RK6 resonant lowpass filter

Dual-triode 12dB LPF

The RK6 is a TOTALLY UNIQUE product, one that never existed before. It is a Sallen-Key two-pole lowpass filter with a feedback loop, to allow it to resonate. There are no integrated circuits or transistors in the signal path. It is a PURE TUBE design, the sort of design that might have been invented in the 1950s if anyone had dared to create a tunable lowpass filter for musical uses before Moog and Buchla. It even uses a coupling transformer to provide the proper phase of feedback signal — purely a “classic good old days” approach, and one rarely seen in the 21st century. It is the most primitive general-purpose lowpass filter the world has ever seen and probably ever will see. No, it does not simulate any other product and it does not sound like anything else.

Two “vactrol” optocouplers tune the filter, and another coupler allows CV control of the filter’s resonance. The RK6 may be used as a conventional lowpass filter or as a lowpass gate, as adjusting the TUNING control and CV input can almost completely cut off the signal.

All specifications are approximate and may vary from sample to sample. Tube used: 17JK8 dual triode. The RK6 uses only 100 mA (200 mA briefly during cold power-up) from +12v and 2 mA from the -12v supply rail. We deliberately run it colder than usual for maximum lifetime. It should work with most available Euro power supplies, even the ones too small to run other Metasonix RK modules. Input impedance (audio) is 100k ohms and output impedance is 3k ohms. (Do NOT attempt to drive a 600-ohm load.) The CV inputs require a 3 mA driver; voltage needed for the frequency CV is 0v for minimum gain and +3v for maximum gain (about 0.95), dependent on the setting of the TUNING control. Tuning roughly follows a Hz/‘v curve; resonance occurs mostly in the mid/treble range. The resonance CV input works over the range of 1.5-2.5 volts, dependent on the setting of the RESONANCE control.