Design
Prototype
Figure 1 - LMS101 Breadboard Prototype
While designing the LMS101, it was important to prototype and test the synthesizer to ensure that the designs function correctly, as well as sounded great!
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To test our designs, we built the LMS101 breadboard prototype.
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We implemented an affordable +/- 15V switching power supply and a power conditioning section to test our design. Regulators, bulk, and decoupling capacitors ensure nice clean power for our more sensitive components.
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We needed to monitor the sound, so one of the first things we implemented was the headphone amplifier. For this, we simplified the Befaco Output V3 Eurorack open-source design to our suit out specific needs [2].
Oscillator
The LMS101s oscillator is produced using the Electro-Smith CEM3340 VCO submodule, which emits sine, triangle, saw, and square waves. It also includes many other peripheral inputs, such as frequency modulation (FM), a pulse-width modulation (PWM) input pin to modulate the square wave, coarse and fine tuning, etc. It was the perfect submodule for our application.
Figure 2 - Electro-Smith CEM3340 Submodule [3]
Wavefolder
The wavefolder is based on a simplified version of the Buchla 259 Complex Wave Generator Timbre circuit, outlined in a paper by Fabián Esqueda and Julian D. Parker [4]. We wired it up and, with the addition of some carefully calibrated input gain stages, we could immediately hear that we had found our synth's powerful voice.
Figure 3 - Simplified Buchla 259 Timbre Schematic [4]
Analog Control Voltage Inputs
We used vactrols (photo-resistive opto-isolators) in the analog section of the synthesizer for the Timbre and the Low-Pass Gate control voltage inputs, typical of Buchla and other additive synthesizers of the 1960’s.
Figure 4 - Vactrol [5]
Figure 5 - Cardboard Test Keys
Keyboard Prototypes
We built our initial keyboard prototype out of copper tape on a carboard box (Figure 5) and it worked surprisingly well. From there, we designed a PCB with various key patterns, and with the help of Landon Brown, Camosun Lab Technologist, had the test board produced quickly on Camosun's in-house PCB milling machine (Figure 6). Despite the drastically different patterns, the test keys performed comparably.
Figure 6 - Milled Test Keys
Q-Touch and Multiplexing
The keyboard uses Microchip’s AT42QT2120 Q-Touch sensors to detect finger contact on the capacitive touch pads. These sensors notify the microcontroller that a key touch has been detected. Once notified, the microcontroller communicates with the QTouch sensor over an I2C bus to determine which keys have been pressed. For our 25 keys, pitch bend slider, and octave transposition buttons, we required three AT42QT2120s. Due to these devices having the same device ID, an I2C multiplexer was necessary for the microcontroller to be able to communicate with each chip individually. We went with the Diodes Incorporated PI4MSD5V9548A I2C Multiplexer.
Figure 9 - Microchip AT42QT2120 Q-Touch IC [6]
Figure 10 - Diodes Incorporated PI4MSD5V9548A I2C Multiplexer [7]
Microcontrollers
For the keyboard, we selected the Pi Pico as our microcontroller because it has multiple of each I/O peripheral and has flexible pin configurations.
For the synthesizer's digital modulation section (Envelope Generator, LFOs, and S&H), we selected the Arduino Nano Every because we required 5 analog-to-digital convertors.
Figure 8 - Arduino Nano Every [9]
Figure 11 - Hand-drawn Synthesizer Schematic
Prototype Sketches
While we prototyped, we simultaneously laid out schematics.
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We began with pencil to paper. Rather than start in software, it was important to test designs and make changes quickly. We also felt like the synthesizer engineers of the 1960s who inspired this project, drawing schematics by candlelight.
We then moved over to Altium to capture our schematic and PCB designs. In Altium, we were able to use hierarchical and flat design processes for the keyboard and synthesizer schematics to suit our needs. Flat design means all schematics stand alone within a project and link to one another, such as for the synthesizer schematics (Figure 12 and 13), whereas for the keyboard, hierarchical design was important because of the data transmission from one section to another (Figure 14). You can see that the green boxes represent sub-schematics, where the nitty gritty stuff is occurring.
Altium Schematics
Figure 12 - Altium Schematic of Digital Synth Section
Figure 13 - Altium Schematic of Analog Synth Section
Figure 14 - Altium Schematic Overview of Keyboard
Figure 15 - Altium Synthesizer PCB Top Layer
Synthesizer PCB
The synthesizer PCB was constrained by the user interface of the top layer. We wanted the I/O jacks to be placed along the left side and bottom of the panel, to give users full access to the controls.
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On the bottom layer, we separated power conditioning, sensitive analog signals, and high-speed digital signals because they all share the same ground planes. Isolating them was important to make the synth sound as good as possible.
Figure 16 - Altium Synthesizer PCB Bottom Layer
Figure 17 - Altium Keyboard PCB Top Layer
Keyboard PCBs
The keyboard PCBs balance ergonomic design while constrained to a very small space. Because of the nature of the capacitive sensors, no electronics could be placed directly beneath the keys.
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For economy of space, we wanted to use as many surface-mount parts as possible. However, 3.5mm jacks do not come in surface mount, which forced us towards designing a second PCB for the through-hole components.
Figure 19 - Altium Keyboard Daughter PCB Top
Figure 18 - Altium Keyboard PCB Bottom Layer
Figure 20 - Altium Keyboard Daughter PCB Bottom
Enclosure
The LMS101 enclosure is a laser cut plywood box designed in Fusion 360, to balance futuristic aesthetics with something more grounded.
Figure 21 - LMS101 Laser-Cut Plywood Enclosure