MousePlugins has released X-Or, a free monophonic bass synthesizer for Windows and Linux.
X-Or is a two-oscillator synth with dual free-running saw waves, a resonant low-pass filter, a filter contour envelope, a loudness envelope, and glide.
I love programming synths, but I rarely reach for dedicated mono synths in my own workflow because I do not make these types of bass and lead sounds that often. If you mostly need basslines, sub layers, plucky leads, and pedal-bass parts, though, a limited synth like this can make sense because there is less to think about.
The basic architecture is pretty simple. OSC A follows the incoming MIDI note, while OSC B can be offset with coarse and fine tuning. The OSC MIX control blends the two, so you can use small detune amounts for beating, fifths for stabs, or octave offsets for thicker bass patches.
The filter is a resonant low-pass with its own attack-decay contour envelope. CONTOUR controls how much the envelope opens the cutoff on note-on, then the cutoff falls back to its resting position.
There is no separate filter sustain or release, which, in my opinion, makes the synth better suited for plucky bass sounds than general-purpose monosynth use.
The loudness envelope is also pretty basic. You get Attack, Sustain, and Decay, with note-off using the same Decay time to return to zero. Soft retriggering helps avoid clicks when new notes arrive while the previous tail is still audible.
X-Or is transposed down one octave by default for pedal-bass voicing, so middle-keyboard MIDI parts land in a lower register. Glide is a nice addition for those acid basslines, and the timing goes from 0 to 2000 ms.
There is also a live chart with Waveform and Spectrum views, plus output meters. Presets can be saved as .xorpreset files, and the plugin includes A/B compare, copy/paste, undo/redo, factory presets, and a resizable editor.
I tested X-Or briefly, and it works fine here. It sounds fine, too.
The part I am less sure about is the interface. I do not know whether this plugin was AI-assisted or not, and the product page does not say either way, but some UI details have that vibe-coded look to me.
For example, some labels appear truncated with ellipses, like “Reson…” instead of “Resonance,” and a few interface borders feel uneven when you look closely. That does not prove anything about the DSP, of course. It may be that the developer can code a synth but used AI help or other tooling for the interface.
This is the confusing part of the current plugin scene. If a free plugin works, sounds good, and is useful, should we cover it even if the presentation feels AI-assisted?
Or should BPB avoid plugins like this entirely unless the developer is clear about how they were built? I would genuinely like to hear what readers think.
X-Or is available in VST3 and CLAP formats for Windows 10/11 and Linux x64. There is no macOS version at launch. The plugin is free and doesn’t require activation.
Download: MousePlugins X-Or (FREE)
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Last Updated on May 29, 2026 by Tomislav Zlatic.
