Firefly Synth 2 by Sjoerd van Kreel is a free VST plugin built for producers who want deep synthesis, flexible modulation and detailed audio routing without entering a fully modular environment.
This open-source instrument combines multiple synthesis methods, per-voice effects, global processing, extensive modulation and an unusually flexible signal path.
Firefly Synth 2 is available for Windows, macOS and Linux in VST3 and CLAP formats. A separate FX version is also included for processing external audio.
What Is Firefly Synth 2?
Firefly Synth 2 is a semi-modular synthesizer and creative audio effect.
It provides detailed routing and modulation while keeping the main synthesis structure available inside one interface. Producers can build conventional synth patches, complex FM sounds, physical-modeling-style textures, evolving pads and heavily processed electronic sounds.
The separate FX build can process external audio through the same broader processing environment.
Four Flexible Oscillators
Firefly Synth 2 includes four oscillators with several synthesis options.
- Basic waveform generation
- DSF additive synthesis
- Dedicated hard sync
- FM synthesis
- Combined string and noise synthesis
- Inter-oscillator AM and FM
- External audio as an oscillator source
This makes the synth suitable for everything from basses and leads to metallic FM patches, synthetic strings, percussion and experimental textures.
Per-Voice and Global Effects
Firefly Synth 2 includes four per-voice effect sections and four global effect sections.
Available processing includes comb filtering, state-variable filters, compression and several waveshaping options.
Because processing can happen at different stages of the architecture, the same effect can behave very differently depending on its position in the signal path.
Echo, Reverb and Delay
The synth provides both per-voice and global echo processing.
The available tools include reverb, feedback delay and multi-tap delay, with additional filtering for shaping the resulting space.
Per-voice processing is particularly useful for creating unusual patches where each note develops its own echo behavior before reaching the global mix.
Deep Modulation System
Firefly Synth 2 includes six per-voice LFOs and six global LFOs, alongside seven per-voice envelopes.
The envelopes use a 16-stage MSEG system with customizable loop and release points. Linear, exponential, unipolar and bipolar behavior is available.
An extensive modulation matrix provides 17 per-voice slots and nine global slots.
This makes the plugin suitable for evolving pads, rhythmic movement, complex FM sounds and patches that react differently across individual voices.
Global Unison and Stereo Movement
The global unison system can influence a large number of per-voice parameters.
This allows producers to create wide sounds that differ in more ways than simple pitch detuning.
Used carefully, the system can add movement and width to pads, leads and experimental textures. More extreme settings can produce highly animated stereo patches.
Microtuning and MTS-ESP
Firefly Synth 2 supports microtuning through MTS-ESP.
The plugin acts as an MTS-ESP client and requires a compatible tuning master when microtonal control is needed.
This makes the synth relevant for experimental composers and producers working outside conventional equal temperament.
Instrument and FX Versions
Firefly Synth 2 is available as both an instrument and an audio effect.
The instrument version is designed primarily for MIDI-controlled synthesis, while the FX version treats external audio as its main input.
The effect build opens additional possibilities for processing drums, vocals, loops, synths and other audio through the filters, waveshapers, compressors, delays and modulation system.
Best Uses for Firefly Synth 2
- Complex electronic basses
- FM and AM synthesis
- Evolving pads and textures
- Experimental percussion
- Physical-modeling-style sounds
- Hard sync leads
- Creative external audio processing
- Modulated delays and effects
- Microtonal composition
Compatibility and Plugin Formats
- Windows: Windows 10 or later, x64, VST3 and CLAP
- macOS: macOS 10.15 or later, Intel and Apple Silicon, VST3 and CLAP
- Linux: x64 with glibc 2.35 or later, VST3 and CLAP
The developer also notes that Linux ARM64 is known to work but is not officially supported.
No Audio Unit, AAX, VST2 or LV2 version is officially listed.
Installation and Registration
Firefly Synth 2 can be downloaded directly from the official GitHub Releases page.
No account, serial number, subscription or license manager is required.
Separate downloads are available for the instrument and FX versions in VST3 and CLAP formats.
Important Limitations
Firefly Synth 2 is a deep instrument with a large number of controls. Producers looking for a minimal preset-based synthesizer may find the interface demanding at first.
The developer also warns that some hosts may not display the real-time visualization correctly with the VST3 version. The CLAP build is preferred when the host supports it.
Renoise is not officially supported because plugin updates may create risks for existing automation parameter assignments.
Free and Open Source
Firefly Synth 2 is free and open-source software released under the GNU General Public License version 3.
The complete source code is available from the official GitHub repository.
Official Download
Official website: Visit Firefly Synth 2
Official download: Download Firefly Synth 2 for free
Official source code: View Firefly Synth 2 on GitHub
Final Verdict
Firefly Synth 2 is a powerful free semi-modular VST3 plugin for producers who enjoy building sounds rather than simply browsing presets.
Its four oscillators, deep modulation system, per-voice processing, flexible routing and separate FX version give it a much broader role than a conventional free synthesizer.
The learning curve is real, but so is the creative potential. Firefly Synth 2 is the kind of plugin that rewards curiosity, preferably with a coffee nearby and several hours mysteriously missing from the afternoon.
