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POLYKIT SYNTH DIY

Semi-Modular Synthesizer

Project Modular Analog Synthesizer Diy Open-Source Pcb Vco Vcf Adsr Vca Wavefolder Schematics Kicad
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An analog, semi-modular, patchable synthesizer you can build yourself

The Polykit X1 is a DIY semi-modular synthesizer that pairs a single Polykit-X voice card with a dedicated interface board: LFO, wave folder, mixer, attenuators, buffered multiple, offset voltages, and power supply. Everything is fully open source—schematics, KiCad PCB layout, front panel, and a 3D-printable case. It is designed to work with the same voice card used in the polyphonic synthesizer project.

Overview
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  • Semi-modular, patchable – 3.5 mm jacks throughout for CV, gate, and audio
  • One voice card – VCO (saw, triangle, pulse), ADSR, multimode filter (8 responses), external input, stereo panning
  • Interface board – LFO, wave folder, 2‑channel mixer, attenuator, buffered multiple, offset voltages, ±12 V / ±5 V PSU
  • Open source – KiCad schematics and PCB, Gerbers, front panel files, STL for case
  • 12 V AC wall-wart powered (e.g. 12 V AC/AC 1 A)

Voice card (Polykit-X)
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The sound engine is the Polykit-X voice card: one PCB that carries a full analog voice. It is the same design used in the 8‑voice polyphonic synth; here it is used once and patched via the front panel.

Voice card specs
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  • VCO – Saw, triangle, and pulse; 1 V/oct pitch CV, FM, PWM, hard and soft sync
  • Envelope – One ADSR; output and CV inputs for A, D, S, R; gate and trigger inputs
  • Multimode filter – Pole-mixing VCF with 8 filter responses: 4P LP, 2P LP, 4P HP, 2P HP, 4P BP, 2P BP, 3P AP + 1P LP, 4P N (based on pole-mixing principles as in classic Oberheim-style filters)
  • VCAs – Internal VCAs for blending, main VCA for the envelope, panning VCA for stereo out
  • External input – Audio in that can be mixed with the oscillator
  • Output – Stereo (left/right) with panning CV

The voice card connects to the interface board via a 36‑pin header. Pinout, BOM, and tuning steps are documented in the voice card repository.

Semi-modular interface
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The X1 interface board adds everything needed around the voice card to make a playable monosynth:

BlockDescription
LFOTriangle–pulse blend; rate and waveform CV; LFO output jack
Wave folderWaveshaping (based on CGS52); wave in / wave out jacks
MixerTwo channels; one input has level control; default input for channel 2 when unpatched
AttenuatorSingle attenuator with +5 V normalled on input
MultipleBuffered multiple (1 in, 2 out)
OffsetsFixed offset voltages for modulation
Power supplyOn-board PSU: 12 V AC in (barrel jack), ±12 V and ±5 V for the voice card and circuitry

Front panel brings all voice card and interface I/Os to 3.5 mm jacks: pitch CV (1 V/oct), gate, trigger, FM, PWM, sync, filter CV, resonance CV, ADSR out, LFO, pan CV, external in, mixer in/out, multiple, and stereo out.

Build and files
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A full bill of materials (BOM) is in the polykit-x-monosynth README; it lists all passives, op-amps (TL074, TL072), regulators (78M12, 79M12, 78L05, 79L05), transistors, diodes, pots, switches, and Thonkiconn-style 3.5 mm jacks.

Tuning
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Tuning follows the voice card tuning procedure (VCO scale and trim). You need a 1 V/oct source (keyboard or fixed voltages), a tuner or tuning app, and dupont cables to reach the trimpots. Minimal set of connections: ground, all four rails (+12 V, −12 V, +5 V, −5 V), PITCH, PWM, ADSR outputs, GATE, LVL2, FREQ_CV, and audio outputs.

Modifications
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The README suggests adding the VCO saw output into the mixer’s default input (e.g. mix channel 2) via a 1 kΩ resistor, by tapping the signal between R1 and U6 pin 8 on the voice card.

Videos
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How I built a PATCHABLE ANALOG SYNTH from scratch on MakerTube

Resources
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Related

Open Source 8-Voice Analog Polyphonic Synthesizer

Open Source 8-Voice Analog Polyphonic Synthesizer

Project Polyphonic Voicecard Open-Source Midi Multimode Arduino
On this page I’m trying to collect all information about my programmable, analog, 8-voice polyphonic synthesizer. I started this project a long time ago, but it finally entered some stage where it is somewhat presentable. Everything you find here is publicly available. Schematics, PCB layouts, and source code are available on my GitHub. Do not expect a finished product in any form. The resources shown here are for enthusiasts who would like to build and program their own polyphonic synthesizer.
Core VCO

Core VCO

Eurorack Modular Vco Oscillator Through-Hole Kit
This voltage controlled oscillator is part of my Eurorack series. It outputs a saw and square signal, and its frequency is controlled by a 1V/Octave control voltage. There are CV inputs for frequency modulation, pulse width and oscillator sync. In addition, there are controls for frequency, fine tuning and pulse width.
Quantize & Hold

Quantize & Hold

Eurorack Modular Quantizer Sample Hold Utility Cv Arduino Dac Open-Source Through-Hole Kit
This Arduino based module let’s you quantize and hold a given control voltage. There are controls for selecting different scales and transposing a note. The module can either continuously quantize a given voltage or hold it when triggered. It uses a 12-bit digital-to-analog converter (DAC) and the software is open-source, so you can adjust it to your liking. It is a great companion for sequencers with unquantized outputs or for quantizing other voltage sources.
Steiner-Parker Diode VCF

Steiner-Parker Diode VCF

Eurorack Modular Vcf Filter Easy-Build Through-Hole Kit
This filter is part of my Eurorack series. The design is based on the Steiner-Parker diode filter, and I stripped it down to a minimal set of components while keeping its characteristic sound. It has controls for frequency cut-off, resonance and CV level. There are distinct inputs for low-pass, band-pass and high-pass, which allow all sorts of filter responses when combining them.