V/Oct Calibration Tool

I'm currently upgrading a few of my first DIY synthesizers by adding MIDI control and perhaps also renovating the front panel. I also want to re-calibrate the tracking of all the oscillators of the first synths I built in order to have a proper volt/octave scale for the MIDI/CV converters to work with. To that end, I built a small V/Oct calibration tool based on an old circuit by Ray Wilson of Music From Outer Space that he mentions in one of his earlier VCO circuits. In contrast to Ray's tool, mine spans a 7 octave range by supplying voltages from 0 to 7 volts.

V/Oct calibration tool. Soon with enclosure.

I started by selecting a couple of resistors from a batch of 50 or so (10k 1% metal film) for equal resistance using a Wheatstone bridge. I managed to find two groups of 3 and 4 resistors that among them produced a midpoint voltage of 0.0mV (when measured using my cheapo DMM), with the two groups being 0.1mV apart. I don't know how that difference translates to resistance, but in any case they all now seem pretty equal to me. :-)

I used these resistors to build a multi-level voltage divider to produce the voltage steps starting with the 7 volts on top of the chain. Incidentally, I get the top 7 volts by simply swapping the 47k and 68k resistors in Ray's circuit (trimmed to precision using a 10k multi-turn trimpot). The calibration tool's power is taken from the synth's power supply. Two LEDs on the front panel show proper connection to both negative and positive supply voltages.

It took a bit more time to build than I anticipated (I did it on perfbord with the component leads providing the connections between parts), but now that it's finished I actually look forward to tediously calibrating all the oscillators.

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.

MFOS Sequencers

The MFOS SoundLab MiniSynth is a voltage controlled synthesizer using a voltage per octave scale. To play it you need a musical keyboard with a CV (control voltage) output. You can buy them, but they're rather expensive. You can also DIY them by salvaging a keybord from a defunct electronic organ. I might do that sometime. Another way of "playing" a synthesizer is by feeding it control voltages through a device called sequencer. That's a device with a load of knobs by which you can set up and then repeatedly step through a number of voltages (usually 8 or 16) that you can feed into a synthesizer in order to get those repetitive bass lines and arpeggios that electronic music is famous for.

A sequencer is actually a fairly simple device to DIY, and there are simple designs galore on the internet, some using only very few components. When I get to it, I'll definitely design and build my very own sequencer, but to play with the SoundLab I wanted something simple fast. So I ordered the PCB of Ray Wilson's 10 step sequencer from Music From Outer Space (actually, I ordered and stuffed the PCBs of Ray's 16 step sequencer before I bought the smaller 10 step one. It's still unfinished but that's another story...). With my soldering skills steadily improving, stuffing the PCB was quick and painless. As usual with my DIY projects it was the housing that slowed me down. I finally decided to craft a small wooden box to house my sequencer. The front panel was to be a thin wooden board that I shielded by glueing a piece of aluminium sheet to the back. The labelling on the front consists of a printout of my design that I laminated, cut, and then stuck to the front (self-adhesive lamination sheet). It ended up way better than I expected.

When I tested the sequencer I found that the muted steps somehow leaked through to the synthesizer: I could still hear notes that were supposedly turned off. After debugging and signal tracing for hours I couldn't find a fault with the sequencer. I just hadn't played with the synthesizer long enough to discover that there's a dedicated knob to set by how much the AR envelope generator affects the volume of the sound. If, for example, set to mid-level the AR (and hence the sequencer's gate) control only the top half of the volume range while the bottom half is just sounding on, all the while changing notes according to the sequencer's changing voltage. Who could have known!

MFOS SoundLab Mini-Synth PLUS

The first Synth DIY project that I more or less successfully completed was a SoundLab Mini-Synth PLUS from Ray Wilson's site Music From Outer Space. I decided to purchase the necessary PCBs after having researched other options, in particular the Simplesizer project on a German Synth DIY site. Unfortunately, that project was abandoned a few years ago, and PCBs are not available anymore.

While waiting for the PCB to arrive and checking various suppliers for the necessary electronic parts I found out about an enhancement to the original design (the actual "PLUS" part). The PLUS enhancements carried the warning that they'll involve several kludges on the board and therefore require having some additional skills that I quite probably lacked. I boldly went for it anyway.

Building it was time consuming because I needed to learn basic skills such as soldering "on the spot" (both figuratively and literally). The desoldering pump was in frequent use. Also, I couldn't quite decide on the cabinet for the synthesizer which slowed things down for a while. Furthermore, I wanted a different panel layout than the one suggested by Ray Wilson. This required that I modified the wiring layout (i.e. the connections between the various controls on the panel).

When I turned on the Mini-Synth for the first time, it appeared to work! Mostly, anyway. The second VCO was silent. That spelled trouble in capital letters. I had no equipment to speak of, and was supposed to signal-trace and find a problem on a circuit that I basically didn't understand. To pinpoint the problem, I reasoned, it might be helpful to tune both VCOs to about the same frequency so I could check each of the circuits and compare the corresponding voltages (I had one functioning VCO to compare the malfunctioning one with). It took me a lot of studying circuit diagrams and a night's sleep to eventually notice that the same wire cannot have 0 volts at one and 7 volts at the other end. A close inspection with a looking glass revealed a dodgy soldering joint at a transistor.

After having fixed that, the SoundLab worked perfectly. I hadn't calibrated it to a proper Volt/octave scale yet. Nonetheless, turning knobs and throwing switches and listening to the noises that it produced was a lot of fun. Most importantly, though, I was hooked.