The RK-4 is an 8hp Eurorack module replicating the basic function of the older R-54 "Supermodule". It is a vacuum-tube (electronic valve) Wien-bridge bandpass filter which can also be used as an oscillator. Tuning is effected by a dual-element Vactrol optocoupler.
TUNING tunes the circuit through it's full sweep range, from below 20Hz to more than 5kHz, manually. RESONANCE controls the resonance when used as a filter, and sets/adjusts the waveform in VCO mode. Adjust RESONANCE to the marked dot on the panel for a reasonably-low-distortion sinewave output. Turning it farther counter-clockwise will increase distortion. Note that these controls have very large control ranges, and their effective range is much smaller than their full rotation. This is normal, it was done to allow for variance of the vacuum tubes and other components.
VCO use is possible ONLY with no audio input applied to the AUDIO INPUT jack. To use the RK4 as a straight VCO, do NOT plus a cable into the audio INPUT.
By applying an audio signal to the AUDIO INPUT, the RK4 transforms from a VCO into a 2-pole resonant bandpass filter, sweepable over the same range (about 20Hz to 5kHz). RESONANCE may be adjusted as needed for a low-Q bandpass response, a very sharp peak bandpass, or oscillation (which can give some unique sound effects and distortions). When used as a filter, the RESONANCE setting has to be carefully adjusted (somewhere near the dot marking) for best results.
The PITCH CV INPUT controls the module's frequency with a control range of approximately 0-4 volts, with the TUNING control offsetting the center frequency in parallel. Thus, the CV input will accept a broad range of effective CV changes. The maximum sweep range is about 8 octaves, and the PITCH CV INPUT does not track conventional solid-state VCO or filter responses (it is approximately linear or Hz/v except in the lowest octave, and V/oct CV will not give accurate pitch tracking). If tracking of other VCOs or v/oct is requires, an RK4 can be forced to track a conventional VCO over a small range by feeding a "hot" VCO signal into the CV input. Tuning is accomplished with a "Vactrol" voltage-variable resistor, and will have the slow response time and odd behavior typical of Vactrols.