We discussed the MiniDexed a few times in other threads already [1] [2]: it’s a RPI-baseed project emulating a classic Yamaha DX7 FM synth, using code from the Dexed VST.
To be more precise, it really aims to emulate a Yamaha TX816, the rackmount studio version with 8 channels, but to achieve that, it requires at least a RPI 2, which makes it a bit of an expensive project. In fact, there are even more powerful versions of this project that use a Teensy, increasing the cost further. It’s very much built in the monolithic synth mindset, expecting to be used with a keyboard, input is MIDI only.
As the RPI has no ADC pins, it makes it impossible to simply hack in CV input without additional hardware, and the developers regard the problem as out of scope.
However, it can also handle a few notes of polyphony on single channel with a cheaper RPI Zero, which is much less expensive.
You might have seen that a few days ago I finished a little MiniDexed module:
I’m trying to go further into converting it to a proper modular module that does things modularly, and would welcome hearing what you have to say as I design it, as I think others might like to use it. This design should be within my comfort zone but still a bit challenging, so you might catch mistakes too.
Here is my general idea:
- RPI Zero based, single-channel, no need for a full orchestra in a modular
- Single 80×100mm board
- Under the magic size to get a good deal from the fab
- Only the LCD is (optionally) mounted directly to the PCB, everything else is panel-mounted and wired to pin headers. There’s not too much wiring on this project.
- I’ll provide synth-printer 3D prints of panels in Eurorack and Kosmo form factor, or you can design your own panel
- Fits a 16hp Eurorack module, 18hp better for comfort
- Fits a 10cm Kosmo module
- Could use a OLED instead of a big LCD and mount the board perpendicular for a smaller build
- Input can mix MIDI In, and duophonic CV/Gate + modulation
- An Arduino Nano translates CV to MIDI
- Hack the code for alternate behaviors using toggle switches with unused pins (ideas: pitchbend for non-quantized V/Oct, automatic fifths, etc)
- It’s all control rate stuff so its tiny CPU should be able to handle it
- The module doubles as a MIDI to CV, and as a CV to MIDI. A DAC module provides precise V/Oct
- An Arduino Nano translates CV to MIDI
- Can take power from +12V using a common buck converter module
- Common THT components and generic Aliexpress modules
- BOM under 30 euros
Here’s what I got so far!
The back of the board is mostly modules, all but the RPI are generic cheapo Aliexpress stuff so there’s always a risk of receiving slightly different variants. I might Tetris them differently but it all fits:
The LCD is mounted directly to the panel using screws and standoffs. Using long pin headers would be a good idea to have enough clearance to wire things up. I couldn’t get the textures to show up on that 3D model but you get the idea:
There’s a fair amount of additional components so it will probably end up pretty cramped with components tucked under the sandwiched boards. Some resistors and diodes might have to be mounted vertically to fit everything, too. Hopefully I can keep it within those dimensions and THT only.
[EDIT: Revised schematic a few posts down!]
