The RK8 contains six bandpass filters, which use vacuum tubes. They are simple “twin T” designs. Each filter is about one octave lower in range than the one to the left—treble on the left, bass on the right.
Each filter may be manually swept with its own separate tuning knob, over approximately one and one-half octaves. Or ALL the filters may be swept together, using either the “MANUAL TUNE” knob, or the CV input, which overrides the knob when a patch cable is inserted. Although it is capable of some conventional swept-filter effects, the RK8 is more useful as a complex equalization system or for unique special effects.
Resonance is separately adjustable for each filter. Usually the treble filters will resonate strongly and oscillate under some settings. The bass filters might not. It’s a strange monster.
Inputs and outputs are compatible with all Eurorack and other modular synthesizer equipment. The output peak voltage is limited to the supply rails and can’t damage other modules.
Don’t assume you know what you are getting here—the RK8 is unlike anything else ever marketed. Also don’t expect it to track VCOs. Its voltage-to-frequency curve is “more or less” hertz-per-volt but can’t really be guaranteed. The six vactrols in each RK8 were hand-matched, but can’t be expected to remain matched, or to track together perfectly.
Again, everything about the RK8 is abnormal. It is not perfectly “clean” and perfectly “distortionless”, and it cannot be made so. Because it uses vacuum tubes.
It is something you have NEVER seen before. It can NOT be compared to any other product, ever made, PERIOD. Do not walk into this with “preconceptions”.
Specs: 32HP width Eurorack module, with circuits protruding about 1.5″ or 38mm behind panel. The RK8 uses six 5702 subminiature pentodes, all Raytheon US military surplus from the 1980s. Power supply required is about 600 mA after warmup and 1.4 amps when started cold from +12v, and 10 mA from -12v. Low-cost power supplies are NOT recommended. A spare tube is included, although soldering equipment is needed to replace any of the tubes. They are run very conservatively and will last many, many years.