Monday, 1 November 2010

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!

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