Boss RT-20 Rotary Ensemble

The RT-20 ($299 retail/$225 street) employs Boss’ COSM modeling technology to mimic the specific acoustic qualities of a Leslie 122 Tone Cabinet configured in three different ways, and then tosses in a Uni-Vibe phaser emulation for good measure. There are lots of rotary-speaker emulators out there, and, given that the RT-20 is priced towards the mid-to-lower end of the spectrum, it boasts an atypically robust feature set.
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You get stereo I/O, individual speed controls for Slow (bass) and Fast (horn) rotors and a Balance control to blend them, a Rise Time control for determining how quickly the unit accelerates and decelerates, separate Effect and Direct levels, and an Overdrive control that can be optimized for guitar or keyboard. Most of these controls are unusually versatile. The Slow control ranges from zero modulation to relatively fast, and the Fast control can spin so rapidly that if it were an actual Leslie horn, the whole cabinet would likely rise into the air. Similarly, Rise Time ranges from snail-paced to nearly instantaneous, and Drive can go from gentle crunch to meltdown.

The RT-20 offers four operating modes: Mode I is clean with just a touch of overdrive. Mode II adds the emphasized tremolo that results from close-miking. Mode III simulates the sound of a Leslie amplified through a Marshall model 1959 plexi. Mode IV runs the Uni-Vibe program through the Marshall emulation. The “natural” overdrive in all modes is apart from the additional—and very realistic—overdrive you can dial in with the Drive control.

Two large footswitches turn the effect on and off and switch between speeds. Pressing them simultaneously engages the Brake function, which stops the rotors quickly or gradually depending on the Rise Time setting. You can also vary the rotor speed in real time using an optional expression pedal, such as the Boss EV-5 ($99 retail/$55 street).

To take full advantage of the RT-20’s sense of motion, you should use it in a stereo configuration—although the pedal also produced some sweet tremolo and chorus sounds when used as a mono stompbox. In Mode I, the RT-20 generates waves of lush lows and shimmering highs truly reminiscent of a Leslie. You can’t get a totally clean sound—which may be intrinsic to the Leslie vibe—but given the Drive control, it would have been nice to have a pristine sonic option. Mode II adds slightly more upper-end movement, whereas Mode III is considerably chunkier, with a fairly realistic rasp (though, not surprisingly, vintage Marshall tones did not ensue). Mode IV’s Uni-Vibe simulation was above average, though a little too polite to get you fully across the Bridge of Sighs.

Aside from my quibble over cleanliness, I was extremely impressed with the RT-20. Its sound is truly inspiring, and I spent hours experimenting with it. At just over two Benjamins street, it’s an exceptional value—even if you only plan to use it in mono.