RK6 resonant lowpass filter
Post Modular
£485
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.
Post Modular's own words, read from their page.
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