The MiMi-a opened

The MiMi-a is built entirely on copperless veroboard. The digital boards are built with the components and wiring on the top of the board and only the power lines on the bottom side, whereas the analog boards have the components one the top and most of the wiring on the bottom side, like conventional circuit boards.

But since this is an analog synth, let's start with the analog boards.

The MiMi-a opened
With cover opened...

There are a number of circuit boards in the bottom of the case, from left to right:

In the lid of the case the keypad is mounted, together with decoding logic glued directly to the board. (Yes, if someone wants to know, it's from a desk calculator.) Also, the display driver board under the display, and just behind the output connectors, the output assigner board for mapping the voices to the six individual outputs.

The voice boards are worth looking closer at, as they took about a day each to build, after the initial design had been worked out on a breadboard. 6 continuous days of soldering (and not just 9-5 either), and I was, well, tired, to put it mildly.

Evident on the voice board are the four capacitors in a row in the filter section, surrounded by transistors, spilling the beans that the filter is actually a Moog design to start with. The signal processing is in a logical flow from left to right, with the oscillators' analog section on the extreme left, continuing with the wave shapers and ring modulator, waveform selector and modulation routing analog switches (partly hidden by the ribbon cable), oscillator mixer and filter-by-oscillator-B VCA's, filter, and finally the VCA on the right.

A MiMi-a voice board stack
Close-up of a MiMi-a voice board stack

But what about the digital electronics? Well, remove the keyboard, and, voila...

The MiMi-a with the keyboard removed
...and keyboard removed
From left to right, the boards are:

From initial design to finalization of the first version, it took about half a year. That includes design, breadboarding, soldering, hardware and software. At that time, the output assigner was not completed, and a number of software modules were not finished, like the VGC and split/layer modes. The software now comprises about 14 Kbytes of code for the main operating system, with a few additional kilobytes for the cassette storage and diagnostic modes, and further kilobytes for frequency (oscillator and filter tracking) tables. Two 27128 chips are used for program storage.

A few updates on the analog boards have also been done after the original design, including the linear/exponential VCA, but no major design changes have been effected.

Back to the MiMi-a page.