Personal Synthesizer

[Personal synthesizer schematic / 5MB]

[mp3 .zip archive / 12.5MB]

I built the personal synthesizer in 2004. It was designed to fit in a small suitcase so I could carry it on to an airplane. I made the whole thing in a week and a half. It was a blur of excitement.

I finished at 3 am, slept for a few hours, and then left with my family to catch a flight to my cousin's wedding. The day after the wedding, I took a five hour bus ride to meet some friends who were on tour. I practiced the new instrument on the bus with headphones. That night I played it in public for the first time. Right away I knew I'd stumbled on something really magic. This instrument was very small and totally self-contained, but had a lot to offer. It became my primary performance synthesizer for the next three years.

The synthesizer has two oscillators based on Thomas Henry's old linear-cv 8038 design. My innovation was to add a control voltage to drive the 8038's waveshaping section. That turned out to be a big part of the magic!

The synth also has a headphone amplifier which uses a transformer. But without the headphones plugged in, the capacitor and primary of the transformer form a current-hungry resonator. So turning "on" the headphone amp gives rise to a pronounced bass boost, or even motor-boating. (Low-frequency feedback through the power supply.)

The mic pre is based on an old Shure microphone mixer. Transformer-coupled circuits have always seemed really magic to me: the information temporarily leaves the charge domain, becomes magnetic flux, and then is converted back to charge again. For this section I used parts recycled off a mystery circuit board I found on a dusty shelf at Jabbour Electronics in Pawtucket RI. Sadly this so-called business has left the building!

I followed the mic pre by the Polyfusion envelope follower (section B, above diagram). However, assigning provenance to this circuit is dubious. The same topology is given as a generic in several old resources, notably the classic National Semiconductor AN-20 (available here). I routed the ef output through a +-1 buffer of course (section A, above diagram).

The synth has a noise generator (section C, above diagram) adapted from a Ray Wilson design. I don't really "like" this circuit (specifically: operating the second opamp open-loop like that just seems wrong) but it works "ok" and I never got around to changing it. I also added a simple 2-pole low pass filter to get a rumbly output. Both noise outputs go through 50k pots (not shown) so you can control how strong the noise outputs are.

Finally, to get to the other really magic part of the personal synth, I eschewed filters and opted instead for a diode-bridge vca, implemented with zener diodes. Theoretically the zeners shouldn't really change things (consider how attenuated the audio input it) but as they say, the results speak for themselves. This is based on a Korg design (MS-50, and also an older opamp-free one from an unknown product), but I removed all the stabilizing/linearizing parts. I apologize for the quality of the schematic but as I said, it was 3 am at that point. Not shown in the drawings is a three-input passive mixer to the diode bridge vca, and also three cv inputs to the bridge (rather than the single one shown).

As it turned out, the most important aspects of the personal synth (from a practical standpoint) were that it fit into a small suitcase so I could bring it everywhere, and its capabilities were both large and small enough that I was able to really develop a relationship with it. A few months after I finished it, I started touring a lot. We had some really great shows together, and a few really terrible ones.

In the good category would have to be the third night I played it. We were in Eugene OR. Instead of a pa I was playing through a big stack of amps. (Probably Prurient's, he had a good concept of heroic size at the time.) Somehow I found out that when I breathed hard into the mic, the protection on the amps kicked in for a second. The sound kind of went away, then it slowly came back. I had these heavy bass interference patterns going below that, so they'd come back all wavery, then I'd exhale again. I remember lying on my stomach on the floor, feeling out of breath and very small but effective, totally entranced by this massive process I could control.

On the bad side would have to be that time I got pissed off in Albany, decided to quit music for good, and threw the poor personal synth on the floor as hard as I could. I had been trying to figure out how to play the song "Let Go" in public. (In point of fact, it was never a good idea. The other time I tried, the most special microphone I ever had was permanently destroyed.) I wanted to leave the synthesizer in the corner of the disgusting club, but luckily my friend Victoria had the good sense to put it in her car. (In the end, I fixed it of course.)

But as much fun as it was getting to travel around and play the personal synth in front of people, the most fun I had with it was playing at home, in the basement. That's where I liked to do experiments, test things out, and make recordings. It had a lucky combination of scientific repeatability and inherent chaos, and it was the perfect size to fit on my lap. That's how I ended up being able to do very relaxed recordings like "New Hard Core", from a 2005 cassette called Wiped Away, originally released on Durable Stimuli, and "The Sun, The Sky, The World", from the Can't 7" released by Ultra Eczema, in 2006.