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RK6 Resonant Lowpass Filter – Metasonix

RK6 Resonant Lowpass Filter – Metasonix

by Ellison Wolf

It’s no surprise that the Metasonix RK6 Resonant Lowpass Filter is like no other filter out there. Maybe it’s just the tube—a dual triode 17JK8 [one that wasn’t in my tube lexicon until now], maybe it’s the circuit—a primitive design [as per the Metasonix website], which was structured to work with this particular tube, or maybe it’s just plain ol’ Metasonix voodoo. Whatever it is, RK6 isn’t your Grandmother, Mother, or Matriarch’s filter. As well, there are no switches, no giant knobs to sweep frequencies, and no blinking LEDs. According to the Metasonix website, the RK6 is “a PURE TUBE design, the sort of design that might have been invented in the 1950s if anyone had dared to create a tunable lowpass filter for musical uses...It does not simulate any other product and it does not sound like anything else.” It’s a brash statement for sure: Cocky, self assured, and kind of bratty; all keeping in line with the Metasonix ethos of doing their own thing. In most other sorts of businesses, “The customer is always right.” Not here. Metasonix is always right, whether you agree or not, whether you like it or not. And the RK6, a finicky filter, unlike any other I’ve come across, has some sweet spots for sure, but you’ve got to play nice and take the time to find them. Kind words can go a long way.

The RK6 is a two-pole (12dB) Sallen-Key filter, the type that is also found in the Steiner-Parker Synthacon, various vintage Korg synths, and Arturia’s Brute-based synths, and has a feedback loop that lends it its resonance capabilities. Layout-wise, front, center, and at the top of the simple metal faceplate, you’ve got the glowing 17JK8 sticking out. Only three knobs adorn the RK6—Reson[ance], Input Level, and Tune—to go along with its four jacks; those being Input, Output, Reson[ance] CV In, and Tuning CV In. Since it’s a two-pole filter you won’t get that Moog-y filter sweep we’ve all grown accustomed to, but you couldn’t anyway, as one of RK6’s more odd/unique (however you want to say it) features is the fact that all of the knobs on the RK6 only have a real, usable range of about the top 20% or so, so actually sweeping the knob isn’t super effective anyway. After a certain point the potentiometer ranges for each knob stop making noticeable changes, both visually and sound-wise—easy to see when going through an oscilloscope and/or spectrograph. This is considered normal operation so save your emails to Metasonix HQ for another time. As well, when I was going through my oscilloscope, patching a simple triangle wave into the input with the resonance down a little (to tame the beast, so to speak) shows that even with the filter open to its max, the triangle wave is more of a shark fin, with a slight curvature and a little high end taken off the top. This isn’t about perfection or exactness, it’s about uniqueness. With the RK6 you’re definitely venturing into uncharted shark-fin infested waters, indeed. Turning the TUNE knob a third of the way counter clockwise filtered out the sound completely, and therefore patching an LFO into the TUNING CV IN and the RK6 becomes a lowpass gate, with a few minor tweaks to the knob settings. With the resonance set from high to cranked, I was able to go from nearly inaudible dog whistles to some very brassy tones that Coltrane might have made while breaking in a new reed or messing around backstage. To get this effect to be more pronounced it helped to CV the resonance and adjust the filter so that it wasn’t quite all the way open. When doing this—with the input less than max—the resonance mostly took over, and changing the waveshape from my oscillator—in this case, the Blue Lantern TPCSlimVCO (review in Waveform #1)—didn’t alter the sound at all. Decreasing the resonance let the oscillator get through and brought about more predictable, typical filter results.

I’m a big fan of misuse and happy accidents, and during my testing was pleasantly surprised when I realized that the RK6 can be patched backwards—that is, using the IN for an OUT, and vice versa—to usable effect. It’s a little less nasal/honking this way, and I figure, if this tube can be used in a way that it wasn’t intended for, then why not the module itself?

One of the things I love about Metasonix products are the variations you’ll get between two of the same model. Since they’re tube based, and there can be some variance between tubes of the same type, no two Metasonix modules will be exactly the same. Add to that, the variances of optocouplers—three of which Metasonix employs in the RK6 (two to tune the filter, and one more to CV the resonance)—and the differences (er...unique qualities) between same modeled modules only intensifies. Some might find this lack of uniformity, this non-homogeneity somewhat troublesome or annoying, but I find it quite the opposite. We all love to think that our “stuff” (whatever that may be) is special, different, and more magical than the next persons, and with Metasonix products there is the possibility of merit to that claim. Either way the RK6, while odd and particular, is nonetheless a unique individual that easily stands out amongst the many many clones, reproductions, or chip based replicants, and that in and of itself makes it worth checking out.

8HP +12V 100mA (200 mA briefly during cold power-up) -12V 2mA

Price $380

metasonix.com