The Lil Sidrassi is a paper circuit designed by Peter Blasser. It has pretty good instructions.
Lil Sidrassi was made into a PC board by Dennis Vershoor. The circuit consists of five transistor oscillators, each running at a fixed pitch, as determined by the ‘hairy’ capacitor. The oscillators modulate each other, going around the ring. I added a three-position toggle switch to each circuit, so each one can operate at one of three pitches. The center position (off) uses only the on-board capacitor. I chose 10nf caps (103) for all the on-board caps, and 33nf for one of the positions on each switch. The other five are 47, 100, 220, 330, and 470 nanofarad. I picked these without listening first, with the result that the pitches came out relatively low. I may change some of these values to get higher pitches and less uniformity.
Assembly
I sandwiched the board between two 4×5 inch pieces of plexiglass.
The top piece holds the nodes and switches, plus the audio output jack.
Here it is together.
Two nodes in each circuit are brought out for touching. I found some brass screws, washers and nuts, situating them in a pentagonal shape surrounding a central node for the output. The five nodes closer to the output node are the oscillator outs; the farther nodes are inputs for glitch modulation. It all operates by touch. The output circuit is only a 2N3819 FET. To my amazement I found an old 3819 in my parts bin, its leads heavily oxidized after lying there for more than thirty years. It cleaned up and worked! The board supports an LM386 chip for driving a small speaker, but I took the output straight from the FET to a jack for plugging into an external amplifier.
Schematic
The schematic shows how the five SPST on-off-on switches are wired to select three ranges for each oscillator. The highest pitch is set by cap x, and then cap y or z can be switched in parallel to x to get two lower frequencies. Table shows the capacitors used (in nanofarad) for each oscillator. Notice how each oscillator’s output modulates the next’s frequency (O1 out to O2 mod, etc.).
Demo
For this demo, I passed the signal though the Synthesis Technology E580 Mini-delay module. The player brings out sounds by touching the nodes. Multiple outputs can mix to the output and some pitch bending is possible. Toggles go into lower, and even sub-audio frequencies.
Live performance
And here’s me, performing it. This is after I changed eight capacitors to get more pitches.
https://www.youtube.com/watch?v=0P18lB4FeX8
I really enjoyed this 🙂 The idea of making touch-synths have always appealed to me. And your personal design I find is very attractive. There’s something quite erotic about the whole thing, is it not? I have just started out myself, and would appreciate if you’d take a gander.
I see you have just started your blog. I hope you’ll put up some sound or video. Please check out Peter Blasser’s paper circuits, I think you’ll like them. I’ve been watching videos of trains in Norway and I would like to visit someday and ride them. Beautiful, beautiful country.
Hi Richard, thank you so much for this explainer! I was curious about how you soldered the other sets of caps in to expand the range of the sid. I’ve seen people use pots and have found some videos of tone controls, but they all lack explicit instructions on what to solder where. I’ve built two of these and would like to expand their sounds on the fly! Thank you so much!
Austin
In reply to Austin Richey:
When reading the circuit/paper design, you can find a component with a “sun”: it means “choose a capacitor with any value”.
The author (Richard Brewster) posted a table (the one written in pencil) with X, Y and Z: those are capacitor values. You can try any combination, repeating it for the 5 (or more) oscillators.