The RK4 is the minimized version of our R-54 Supermodule, boiled down to a single 8hp module with a single vacuum tube.
It uses a triode-pentode in a unique Wien-bridge circuit, to provide clean sine-wave oscillation. Using a Vactrol, the pitch of the RK4 is sweepable from less than 20 Hz to more than 2 kHz, continuously, via front-panel control or a CV input. By adjusting the waveshape/resonance control, the sinewave signal produced by the RK4 can be trimmed to less than 10% THD (depending on operating pitch)—with NONE of the problems inherent in solid-state oscillators and sine waveshapers. Because the Wien bridge contains no piecewise sine shaper or amplitude-control circuit or component. The tube itself performs that job. So the distortion products are entirely low-order, unlike the “sine waves” that solid-state VCOs or samplers produce.
Turn up the RESONANCE, and the oscillator output gives a unique soft-clipped square-wave sound. The simplicity of the RK4 will fool you–its behavior is odd and radical, and years will be required to fully explore all of its quirks.
Now for the special trick. Plug an audio signal into the audio input. Instantly, the RK4 transforms from a VCO into a bandpass resonant voltage-controlled filter, with the same range as in VCO operation — 20 Hz to >2 kHz. Its behavior and sound are unique. Again, you will be kept busy for years exploring all of its strangeness.
CV response: Hz/v. Maximum output signal, 12 v p-p. Power requirement: +12v dc, approx 400 mA, and -12v at 10 mA. As with all our modules, it draws about 2.5 times the operating current during “cold” power-up, and you must insure your power supply can handle the inrush current. See our support section for more information and power supply suggestions.
(All specifications are approximate and may vary from sample to sample.) Tube used: 6GH8, 6U8, 6CQ8, ECF82 or other compatible types.
AUDIO DEMOS Provided Courtesy of Zerosum Inertia
Livewire AFG to RK4 modulated by VCS
Binary Zone sequencing AFG to RK4 with Binary Zone inverted out to RK4 CV in
RK4 by itself slowly increasing the resonance showing off the waveform effects
RK4 FMed by AFG sine and Triangle(total mayhem)
RK4 to RK3 waveshaper to RK2 VCA
The Metasonix RK-series of vacuum-tube modules is intended for use in any “Eurorack” standard modular synthesizer system. All RK-series modules are 8HP (40.7mm) wide and fit in any Doepfer(tm), Analogue Systems(tm), Monorocket ™, or other compatible cabinet. They are powered entirely from the cabinet’s internal +-12v dc power supplies, using Doepfer compatible 16-pin power connectors. Their circuits protrude behind the panel less than 31mm (1.2 inch). All vacuum tubes are NOS (new old stock) types from classic American and European manufacturers that are run very conservatively for long life, and are readily available from distributors. The 6GH8/6U8/6CQ8 tube protrudes from the panel for visibility and cooling, less than 40mm (1.58 inches), and unquestionably fits inside a Doepfer A-100 suitcase cabinet lid or into a Doepfer mini case or Monorocket case cover. All audio inputs and outputs and CV inputs/outputs are 100% compatible with other synthesizer modules. Plate voltage is provided by a tiny switching supply producing clean 48v regulated at low current, which is very safe and will not injure the user. Potentiometers are top-quality Alpha RK12L types, and 3.5mm phone jacks are special Kobiconn vertical-mount types, much better quality than the cheap ones usually seen in Euro modules. Each RK-module is fully protected against reverse supply voltage, overvoltage, undervoltage, or incorrect hookup to external equipment. So don’t worry! As with all our products, the RK series is totally handcrafted in Northern California, USA.