Wednesday, 14 September 2011

MFOS SoundLab Ultimate and Expander

After having completed the MFOS SoundLab MiniSynth Plus, the PAiA FatMan, and the 10 step sequencer, I desperately needed another project. I had already found the SoundLab Ultimate and Expander half-modular synthesizers on Ray Wilson's site, and with a series of projects successfully completed I considered myself experienced enough to go for a bigger project. I bought both Ultimate and Expander PCBs including front plates -- knowing that it was usually the front panel design that would slow me down.

I tend to follow construction guides to the letter, for example using 5% resistors wherever the designer used them even if 1% ones might be better. As usual I started soldering components according to increasing height and delicacy, first resistors, then diodes, IC sockets, capacitors, transistors, and so on.

The SoundLab Ultimate is a precision synthesizer with 3 temperature-stabilized oscillators. To guarantee stability it requires matched transistors for the current converter circuits. You can buy matched transistor ICs such as the SSM2210 or LM394, but they are very rare and very expensive. A short Google search reveals several circuits showing you how to match transistors for equal VBE, where less than +/- 2mV is considered "equal" enough. If you buy transistors by the lot and receive them taped, you'll find that most of them will be close enough anyway. I followed Ray Wilson's transistor matching guide and ended up having two sizable batches of matched transistors, each within a 2mV margin, more than enough for the Ultimate and the Expander.

I glued them pairwise together and stuck a tempco resistor on top. I should have put a cable tie around them for insulation and better hold. Perhaps I'll do that when I do a few modifications. The whole contraption was then inserted in a socket that I unnecessarily soldered there.

The noise generator requires a transistor whose thermal noise is then amplified. Ray suggests selecting among a couple of transistors the noisiest one. I did that and found one that indeed exhibited a sharper hiss. I soldered that one in place. That was the last component to be soldered in.

I spent a while researching various cabinet designs and found several that I liked. In the end I went with an early design with both Ultimate and Expander side by side with the auxiliary panels below but accessible from the front. I bought some timber boards that I hand-sawed to size. Below the two openings for the front panels I added a small board with holes cut for the two auxiliary panels. With the basic cabinet complete I stained everything black and sprayed the thing with a few coats of mat lacquer. It looked quite professional.

To fix the PCBs in the cabinet, I designed a rotating PCB holder allowing me to inspect the circuit and then fold the PCB upright (see picture). Unfortunately, Ray Wilson's suggestion (while doing a different project) to attach the PCB to the back of the front panel came too late. It would have simplified things a lot.

In the construction guide, Ray warns that some of the wires connecting the PCB and front panel might radiate audio signals that are picked up by other wires. That's why he suggests to tightly twist signal and ground wires for those connections. However, I found a few lines that still "sing through". I bought shielded wire that I'll replace those wires with. Also, the blue LEDs are very cool, but perhaps a tad too bright. I might replace those, too.

The power supply is based on an MFOS Wall Wart power supply board (bottom of picture above) that converts a 12VAC input into the necessary stabilized -12V/0V/+12V DC voltages.

All in all the Ultimate/Expander pair is a great analog synthesizer with powerful oscillators and great filters, and very versatile because in addition to the fixed internal wiring it's also a modular synthesizer that can be patched up for an infinite variety of sounds and effects.

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