not very practical to add more voices to the JP4. There is nowhere to put them in the case, and the keyboard scanner only does 4 voices. It would be possible to build a 4 or 8 voice rack or boat synth using the new cards and the new patch programmer we are working on. Best Regards Rob
Russell alderton
Hi there,
Just leaving my comment that if these cards ever became available I would probably be up for 5 of them for my tech to build.
Regards
Russ
autoy
I would buy 4 of them to make an 8-voice JP4 if someone can figure out an expansion. Perhaps a few more as backup.
Best of luck with this project.
autoy
@amsynths definitely challenging but there’s a lot of air inside the JP4. I can think of at least a few ways to place more cards inside, at least a couple of them for sure. You are right about the key assigner though, it would have to be a new board altogether, like the one Lazlo Kovari is developing for IO V2 now. But 6-voice Jupiter-4? Where do I sign?
Allie
Also leaving a message to say I will buy 4 if made
ua726
nice one Rob, think I might’ve been watching that JP4 voice board too 🙂
Any chance you could measure the voice card for us? I had a card drawn up but had second thoughts as to the size of it.
cheers
amsynths
Hi, Jupiter 4 voice card is 195mm x 90mm and mine will be fitted to a Moog 60HP case….best regards Rob
ua726
thanks Rob, I was pretty close in the end.
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amsynths
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Enrico
Wow! I will go for 8! (ten years ago I bought 8 jp4 cards.. 4 of the first version and four of the second one, and also a single controller board)
Considering that you also made a great jp4 oscillator board..why not put that extra osc (pro mars option).
considering that the cards are ready for external cv control, maybe multiple single voice modulescan be chained and controlled from an external module with memories (similar to the sem by oberheim).
Allie
Hi – just wondering if there is any update on this? Thanks
amsynths
Hi, The Jupiter One is at prototype stage, should be ready in May. Rob
Marc rubin
Rob!!!!!
I love you! Please let me know when the voice cards will be out. It would be really handy if you have a midi routing card to send to each voice and a little mixer as well. It would also be handy to be able to control the boards from a single master control surface like the original. Setting each voic3 to the same settings could be challenging. I’m thinking leave the pots out and control via cc. Be much cheaper and you could put 4 voices on one card. Anyway very happy you are doing this. This is such an overlooked synth.
Marc rubin
Last thought. If you do all that. We can make up a copy front panel and make clones like the 2600. Be easy to use some random midi keyboard in the housing to drive it.
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