Metasonix RK4 Filter / VCO

The RK4 is a compact version of the unfortunately no longer available R-54 Supermodule. Like the older brother, it can be used as an oscillator and bandpass filter. During VCO operation, the RK4 emits an exceptionally clean sine wave, thanks its triode-pentode which was placed in a Wien-bridge circuit. By turning a potentiometer, the sine can be morphed into a square wave. The frequency response ranges, in both VCO and VCF operation, from 20 Hz to 2 kHz. Pitch respectively cutoff can be controlled via a knob and a Hz / v connector. An audio input plus level potentiometers complete the module.

In use, the RK4 shines with beefy, warm tube sound. Frequency modulations lead to fierce, yet charming results. Utilized as a filter, the circuit is a dream as well. The bandpass is wide and cuts off frequencies very smoothly. Hi resonance values produce shrill screams. – A truly unique module with tons of character!

While being used as an oscillator, the RK4 is controlled via its pitch CV input (Hz / v). For tuning adjustments, there is a potentiometer. The resonance knob can be employed to morph the waveform from sine to square. Another potentiometer edits the output volume.

For utilizing the RK4 as a bandpass filter, an audio input with level control was implemented in the module. Its tuning potentiometer sets the cutoff frequency, the pitch CV input can be used for modulations.