Meet Mitxela's Flash Synth: The World's Smallest Synth Gets Slightly Larger, A Lot More Powerful

Mitxela's Tim Alex Jacobs has partnered with Aaron Andrew Hunt of H-Pi Instruments to revisit the World's Smallest MIDI Synth project, creating a slightly larger device — but one still barely larger than a DIN plug — which boasts 16-channel polyphonic tuned output straight into headphones or speakers, no external power required.

Jacobs' original World's Smallest Midi Synthesiser project was completed in August 2015, packing an ATtiny85 microcontroller, piezoelectric buzzer, and supporting components into a cut-off DIN plug. Connected to a standards-compliant MIDI keyboard, the tiny synth — held together with electrical tape — would happily respond with sound, no external power required. Having seen the original project, Hunt commissioned Jacobs to build something a little more saleable — and the result is the Flash Synth, a more powerful and robust design with 3.5mm audio jack output.

"As I concluded with my last attempt at a polyphonic synth cable, the solution is to throw more processing power at [the project]. Since there was no harm in trying, I threw an STM32F103 onto a board to see if we could power it with the midi signal. We could not," writes Jacobs. "I next made a search for a more appropriate chip, and given the specification initially settled on the STM32L476RG. This has 1MB of flash memory, a built in DAC, everything we need. Pretty astonishing what that chip can do, it's a super duper micro. The datasheet gives a power consumption in terms of μA per MHz."

Having iterated through a few prototype designs, including one with a compact QFN part soldered upside-down onto protoboard, Jacobs completed the software which would drive the synth — complete with lookup tables for base frequencies, pitch bending, and for sine-wave lookup as a means of freeing CPU cycles for synthesis operation. For the production-ready hardware, Jacobs turned to flexible PCB technology in order to fold the circuit and fit it into the MIDI connector.

"My method of designing flex PCBs is to first make a paper template. Having your own laser cutter really pays off for something like this, being able to quickly export the PCB and chop it out of paper exactly. Stacks of thin card simulate the components. The first flex PCB had some problems I didn't anticipate, because I'd opted not to add a stiffener board under the chip. The solder joins were prone to cracking when the board was flexed. I fixed this by adding relief into the design of the ground plane, so that the flexures were more clearly positioned. The first design probably would have been fine as we're not planning to open and close this more than once, but it's worth having the safety factor."

The final step was the housing: a robust metal DIN plug makes up the MIDI connector plus housing for the flex PCB, while a 3.5mm audio jack is situated at the other end of a custom-machined metal middle. While the result is somewhat longer than the original Smallest Synth, it's also a lot more durable — and allows the audio output to be redirected to headphones, speakers, or an external recorder, and all without any external power.

Hunt has begun taking pre-orders for the Flash Synth at €129.99 (around $144) on the H-Pi Instruments website; a detailed write-up on the project can be found on Jacobs' own website mitxela.com.