This vir­tual take on RFS’ rare French polysynth is al­ready more pow­er­ful than the orig­i­nal hard­ware, but there’s al­ways room for im­prove­ment

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A mod­elled em­u­la­tion of the exquisitely rare RSF PolyKobol 2 ana­logue syn­the­siser, PolyKB hasn’t seen a full-length ap­praisal in since ver­sion 1, way back in 2010 (9/10, 154), so here we’re re­view­ing the lat­est ver­sion of it as a whole, rather than just look­ing at what’s new. For own­ers of v2.5, which was a free up­grade from v1 (in­tro­duc­ing a new GUI, high- and band­pass fil­ter modes, the abil­ity to use the se­quencer as a mo­du­la­tion source and more), the €39 up­grade fee gets you a third ex­tra os­cil­la­tor and sec­ond fil­ter (the lat­ter bring­ing with it dual fil­ter in­put se­lec­tion and out­put mix­ing, plus an ac­com­pa­ny­ing en­ve­lope), a sec­ond 2x4 User mo­du­la­tion ma­trix, and sep­a­rate time mul­ti­pli­ers for each en­ve­lope.

So, PolyKB III is a three-os­cil­la­tor, dual fil­ter ana­logue-style polysynth with a ma­trix-driven mo­du­la­tion sys­tem, a built-in poly­phonic se­quencer, a hand­ful of ef­fects, and an in­ter­face that’s more in­tu­itive than it looks. The de­sign of the os­cil­la­tors is one of its defin­ing fea­tures, as their wave­forms are con­tin­u­ously mor­phable from tri­an­gle through saw, then dou­ble saw, to a square wave that pro­gres­sively nar­rows down to a thin pulse. The knob that con­trols this mor­ph­ing can, of course, be mod­u­lated for all sorts of shift­ing tim­bres.

Os­cil­la­tors 2 and 3 each also fea­ture hard sync and de­tune against Os­cil­la­tor 1, and can be set loose from key­board con­trol to be­come LFOs – Os­cil­la­tor 2 has a Low but­ton for slow­ing it down to an ap­pro­pri­ate speed range for this, but for some rea­son Osc 3 doesn’t. Up to 16 voices of polyphony are on of­fer, in­clud­ing up to six Uni­son voices, ran­domly de­tuned with the Drift knob in the Key­board/Arpeg­gia­tor panel.

The two zero-de­lay feed­back fil­ters are iden­ti­cal 24dB/oc­tave mod­els with low-, hig­h­and band-pass modes, and over­drive cir­cuits that can be placed pre or post. The in­put into each fil­ter is de­ter­mined by the Source but­tons be­low it, of which there’s one for each VCO and the noise gen­er­a­tor, and an ad­di­tional one for rout­ing Fil­ter 1 into Fil­ter 2; and the out­puts of both fil­ters are mixed us­ing a ded­i­cated knob. It all adds up to a highly flex­i­ble fil­ter­ing setup, ca­pa­ble of both par­al­lel and se­rial flow, or a com­bi­na­tion of both.

Mods and rock­ers

PolyKB III’s mo­du­la­tion ar­chi­tec­ture is ne­go­ti­ated over three as­sign­ment screens: Wired Mod, User Mod and Voice XY. Pro­vid­ing the sig­nals are three en­velopes, two LFOs, the Step Se­quencer (se­condary to its note play­back func­tion­al­ity – see be­low), and the usual MIDI mes­sages: Ve­loc­ity, After­touch, Key­track­ing and Mod Wheel. The en­velopes are hard­wired to the amp and fil­ters (but also avail­able for other things, nat­u­rally), and Envs 2 and 3 are loopable. All three can be mul­ti­plied by fac­tors of 2, 3 or 4, too, for su­per-slow ramp­ing.

The LFOs each out­put the sum of up to five wave­forms – sine or tri­an­gle, square, ramp up, ramp down and sam­ple+hold – ac­ti­vated by the but­tons next to each one. They can also be ‘lagged’ and faded in.

Switch­ing be­tween the mo­du­la­tion as­sign­ment pan­els (as well as the Ef­fect, Step Se­quencer, Space XY and Key­board/Arp ed­i­tors) is done with the fader at the far left. The Wired Mod screen is a but­ton ma­trix in which five modulators (LFO1, LFO2 or Noise, ADSR2, VCO2 and Ve­loc­ity) are as­sign­a­ble in par­al­lel to any num­ber of tar­gets from a fixed list (os­cil­la­tor pitches and wave­forms, fil­ter cut­offs, global en­ve­lope At­tack and De­cay, and the amp). Click an as­sign­ment but­ton once for neg­a­tive mo­du­la­tion and twice for pos­i­tive, and ac­ti­vate the Wheel but­ton for mod wheel depth con­trol. The User Mod screen uses the same but­ton ma­trix con­cept to as­sign two pairs (up­graded from one pair in v2.5) of user-se­lected Sources to up to four user-se­lectable Des­ti­na­tions each.

The but­ton-driven as­sign­ments make the Wired and User mod­u­la­tions easy and fun to mess around with, but with the depths of all as­sign­ments for each source gov­erned by a sin­gle Amount knob, frus­tra­tion fre­quently arises – this is a com­plaint lev­elled at PolyKB since day one, but we guess XILS-lab aren’t plan­ning to ad­dress it. The re­main­ing mo­du­la­tion screen, Voice XY, is dis­cussed in X and Y.

Step­ping out

For in­ter­nal note gen­er­a­tion, PolyKB III in­cludes an arpeg­gia­tor and a step se­quencer. The arp fea­tures ed­itable chord se­quenc­ing and a poly­phonic mode, and de­faults to Ran­dom mode when no other di­rec­tion is cho­sen. The poly­phonic 128-step Se­quencer can play back and edit up to eight voices at once, with notes and ve­loc­i­ties recorded or drawn into a re­lent­lessly fid­dly but ef­fec­tive pi­ano roll-style in­ter­face. Se­quences are save­able as pre­sets, and as of v2.5, the Se­quencer’s pitch and ve­loc­ity out­put can be pressed into ser­vice as mo­du­la­tion sources, too.

Fi­nally, the Ef­fects page houses De­lay, Phaser, Cho­rus and two-band shelv­ing EQ mod­ules, all of­fer­ing enough con­trol to get the job done and sound­ing quite de­li­cious.

Pretty Poly

PolyKB III is a gen­uine one-of-a-kind synth that moves and sounds quite un­like any other. The wave mor­ph­ing is clearly a killer fea­ture, and the mo­du­la­tion ma­tri­ces, step se­quencer and XY ed­i­tors all play their parts in defin­ing the char­ac­ter and feel of this unique in­stru­ment; but it’s the sheer qual­ity of the os­cil­la­tors and fil­ters that keep us com­ing back for more. They sound daz­zlingly ‘real’, ooz­ing ana­logue liq­uid­ity and vibe, whether singing or scream­ing.

There’s re­ally noth­ing PolyKB can’t turn its hand to, from thump­ing basses, writhing leads and del­i­cate bells to elab­o­rate poly­phonic se­quences, dense arps, tex­tures, sound­scapes and any­thing else where mo­tion and dy­nam­ics are key. And de­spite the age of the hard­ware be­ing mod­elled, PolyKB III has a sur­pris­ingly mod­ern sound, pre­sum­ably thanks to the many mod­i­fi­ca­tions made by XILS to the orig­i­nal blue­print. This is a synth that any elec­tronic pro­ducer ab­so­lutely has to try, and a no-brainer up­grade for ex­ist­ing users.

“This is a synth that any elec­tronic pro­ducer ab­so­lutely has to try”

STEP SE­QUENCER Poly­phonic and avail­able as a mod source, the se­quencer is bril­liant but awk­ward ED­I­TOR SE­LEC­TOR A slider isn’t our first choice for switch­ing screens, but there it is… ARPEG­GIA­TOR A sim­ple arp, with ex­tra con­trols in the Key­board/Arp...

The mod ma­trix as­signs Sources to Des­ti­na­tions us­ing but­tons, but why can’t we tweak the depths in­di­vid­u­ally?

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