DIY Synth Page 2: Digital
Expanding the Analog Eurorack synth into the digital domain
The Blog for this project

The Schematics, PCB files, and Simulation models are here
Back to the analog part

I put down the synth project for the summer and fall of 2016 while some interesting things happened. I learned to use DipTrace. Now instead of doing board layouts in the proprietary ExpressPCB, I use DipTrace instead, a nice full-featured CAD Schematic and PCB layout tool. It is low cost (free for simple boards) and easy to learn, but powerful. It generates Gerber files, pick and place files, and other manufacturing files, Unlike ExpressPCB, which generates none of these files and requires that you purchase boards only from ExpressPCB. Back in the early days, the ExpressPCB boards were low cost and simple. Nowadays, they are no longer the low cost supplier since there are several low-cost, mostly Chinese PCB manufacturers.

I also considered Eagle, Altium, and KiCad. I personally cannot stand the Eagle user interface. I've tried, but no. Too many non-intuitive mouse clicks cause parts of my brain to fight it. Altium's free version (Circuit Maker) requires that you share all designs on-line and I had trouble creating new library components with it. KiCad is OK, but I was able to learn DipTrace much faster than KiCad. I manually converted an old PCBExpress board to DipTrace and it went very well. So Diptrace it is. In the past year I have laid out about 10 designs, and built 6 of them. I order boards from Seeed Studio and PCBWay (low-cost Chinese suppliers) and Advanced Circuits (more expensive and faster American supplier).

Teensy Audio

I also discovered Teensy processors at These low cost modules use the Arduino development tools, come in convenient and small DIP packages, and have processors ranging from the Atmel 8 bit to 200MHz ARMs. The one I like is the Teensy 3.2, 72 MHz arm processor. In addition to having many excellent code libraries and examples, the designers, Paul J Stoffregen and Robin C Coon, developed a very cool audio library and hardware audio interface that can generate and process CD quality audio along with their Teensy Audio Board. And even cooler, applications using the audio library can be programmed using a nice simple graphical programming tool that takes care of many real-time audio tasks like digitizing, recording and playback, filtering, effects, waveform and musical instrument synthesis, and many others. Buy a Teensy 3.2 , and an Audio board, and have some serious nerd fun. Really, this is the first time that a hobbyist or Arduino person can build meaningful, real-time CD quality audio DSP applications. Very cool.

My first Teensy audio project was to load up and run Peter Teichman's roto2, an small simulation of a Hammond drawbar organ. It works swell. He nicely addressed the ability to use USB midi, build a 8x polyphonic instrument using the basically monophonic Audio Library, and built an excellent vibrato module. Here is a demo of roto2.

I wanted to add regular old 5-pin MIDI to it and it was pretty simple to do so. The callback function for MIDI and USB MIDI are about the same. I searched for a 4 octave keyboard with 9 sliders that could be used to control the Hammond drawbars. I settled on the $120 M-Audio Oxygen49 keyboard, and it turns out it is a USB only device, no 5 pin MIDI. I learned that most newer low-cost keyboards don't have a Midi connector.  When I looked up Midi USB converters, I found a very cool Midi USB Master from the nice people at HobbyTronics. This little board acts as a MIDI host to USB adapter for $15. This worked swell.  Here is my full Hammond organ prototype with USB Midi master. Teensy is mounted above the audio adapter, the 3 wire cable is the stereo audio output. This setup is powered by the Teensy USB connection which is also a USB serial port for debug, as well as the Teensy download port. The board with the blue LED is the USB master, which is driving the Oxygen 49 Keyboard. On the right I show the HobbyTronics Midi adapter which also works fine, but I'm not using it at this time.

roto2 hammond organ on teensy

Ornaments & Crime
While messing with Teensy, Steve turned me on to a cool open-source synthesizer module called Ornaments & Crime, a very clever analog synth control voltage processor. It has about 10 powerful operating modes, each with many parameters. It can be a powerful multi-channel quantizer, can generate many different mathematically and sonically interesting control voltages and sequences, and has a mode with quad Turing Machine sequencers.

I ordered O&C boards and panels, plus the various components, and build it from instructions on-line. It is very powerful. It sports 4 16 bit DACs to generate very precise CVs. It has a nice User Interface (UI) using a bright blue OLED graphic display and two encoder knobs. It takes a bit of menu diving to get at the settings, but the UI is well done and intuitive.

The parts are mostly 50 mil pitch ICs and 0805 passives, with a few tiny exceptions so building is fairly easy for someone experienced with SMT. It was pretty easy to build, install the code from Arduino, and calibrate. Runs great. I built 2, one for me and one for friend Steve D.

The challenge I had building O&C was to find the exact right 1.3" OLED. Unfortunately the pinout that the designers selected is pretty rare and must be ordered from China. I ordered 2 of the wrong ones, waited until they arrived from China, realized my mistake, and then ordered the right ones from Ali Express. Meanwhile I built a simple wiring adapter so I could temporarily plug in the incorrect OLED. The panel won't fit this way though. When the right OLEDs arrived, they plugged in and worked swell. I like the super high contrast of the blue OLEDs.

Here is O&C running the 1.2 firmware with the excellent sequencer.

Ornament & Crime

DIY Braids
Braids is a Eurorack "Macro Oscillator" designed and sold by Mutable Instruments. They bill it as capable of replacing a rack full of analog gear. It has dozens of different and interesting waveforms available, and each type of waveform can be modified by either knob or CV to affect its pitch, FM, "Color" and Timbre". It also has an envelope generator included. Steve uses a Braids as his main oscillator and I have wanted to play with one. I couldn't bring myself to pay $400 just to play. Since the design is open-source, and boards, panels and firmware are all available, so I built up a couple.

Armed with the experience of building someone else's microprocessor-based Eurorack modules (O&C), I found some clone Mutable Instruments Braids boards for $7, the panels for $30, and the other parts for about $60. Ideally you install the development tools, build (compile) and program the firmware, but I was put off by a statement on the Mutable Forum that the build process is difficult under Windows. Step 1: Install Linux. Step 2: install many development tools....  I located a pre-built version 1.5 on-line which worked perfectly. Then I used the excellent Mutable Instruments update facility to update the module to the latest, v1.8. I already had a programmer for the STM32F processor, and needed only a $12 adapter for the tiny JTAG header. 

Braids uses several fine-pitch parts and 0603 discrete components, so I don't recommend it as a first surface-mount project. Get comfortable with 0805 soldering first before you attack your first 0603 project. Me, I'm pretty advanced at SMT hand assembly and had no trouble building the boards.  

The only tricky part of Building Braids is that the encoders that Mutable uses are a special order and so not available. Encoders have many, many options: Pulses per rotation, pulses per click, click or no click, pinout, rotation direction, shaft length and type, bushing length and type, Turns out that the two big encoder suppliers, Panasonic and Bourns generally rotate in opposite directions. This can be fixed in firmware if you can compile the firmware. I couldn't find a compatible Panasonic part, but I found a similar TT part EN11-HSM1BF20 that works fine.

Here is DIY Braids.
DIY Braids

Buffered Mult
With all these new trigger and CV inputs ( 4 VCOs, Braids, O&C, Turing Machine, Drum machine, CV-> Midi) I was struggling with how to get inputs to all these modules.  Fortunately Synthrotek came to the rescue with the '07 MULT, a nice low cost and very dense buffered Mult. It makes multiple copies of up to 4 control voltages or trigger signals, and it does it precisely using TL07x op-amps. And it is low cost. I paid $25 for the boards and front panel, about $10 for the 18 jacks, and had the rest of the ICs, caps, resistors in my stock.. Ii had it built up in an hour +. Here it is in all its dense wiring glory.
DIY Braids

New Drum machine
I wanted a flexible and good sounding Drum machine with a sequencer. I considered Eurorack, but a Eurorack full drum set gets expensive quickly and takes a bunch of rack space.  I had a big Yamaha Midi Drum machine, but it was too big for my space. The Korg Volca Beats has all I wanted in a low cost $150) and small package.  It accepts an external trigger to keep the beat with an analog synth. And it it fun to setup and make beats.  I plan to allow hardware triggers of individual drums by building a CV and Trigger to Midi circuit out of an Arduino. Damn, this thing needs a good cleaning!

Korg Volca Beats with cat hair

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Last Updated: 7/12/2017