At this point in his career, Eric Barbour of Metasonix has moved on from his early, rowdier days, when their modules had zany names with pseudo-obscene doodles adorning their screaming canary yellow faceplates. Now in its place are understated, cohesive, interesting, yet still just as enigmatic modules and synth voices. Basically, all of the wackiness that defined early Metasonix products and were used to personify the company in its infancy, are now internalized. They’ve matured, so to speak. There are still plenty of unpredictable, adolescent Metasonix-y aspects under the hoods of their modules, and the relatively simple looking RK5 [See? No crazy name?!]—their dual low pass gate module—is a good example of this.

One of the main points of interest in this particular module is that a tube based LPG has never existed before this. By the time Don Buchla invented the low-pass gate in the 70s, tubes had long been pushed aside for technological applications, and so the RK5—like a lot of the Metasonix line—has no real predecessor in that sense. Live and die by the tube, as per the Metasonix credo.

As for what tube to live and die by, the RK5 is built around a 17JK8 dual triode tube, and consists of two completely independent channels, both made up of a Sallen-Key filter with only an input, output, and CV for each. There are no knobs, sliders, LEDs, buttons, or anything on this, something I find really refreshing. Once you know what this does, and it doesn’t take long to figure it out, you can file it away and know what you’re getting. It’s a decidedly old school approach where the technology—or lack of—is the fundamental controlling device. The magic happens automatically either behind the plain metal faceplate, or in the gassy chambers of the tube.

Each of the two channels uses one of the two triodes the tube has available as well as two Silonex optoresistors per channel for its operation. Because of this [and other reasons that we’ll get to in a second], the RK5 naturally outputs an envelope only slightly related to the envelope patched to the CV in. The reason that the output is only slight related is that somewhere around 2 volts the gate stops operating and the tubes natural decay takes over. I wanted to find out at exactly what voltage the gate is activated, so with multi-meter in hand I ran some DC offset into the CV IN and measured when it opened up, which was about 2.94 volts! So when feeding it a typical ADSR envelope, the gate doesn’t open until almost 3 volts, bypassing some or all of the attack stage, and depending on settings, more. On top of that, as soon as the voltage drops below the nearly 3 volt threshold, the RK5 shuts the front gate. What this means is that any envelope being fed into the RK5 will have the everything under about 3 volts completely ignored. This is very different from your typical LPG. This also means that there is almost no attack slope coming out of the RK5, no matter what the envelope going in says. And if you feed it an envelope with too slow of an attack before it resets so that the RK5’s gate never opens? Silence. While this might sound like you’re not getting enough band for your buck [I want all the voltage!!!!], it’s just one peculiarity is that gives the RK5 some of its unique sonic characteristics. Another thing that adds some special sauce to the RK5 is that it has a ceiling of 5 volts for the CV IN. That means that any voltage higher than 5 volts that’s being fed into the CV IN, registers merely as 5 volts, kind of like a voltage hold/flatlining sustain of the outputted envelope, which in turn means that if you were to feed an envelope into the CV IN that went from 0-7 volts, peak to peak, the RK5’s response would only process it from 2.95ish to 5, with anything above 5 volts registering the same as just 5 volts and anything below about 3 volts, totally ignored. With the voltage ceiling the RK5 acts almost like a limiting compressor except the sound isn’t being squashed, just the control voltage.

Metasonix modules are known for a bit of their wild west approach, in that there is an imperfect, unpredictable, un-uniformness to their offerings, and the RK5 is no exception. Because of the fact it’s tube based, and each tube has its own characteristics along with the natural variances of optoresistors, the RK5 injects byproducts into the sound and no two RK5s will sound the exact same. As well, no other LPG I’ve heard sounds like this one. You want unique? You want one-of-a-kind?? Here it is, embrace it.

Feeding an envelope into the CV of Channel 1, the RK5 had a pleasing, mellow, and smooth output curve that worked very well for big boomy bass drums and slow moving arpeggios. One reason it works so well as a bass drum is because the RK5 adds a low bump to the output, a little extra thump that is a byproduct of its design, a nice bonus for low-end applications. Since there are no controls on the RK5, in order to change the sound, you can only mess with the decay time of the CV in envelope to vary the sonic qualities of the output quite a bit, going from a quick pluck to a sustained burst, or change the sound after it exits the RK5.

Channel 2 has got a much faster decay, and therefore switching the IN/OUT/CV from Channel 1 and patching into Channel 2, you get an output about 17dB lower than Channel 1 with the same settings. This is not an error, malfunction, or a sign that your tube is broken, and to be sure, I checked with Metasonix HQ to make sure this was normal operation, and was assured it was. Channel 2’s characteristics are much more pronounced with its quicker decay, and therefore much of the low end winds up being chopped off with a little extra distorted grit added to the top end. This worked amazingly well for creating a snare sound, and I realized that the RK5 is perfectly suited for main drum duties: Channel 1 kick, Channel 2 snare. It was easy enough to make up for the lower volume coming from channel 2, and when I put a high octane VCO through Channel 2 [Erica Synths Fusion 2], I didn’t have to add or subtract anything to get the levels I wanted. I suppose one might be bothered by this lack of perfect symmetry in the module, but whether it’s an effect of the tube, optocoupler, or good old fashioned Metasonix Voodoo, more and more I find myself embracing these kinds of peculiarities in the instruments I choose to play. Unpredictability as an instrument.

Metasonix modules are handmade and might not be cheap, but they’re also not crazy expensive, and are completely unique in the world of modular synths. In the case of the RK5, even unique from one specimen to another, this uniqueness is a great part of their charm and perhaps the major selling point. While there are plenty of LPGs out there, most—if not all—are more conventional and predictable. None of them are quite as charismatic and idiosycratic as the RK5.

8HP +12V 100mA [200 mA briefly during cold power-up] -12V 2mA

Price: $350

metasonix.com