The Commodore, Moog, MidiBox project

Posted on Aug 08, 2016 by rutgervlek

After my first DIY synth endevaours, I felt encouraged by the result and my mind was full of new ideas for future instruments. After the lovely vintage, but monophonic sound of the Ocean, I felt I wanted to do something very different. Something a bit grittier, polyphonic and with a hybrid (analog+digital) architecture.8

At the same time I discovered the amazing efforts of Thorsten Klose, in setting up the MidiBox community and sharing a wealth of circuit ideas, code and knowledge. One of the projects on his website was (re)using an old sound IC from the ancient Commodore C-64 computer. Using this sound engine, he built a new synth around it with the help of a micro controller. Many people followed building a monophonic (single SID) version of this, and it sounded like fun and very useful for computer-game style effects and simple melodies. Musically, I wasn’t yet convinced this would be my new instrument, until I heard Thorsten’s demo of an experimental polyphonic mode using a few SIDs in parallel. Now it began to sound like something I would use, especially given the amazingly deep synth engine he had built around it. Shortly after, I heard another demo. This time it was a single SID chip playing through a Moog ladder filter. While the SID’s internal filters do work, they have a rather harsh sound. I was surprised how the sound of the SID changed into gritty, yet warm when played through a Moog filter.

That’s when I decided to build one myself. A 8-voice polyphonic SID synthesizer, based on the MBSID V2 platform, with the addition of 8 parallel Moog ladder filters. As my room was getting rather full of synthesizers, I decided to make it a rack mounted version.

At that time, the only way to achieve enough processing power for the entire MBSID V2 engine to run polyphonically, was to use several micro controllers. About two SIDs could be controlled from a single micro controller, and that’s when I decided to build a stereo architecture. Basically this made the synthesizer either 8-voice polyphonic, but mono (single audio output), or 4-voice polyphonic in full stereo. I was having high hopes for this last mode, as I was guessing that small differences in the analog circuitry would provide a nice feel of “width” in stereo. It turned out even better than I expected (check out those full-stereo string sounds in the demo)!

While 8-voice polyphony was possible with this multi-core design, it was far from practical, and the sheer complexity of the entire project has caused the occasional headache. In the end, it worked out though!  With the help of Thorsten Klose, I ironed out some minor bugs and did a few small modifications to the firmware to make it run on my specific hardware setup, and the whole thing came to life.

However, within a week of playing with the new instrument, it became clear that the synth engine was so deep, and the front-panel so minimal, I could never grab the full potential of the synth when programming it.

It turned out that I wasn’t the only one being overwhelmed by the deep synthesis engine of the MidiBox line. I got in touch with a few others, and had a look at an early attempt to a software MIDI controller. I decided to start from scratch and build a full software editor in Java for the synth. One that was also capable of organizing the patches and banks for it, making it easier for users to share patches or backup sounds. As with all projects, dreaming about it is relatively easy, but the final package took many, many hours. Once the editor was there, it was also a great tool to iron out the last bugs and add more MIDI features to the synth’s own firmware. The software was made available open-source through the MidiBox forum and I’ve been using it on a regular basis since.


The bare custom PCB


Power regulators, fuses and heat sinks in place (bottom left)


The 2HE 19″ case with the MIDI sockets and transformers fitted


The LCD board with home made illuminated push buttons and a rotary encoder (right)


The main PCB ready and in place. The rows of red capacitors are for the 8 ladder filters. On the top side: the 8 SID chips and the 4 PIC microcontrollers.


Frontpanel arrived! Wiring up the front and back panel connections


Same view from the other side (back of the LCD board)


The finished TR-3 polysynth…



ps. If you wondered why it’s called TR-3, have a look at first names of the people who’s creative endeavors met in this instrument: Thorsten Klose, Rick Jansen (who drew the Moog ladder schematic), Robert Moog, Rutger Vlek (who?).


Close-up of the LCD, buttons and rotary encoder


The MBSID V2 editor and librarian software