OS-251 is a pure digital lo-fi synthesizer plugin for Mac, Windows and Linux. Available in VST3, AU and LV2 plugin formats for free.

OS-251 screenshot

Digital lo-fi sounds

OS-251 is a "JUNO" style digital subtractive synthesizer with a simple yet powerful DSP algorithm. Unlike other plugins that use analogue simulation, OS-251 uses digital algorithms to give warm, lo-fi sounds. The design of the oscillators and the selection of parameters were influenced by the JUNO-106, making it easier to express a thicker, chunkier sound.

The oscillator has alias noise (specific to digital signal processing) famous on classic samplers like E-mu SP-12 which adds lo-fi feeling to the sound. You can enjoy the powerful sound at modern sampling rates with your DAW.

Easy to use

Tired of overwhelming synthesizers with hundreds of parameters and presets? Then OS-251 is for you! OS-251 is equipped with only carefully selected parameters, allowing musicians to concentrate on music production. You can see every parameter at once, so you can have a stress-free experience. Because the number of the parameters is limited, you can create the desired sounds yourself quickly and you don't have to spend your precious time choosing presets.

Key features

  • Up to 24 voices of polyphony
  • Oscillators (Sin, Square, Saw), with Shape parameter
  • Square sub oscillator
  • Noise oscillator
  • Biquad low-pass filter (LPF) with Frequency and Resonance parameters
  • Biquad high-pass filter (HPF) with Frequency parameter
  • Envelope generator which can control LPF and amplitude
  • Low-frequency oscillator (LFO) which can control LPF, amplitude, and Shape parameter
  • LFO sync
  • Chorus
  • Unison
  • Flexible pitch controls (pitch bend width, octave, semitone, fine tuning)
  • Preset manager
  • Clipping indicator

Sound Examples



You can download OS-251 from GitHub for free!!

Currently, LV2 is only supported on Linux.

System Requirements

  • OS: macOS (10.12 or later), Windows (10 or later), Linux
  • Software: VST3 / AU compatible host application (64bit)

Change Log

Version 1.2.1

  • Fixed broken oscillator gain parameters

Version 1.2.0

  • Added 'unison' parameter
  • Added clipping indicator
  • Improved performance by stopping calculation of oscillator value when its gain is zero

Version 1.1.1

  • Added native support for Apple M1
  • Avoided to take keyboard focus (Now you can use DAW's keyboard shortcuts when the OS-251's UI is focused)

Version 1.1.0

  • Added proprietary preset manager
  • Added LV2 version for x64 Linux
  • Changed the maximum number of voices from 10 to 24
  • Fixed noise in the chorus effect
  • Fixed wrong 'shape' value when a note starts
  • Smoothed HPF frequency parameter to suppress noise when preset is changed
  • Adjusted font size for Windows

Version 1.0.1

  • Fixed the problem that OS-251 didn't work in macOS 10.12, 10.13, 10.14