The Pro-Mini

Pro-Mini Monophonic Synthesiser
The Pro-Mini is based on the PCB from a Sequential Circuits Pro-One whose case and keyboard are long since gone. Originally the board seemed to have some problems: sometimes the pitch dropped a quarter of a tone for no appearant reason. Didn't seem like a big deal at any rate, but I couldn't figure out what to do with the board, so it lay around unused for quite some time.

Then after a while, a few things started to come together:

In the 'surplus' box at a local electronics shop, I found a cheap keyboard, which looked like it'd been extracted from a Roland SH-1000 or similar machine.
I was given a number of round plywood disks which were left over when someone needed to make a number of large round holes in a sheet of plywood.
Another electronics shop was selling out aluminium cases for an amplifier/radio kit they'd once sold.

With all this, I decided to build a Pro-One using the PCB I had, with some improvements:

More compact. The original Pro-One is a bit of a clumsy beast.
Three audio oscillators. The original Pro-One has only two, but the LFO is based on a CEM 3340 oscillator chip just like the audio oscillators, so it would not be a big deal to have three audio oscillators just like a Minimoog.
Moog filter. Although I wanted to keep the original Curtis filter from the Pro-One, I wanted a Moog filter as an option, if nothing else to be able to A-B compare the difference between the filters.
Better digital interfacing. The Pro-One has a simple parallel interface, but it is not brought out to the outside of the machine.
Three arpeggio modes: up, up/down and down. The original Pro-One has all three, but only two can be accessed without patching the PCB.
Some other slight improvments, like avoiding the cheap slide switches in the original, and have more octave ranges on the oscillators.

Also, I wanted a much more compact machine than the original Pro-One, which is a bit of a bulky machine. There's quite a bit of air in there. In contrast, the insides of the Pro-Mini are quite cramped, making servicing a trying task.

In order to fit everything inside the new case, and because of the extra features I wanted, I had to ditch the original front panel layout of the Pro-One. This meant that I had to remove all the pots and switches from the Pro-One PCB, and design a new front panel from scratch, with new components. Besides, I figured the most likely cause for the pitch instability of the unit I had was a faulty pot or switch, so replacing these components seemed worth while. The PCB was patched to accommodate the changes in the LFO, increased number of oscillator ranges and external digital interface. A Minimoog filter was built from the original schematics on a piece of veroboard, and in turn mounted on the main PCB.

I like to implement slightly odd wheel designs, mostly because I lack the expertise and equipment to do it properly... In the Pro-Mini I've used circular pieces of plywood which I got hold of. The plywood has been lacquered with several layers of lacquer to give it a durable surface. Unusually, the potentiometers have been mounted on the wheels, with the pot axles being anchored in a wood block attached to the case (the same design was used in the MiMi-a synthesiser.

Pro-Mini plywood wheels

During the reconstruction: disaster! While I was testing I accidentally brushed a wire carrying +15V against a couple of PCB traces, instantly destroying the CEM 3310 envelope chips! Luckily, at the time, PAiA were still selling Curtis chips (this was a couple of years ago), so a quick phone call to the nice folks at PAiA got me back on track about a week later.

Detail of front panel, showing Moog switch

The end result is a nice sounding compact synthesiser which sounds exactly like a Pro-One of course, but with the additional feature of having three oscillators and optional Moog filter. The Moog filter is a nice variation, but it must be said that the Moog filter alone does not fully characterize the Moog sound. The Pro-Mini still doesn't feel like a Moog synthesizer just because the Moog filter button is depressed. Incidentally, the name Pro-Mini is a slightly clumsy combination of 'Pro-One' and 'Minimoog'.

Rear view showing digital interface connection

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