Boss RT-20 Rotary Sound Processor
A stompbox that chases a 150-pound Leslie cabinet, and mostly catches it, if you can live with six AA batteries.
Overview
The RT-20 is Boss's attempt to bottle the Leslie rotating-speaker cabinet in a compact pedal. It came with a $299 price tag and a display that spins. It's built on Roland's COSM modeling technology, and it simulates the whirling tremolo that defined the Hammond organ sound for decades - the effect that also turned up on records by guitarists like Eric Clapton and George Harrison.
The control set is unusually deep for a pedal. A Mode knob selects from four rotating-speaker algorithms: standard rotary (a natural Leslie 122 simulation), enhanced rotary (a 122 with exaggerated tremolo), enhanced rotary with stack-amp overdrive (a 122 with Marshall-stack distortion), and a Uni-V setting that pairs a Univibe pedal simulation with stack-amp distortion. Separate knobs set the slow and fast rotor speeds. A Balance knob blends the bass and horn rotors, and a Rise Time knob controls how quickly the rotors decelerate when switching from fast to slow. There's an Overdrive knob for adding grit, plus separate Effect and Direct knobs for wet/dry mix and overall level.
Two footswitches handle the live work. One turns the effect on and off, the other toggles rotation between slow and fast. The twin footpedals also let players control the virtual horn and bass rotors independently. An optional expression pedal allows real-time rotation-speed control, and a headphone output on the back makes silent practice possible. There are two unbalanced ¼-inch inputs and two unbalanced ¼-inch outputs, with a switch to select either keyboard or guitar input. The unit works in stereo or mono.
That spinning multisegment LED display is the pedal's most divisive feature. The power question is real, too: the RT-20 runs on six AA batteries rather than the single 9V found in most pedals. Alkaline cells are estimated to last 12 hours, which makes the optional power adapter close to essential for regular use.
Reviews were mixed but warm. The overdrive drew criticism for sounding harsh and edgy. Jazz guitarist Pete Fogle reportedly loved the unit in a guitar context, where the slow rotor speed felt like a '60s-era phase shifter and the fast setting produced a wobbly tremolo with a psychedelic edge. Players who work with real Leslie cabinets may hear the RT-20 as a tribute rather than an exact replica.
Later players kept reaching for it. EQ Magazine (January 2009, p.34) reported that Porcupine Tree's session work put a Boss RT-20 Rotary Speaker Simulator alongside a vintage Leslie cabinet to shape guitar tones. That treats the pedal as a studio color rather than a cabinet substitute. For anyone chasing the rotary sound without hauling a 150-pound cabinet, the caveats are the battery appetite and an overdrive circuit that divides opinion.
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