Aria’s Honking Synths »

Sea Moss Raft

This Lunetta-style CMOS synth was one of my first DIY projects: flawed, but very educational to build. It honks good, but isn’t music-capable.

Take a listen:

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I barely knew how to make a LED emit light rather than smoke when I started building these. Nobody should attempt to reproduce this flawed build as-is, and no files or build instructions are offered, but I hope it will inspire you. I stopped adding new modules to it once I felt ready to start building bigger, in the Kosmo format. It’s probably as complete as it will ever be.

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Lunetta’s CMOS synths

Stanley Lunetta, who pioneered this style of circuit, called his synthesizers sound sculptures. The rest of us, we call them Lunettas.

A cube about 30cm wide covered in knobs and banana plugs on all visible sides, with a bunch of cables everywhere Photo and build by Stanley Lunetta

Lunetta synths focus primarily on exploring the interactions of the CMOS-4000 series of chips, which deal with digital logic. Compared to more advanced synth DIY endeavors, they are easy to build and understand, cheap to make, and reward experimentation.

Stanley’s website survived him, and shows a few pictures of his sound installations. There’s also a few circuit schematics, but without much explanation for beginners.

A more accessible introduction to musical abuse of CMOS chips would be Elliot Williams’ Logic Noise series: most of the circuits I used come from those articles. To dig deeper, explore over a decade of archives in the Lunetta forums on electro-music.com, and read the in-depth circuit analyses of the Castle Rocktronics modules.

The 4000-series

Those little critters dropped in the late 60’s to do a bunch of binary maths. You feed the chips some juice, give them some inputs, they think really hard, and they pronounce a judgement, rendered on the output pins. There’s like over a hundred chips, and they all do different stuff.

Their inputs and outputs are generally digital: they only care about two voltage levels, on and off. So if you power them with 12V, inputs are high when they are close enough to 12V, low when they are close enough to the ground, there’s no in-between state.

And if you make them output on and off voltages at the correct speed, flipping from high to low something between 30 and 15,000 times per second? Then it’s a square wave you can listen to! Assuming the maximum voltage isn’t too high, you can simply plug in some cheap amplified speakers and give it a listen directly. The change in voltage causes the speaker to move, the rate of change being the pitch you hear.

In a Lunetta circuit, it’s mostly those kind of square waves that circulate. Slow square waves can be used for sequencing. Fast square waves get used as audio. Nothing has a definite purpose, any output can go into any input, any chip can process signals fast enough to be used as an audio effect. Very often, a Lunetta module is nothing more than a chip protected with diodes and resistors, its pins exposed almost directly to the user.

Wanna hear what I said with a lot more big science words? Use one of the resources I linked you, because that’s as far as I’ll explain on this page! Get yourself a more legit teacher if you wanna know what the chips actually do.

Design

A few modules of the synth are connected to re-usable plastic breadboards using DuPont wires
All the guts of the synth are exposed as bare PCBs with male header pins, patched with female DuPont cables. The small size makes it unsuited for live patching, but exceptionally well suited to experimentation with breadboards.

As there are no panels, the aesthetics of the PCBs are important. They must also double as a reasonably ergonomic user interface.

The system runs on +12V DC. It still works at half that, but the values are tuned for +12V, as pitches and envelopes sound different at lower voltages. There is no dedicated power module, it runs off a bench PSU.

The system flipped upside-down, showing the wires soldered underneath the prototype boards
To keep the patches readable, most of the hardwiring is run below the boards.

The rafts were printed with PLA. The models were made with FreeCAD; it was my first serious project with it, and I found the software extremely difficult to learn and to use, even by the standards of someone used to professional software.

The modules tile to an approximate 60×60mm grid. This allows integrating 100×100mm PCBs in 120×120mm modules comfortably, the maximum size for which some PCB fab houses cut you a special deal for minimum quantity orders.

The modules are held together by their feet with little holding rings that look like buttons, taking countersunk screws. The holes in the feet of modules are reinforced with heat set inserts.
3D rendering showing how the models are assembled
The way the modules tile was probably the most interesting aspect of the build, though the 3D files were poorly made, as FreeCAD made it difficult to get anything done.

Custom Prototype Board

That was my first original printed circuit board design!
Inspired by commercialy available Permanent Prototype boards.

A few protoboard PCBs: they are mostly a grid of holes to which you can solder components.

It’s 99×99 mm. There’s built-in reverse voltage protection, a status LED, a main decoupling capacitor, room for 6 potentiometers or tactile buttons, and two half-breadboard layouts stacked over each other, with power rails.

The footprints at the bottom can be populated with pushbuttons or potentiometers. They are not connected to the rest of the circuit, but the topmost pad of each group can be used to run wires.

This is a picture of Revision 1, which has a few flaws (such as the status LED shorting if populated). I also made a Revision 2, which fixed those issues, but never put to use.

Due to their limited usefulness outside this project, and their lack of documentation, the Gerber files are not offered.

I later turned this design into more polished prototype boards for Eurorack and Kosmo.

The Modules

40106 Oscillators

6 voltage-controlled oscillators: 2 LFO, 2 in the lower audio range, and 2 in the higher audio range. Square and triangle wave outputs.

Two modules on small 3D printed rafts. They are mounted on protobards, the largest is 10×10 centimeters. There's a bunch of knobs and small wires flying everywhere.
The board on the left is revision 2. The one in the center, with the pots mounted on a sloped module, was revision 1, and never quite fully worked. Revision 1 was eventually dismantled to reclaim the pots and chips: I can only spare the space to store so many of my beginner efforts.

When using all inverters on a 40106 like this, they have a fairly noticeable degree of crosstalk.

To fill the space, there are also rows of header pins connected together to serve as mults, patchable light dependent resistors, pushbuttons, and a pair of vactrol lowpass gates. The vactrols are handmade with a LDR and a LED in heat shrink tubing. They never quite worked right, as many outputs in my system can’t push enough current to make them light up enough.

4017 8-step counter

Reset is hardwired to limit it to 8 steps, which is more useful in conventional music than the chip’s maximum of 10.

This module has a chip and 8 LEDs in a row, one lit.

4040 binary counter

This chip can be used to divide a clock or to generate sub-oscillators. Whoever came up with its pinout was a weird sad pervert.

This module has a bunch of wires soldered above a chip, and lots of small LEDs lit above it.

Percussions Module & Switching Sequencer

A collection of 4 circuits on a single protoboard

Build on my custom protoboard, this module is very busy, with 4 chips, a bunch of pots, LEDs, and resistors.

⓵ Daisy-chainable XOR exposing the pins of a 4070
⓶ Hi-Hat envelope with choke. Inspired by a circuit from a Logic Noise post, just don’t ask me how it works, I came up with it through experimentation.
⓷ Gate to Trigger circuit based on Ken Stone’s Cat Girl Synth 24, meow. While I got it to work on breadboard, I couldn’t get my finyal build to work, so this part is provides visual ornyamentation.
⓸ Switching sequencer with oscillator, based on a Logic Noise article as always. Uses only 2 gate inputs and 4 pots, but the chip could support 3 and 8.

Output Module & Mixer

Based on the Castle Rocktronics schematics.

Two modules on small 3D printed rafts. They are mounted on protobards, the largest is 10×10 centimeters. There's a bunch of knobs and small wires flying everywhere.
Built for a 60×40 mm perfboard, drilled to fit a mini-jack, with grooves for the potentiometer routed out using a rotary tool with a cutoff wheel.

A mixer lives on a separate board.

Wire tree!!

One of the modules is a very tall rod, with a 3D printed adapter at the top, from which DuPont wires are hanging.
Miles “Tails” Prower plushie for scale.

What’s next?

This raft was abandoned ashore, but I’d like to eventually explore CMOS circuits again, now that my skills have improved. The Lunetta style is endlessly fascinating, and I’d like to integrate it better to my Kosmo modular.