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Xolotl

Xolotl is an open source "harmonic" synthesiser plugin available for Mac OS (VST3 & AudioUnit) & Windows.

Harmonic oscillator

The “oscillator" consists of up to 32 independently pitched waveforms. These can be standard synth waveforms, multi-samples (expanded version only) or single cycle waveforms (expanded version only). By default the pitch increments through the harmonic series, but can be un-quantised using the step knob.

Flexible 16 Stage Sample and Hold

32 waveforms need at least 32 sample and hold effects, right? maybe not under normal circumstances, but this one's a bit different. Depending on how it’s configured, it can act as an effect, sound generator or modulator.

32 Stage Multimode Filter

A bunch of filters with independent control of the cutoff. There's a ton of sonic potential here. Comb filtering, phaser effects and some very tasty interaction between the S&H section.

Finally..

At the end of the signal chain is simple ADSR envelope and pan spread control.

An expanded version with additional 3.5Gb of samples and presets is available for a small fee, in addition to introducing several sample playback modes.