Review: XILS-Labs PolyKBIII

This software synth based on an obscure classic ticks all the boxes. But what's the story behind the PolyKBIII? And more importantly, how does it sound?  

French developers XILS-labs continue their quest to build faithful emulations of rare vintage synths for the modern DAW. PolyKBIII is a virtual reproduction of the powerful yet often overlooked RSF PolyKobol 2 – one of the first analog synthesizers to hint at wavetable synthesis with its unique morphing oscillators. Of course, it wouldn’t be an XILS-Labs synth without quite a bit more than that under the hood.

PolyKBIII in action

PolyKBIII in action



Starting at the core, PolyKBIII’s trio of oscillators provide a wide range of lush analog tonality. In addition to standard frequency and volume controls, the wave morphing knob glides smoothly from triangle to saw, square, and pulse, providing a wide variance of harmonic results between each; setting each oscillator to a discrete wave shape somewhere between the standard waveforms provides all kinds of unique timbres right out of the gate, presenting an ideal target for modulation.

A dedicated noise oscillator can be toggled between white or pink flavors with optional high frequency filtration and three fixed volume amounts. The second and third oscillators can be Synced to the first for added tonal sharpening, and can also be decoupled from Keyboard input for atonal accompaniment. The second oscillator has a Low setting that activates an LFO-like sub-oscillator, while the Tune toggle can detune the last two oscillators from the first for increased phasing, generally enhancing the analog impression.

Self oscillating

A pair of rich yet smooth fully self-oscillating filters can simply be set to low, band, or high-pass circuits. Compared to the now-typical menu of 10 or more filter types in many virtual instruments, I found the limited but impeccably tuned selection to be a refreshing change. Each filter can respond to a dedicated ADSR envelope along with keyboard input, while a Drive knob – available in pre or post-filter circuit options – helps add punch and grit as needed. The ability to select which of the three oscillators and noise circuit go to which filter provides welcome flexibility. The two filter circuits can then be mixed in parallel via the Filter Blend dial, or, if you prefer to use them in serial, you can simply route the Filter 1 output directly into Filter 2.

Standard modulation sources come in the form of three ADSR envelopes and a pair of LFOs. The first ADSR is hardwired to the main amplifier output, the second is linked to the filter circuits, while the third is fully independent – though all three can be deployed for modulation of other parameters via the open-ended modulation routing explored below. All three envelopes can also receive Keyboard input, or be augmented by x2, x3, or x4 Multiplier values, easily generating extra-long pads and drones. Better yet, the second two envelopes can be Looped for additional LFO-style modulation mania.

Speaking of modulation, the pair of host-synchronizable LFOs pack some intriguing options that might escape first glance. Delay and Fade dials are combined with optional re-triggering, while the default Triangle can be toggled to a Sine wave, or switched to Square, Ramp Up, Ramp Down, or Sample & Hold. Where things get interesting is by holding down the Alt/Option key, which allows the simultaneous selection of multiple waveforms to create compound modulation shapes that inject otherwise predictable oscillation patterns with uncommon geometries. 


Two main routing areas allow every parameter discussed so far to be modulated one way or another. The first of these is the Wired Modulation area, whereby LFO1, Noise or LFO2, ADSR2, and VCO2 can all be assigned to positively or negatively modulate a set array of parameters; this is also where Velocity can be assigned to the Filters, Amp, or Envelope settings.

Wired Modulation

Wired Modulation

Even if this were the only available modulation mode, it’s likely you’d be able to make do, but XILS-Labs have provided a User Modulation section where up to four sources can be used to modulate up to four destinations apiece, with each source modulation amount controlled by its own dial. Both LFOs and all three Envelopes are available as modulation sources here, along with various Sequencer parameters. Nearly all continuous Oscillator, Filter, ADSR, and LFO parameters are available as destinations. Taken in combination with the Wired Modulation routings, it’s hard to imagine any desired modulation being out of reach. Toying with various assignments quickly unfolded wildly evolving soundscapes perfect for engaging atmospheres in a variety of styles.

Further per-voice modulation comes in the form of the Voice XY pad, where each pole of the two axes on an XY grid can be assigned to control the amount of any continuous oscillator, filter or envelope parameter. Each voice appears as a red dot that glows green when triggered and can be dragged to different locations in the grid, providing each voice a distinct tonality from the others, all within the same patch – a truly unique feature worth experimenting with if you’re interested in the sort of dynamic layering it makes possible. 

Voice XY

Voice XY

The Space XY likewise places each voice discretely in dynamic stereo locations with a pair of movement modes along with Angle, Width, Speed, and Amount controls, creating expansive results perfect for cinematic pads and other wide tones. A suite of nicely-tuned analog modelled effects includes a Stereo Delay, Phaser, Chorus, and two-band EQ, which is great for added emphasis and boost where needed, particularly on the low end.

Factor in a powerful 128-step sequencer, mono or polyphonic Arpeggiator, thick Unison modes, analog Drift options and XILS-Labs' handy A-B patch building toggle, and it’s clear PolyKBIII is a potent, full-featured instrument that faithfully emulates the synth that inspired it while firmly pushing forward with a wealth of handy features for the modern producer. Some of the button combos, based on the original synthesizer, take a bit of getting used to, and can sometimes feel restrictive with certain parameters, but taken in context, this is a fairly minor quibble. Anyone looking to spark their imagination with a virtual analog emulation full of inventive modulation sources and routing options should certainly give PolyKBIII a try.

Price: €119 (Introductory Pricing)

Pros: Impressive virtual analog sound, easy A-B editing, abundant modulation options, powerful sequencer, great stereo imaging, unique morphing oscillator architecture. 

Cons: Not competitively priced or particularly CPU-efficient; GUI leaves some room for improvement.



Noah Pred is a Canadian record producer, sound designer, technologist, DJ, and Ableton Certified Trainer living in Berlin, Germany. Releasing dozens of records and touring extensively since the '90s, he currently teaches a wide variety of techniques for stage and studio at the BIMM Institute. For more information, please visit: http://... Read More


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