Review: RPM-1 Leslie Simulator

by Bruce Wahler

Updated 7/5/96

The following are facts and subjective impressions about the DigiTech "RPM-1" Leslie Simulator, which I have owned and used for about four months. I use the RPM-1 to process a Voce V-3 organ module and, occasionally, guitar sounds from an Alesis S4 in a two-guitar rock band which plays small to medium sized clubs.


The RPM-1 is a Leslie simulator which simulates the rotating speaker audio characteristics electronically. The RPM-1 uses a combination of analog, digital, tube technologies to accomplish this feat. The unit is lightweight and rack-mountable in a single rack space.

As with any Leslie simulator, there's room for improvement in the RPM-1's design, but I believe it is the most convincing electronic simulator to date. My review criticizes the RPM-1 on some points, but it's an excellent choice for keyboard players who turn pale at the thought of hauling around a Leslie speaker cabinet to gigs, and for those who find stage mic'ing of a rotating speaker or emulator not to be a useful option.

As the RPM-1 is a small rack unit, it has a number of advantages over a real Leslie:

the RPM-1 is more portable;

when mated with the proper amp/speaker system, it's louder than any Leslie;

it needs no maintenance, except for replacement of the 12AX7 tube every few years;

PA reinforcement requires no microphones;

it can simulate a range of Leslie types: single speed, dual speed, stopped rotor, etc.;

the effect can be by passed (a distinct advantage for synth and guitar players).

Compared to other electronic Leslie simulators, the RPM-1 sounds more "alive" and musical: less like a flanger or phase shifter. It's not a perfect replacement for a 122/147, but it does provide the musically pleasing Doppler rotation and vibrato effects that are integral to the Hammond sound. Best performance can be found using a stereo rig with quality speakers. The RPM-1 has no MIDI support or user-modified "algorithms," which may or may not bother you.

Physical Description

The RPM-1 occupies a single rack space (19" x 1.75") and is only about 6 inches deep. Unlike many effects, the RPM-1's power supply is internal; there's no wall wart to bother with. Although the unit uses a pre-amp tube, the only ventilation holes are in the front panel, and there are no limitations about mounting over/under other pieces of rack gear, except a warning not to mount the unit over a power amp. At 6 pounds, the unit is very light, and has a "retro" look to it: lots of knobs, green-brown wrinkled paint job, and white silk-screen lettering.

The front panel features Input Level, Drive (distortion), Horn Speed, Rotor Acceleration, Spread, and Horn/Rotor Balance knobs (described below). The unit provides 1/4" jacks for input (mono or stereo), output (mono, stereo or 3-way), a speed/brake/bypass pedal (included), and a control input for using a volume pedal to manually set the rotation speed. All jacks are located on the rear of the unit, making rack operation a breeze. The power cord is removable, but uses the 3-pin AC connector popular on both PCs and Macs, so replacements can be found at your local Radio Shack, in a pinch. (I've always hated proprietary-style AC cords; they're hard to find on a Saturday night before a gig.)

The RPM-1 is manufactured by IVL in Canada for DigiTech.

Signal Routing

The organ, synth, guitar, etc. is plugged into standard 1/4" jacks in the RPM-1's rear panel. Provision is made for stereo sources, and the inputs are kept isolated when Bypass mode is used (not true of some effects boxes). The input is run through a 12AX7-based pre-amp section, the frequencies above and below 800Hz are separated electronically, and then processed by a pair of rotation simulator circuits. The output of the unit is sent to three 1/4" jacks which supply Mono; Left/Right; or Left Horn, Right Horn, and Rotor outputs.


Adjustment of the Input Level and Drive controls, collectively, set three levels: the output of the unit, the amount of tube distortion in the sound, and the balance between processed and bypassed sound levels. The correct mix takes a little experimentation, but once the right level is found, further adjustments are minor.

The Horn Speed control allows adjustments which simulate the belt and pulley options on a Leslie's horn drive system, allowing users to set the preferred rotation speed. The rotary control provides a lot more adjustment than on a Leslie, but I suspect most users will adjust it once to find a preferred speed, and then leave the control alone.

The Rotor Acceleration control sets the degree of bass rotor lag seen when changing speeds. Fully clockwise sets the lag to the same as the horn (~1/2 second), while a fully counterclockwise setting causes the rotor to stop and not respond to the Speed control.

There is disagreement on what the "correct" acceleration times are for a Leslie. It is obvious that the RPM-1 was designed to mimic a unit at or near peak condition. If, like myself, you are used to playing through older Leslies, these settings may seem wrong at first. After closely listening to the organ on various recordings, I have come to the conclusion that the RPM-1's settings are reasonable for a mint condition Leslie.

Bass-pedal playing organists who feel the pulsing bass detracts from their performance will like the inclusion of a stopped-rotor setting. For my liking, however, this control doesn't have enough range: all settings below about 9 o'clock stop the rotor, yet the 9 o'clock setting only provides about 4 - 5 seconds of lag - a bit too short to simulate an older Leslie. The clockwise setting is so short as to be of little use, and the horn lag is a fixed value. I wish there were more adjustments available to the user.

The Spread control defines the placement of the two virtual "microphones" on the horn, from 0/0 degrees to 90/270 degrees (the rotor "mic" is always at 0 degrees.) Stage size will dictate adjusting this setting at each new gig, as its ideal setting is directly related to speaker placement.

The Horn/Rotor balance control does exactly what it implies, and allows simulation of various mic'ing techniques. The owner's manual also suggests using it to change the tone color of the sound source; personally, I think that's a poor substitute for drawbars. Anyway, there's plenty of play in this control to find your own favorite setting.

The indicators for Speed, Brake, and Bypass functions are actually lighted switches; push the Speed "light" and the rotating speed changes, etc. The speed indicator flashes at two rates: about once a second for slow speed, and around 2-3 times/sec. for fast speed, giving a visual indication of the current setting. The Brake switch allows the rotor to spin down to a stop, as early Leslies did. (The Speed setting has no effect when the Brake is on.)

The RPM-1 also comes with a standard DigiTech FS300 footswitch box containing three foot switches and a cord with a 1/4" stereo plug. One switch toggles the rotors between fast and slow, the second one switch controls the Brake. The third switch bypasses the entire signal processing chain.

This pedal, designed with guitar effects processors in mind, is not well suited to the requirements of organists. For one thing, the order of the switches seems to be meant for right-foot operation: the speed control is on the far left, while the bypass switch is on the right. Anyone who grew up on a B- or M- series will tell you that the right foot controls the volume! Also, the switches are less than three inches apart, making blind stabs under the keyboard an adventure. This problem is magnified by the fact that the Bypass function is electrically the same as hitting both of the other two switches. Guess what happened the first time I accidentally ran my size tens over the Speed and Brake switches at the same time? A new switch box is in order. Luckily, the circuit for the box is simple, so you may want to obtain a larger version, or at least modify the switch order to suit your needs. Die-hard Hammond players may opt for creating "console"-style switches, anyway.

I have yet to find a use for the manual speed setting input, which overrides the speed settings and built-in speed change delays. I imagine that someone, somewhere needs a mid-speed Leslie effect that synchronizes to a sequencer-driven beat or something, but I bought my RPM-1 to simulate a real Leslie. I don't want to be bothered with sweeping the speed from Chorale to Tremolo; that's the RPM-1's job.


What does the RPM-1 sound like? Well, it sounds a lot like a Leslie, or to be specific, it sounds like a brand-new, mic'd Leslie Model 122/147, with the deflectors still in the horn, and maybe even the back wood panels in place. This may or may not please you. Many rockers equate the Leslie Sound with an aging Model 122, deflectors off, with a pair of mics shoved as far into the cabinets as practical. It ain't really here, boys and girls. If, on the other hand, you prefer a more FM-rich sound, and you like to keep your Leslies well-oiled and maintained, read on.

The RPM-1 requires a high-fidelity stereo sound system to perform at its best. Don't expect miracles out of a Peavey KB-series, or even the small Barbetta amp. I use a 175W/channel FET amp, a low-noise mixer, dual 15-band EQ, and a pair of 15" bottoms with bi-radial horns. If you can't hack a similar system, consider running through the band's PA system instead. Bottom line: your amp/speaker combo should sound great playing background music, if you want the best results. To my ears, when used with such a system, the RPM-1 sounds like the real thing, give or take a bit.

The cabinet simulation -- tonality, crossover point and slope, etc. -- is excellent. Unlike complaints I've heard about other Leslie simulators, the RPM-1 does not take away the punch of the highest octave of the organ. In fact, if the price was $200, and the Doppler Effect was awful, I'd still buy it for the cabinet sound. But, they got the Doppler right, too -- and the clockwise/counterclockwise rotation phasing, to boot.

Two of the best "proofs" are:

888000000 Grab a handful of bass keys and glide up, changing from slow speed to fast;

808808000 Play a line in the second highest octave on fast speed.

I haven't been able to arrange an A-B comparison of the RPM-1 to the Motion Sound PRO-3, so I don't feel qualified to comment on the "image" quality of the two products. Others claim that the RPM-1 cannot produce the same illusion as the physically-modeled PRO-3, and I'm inclined to believe them, having heard the PRO-3 unit in a separate setting. Then again, the RPM-1 was has some real advantages over mic'ing a rotating speaker in a loud stage setup, and its crossover specifications of 800Hz and 12dB/octave are more accurate than the PRO-3's 700Hz and 18dB/octave. I have had the opportunity to compare my Voce V3's Spin circuitry to the RPM-1: I think the RPM-1 is a clear winner -- for its tsound -- despite the Spin's greater programmability.

The 12AX7 provides a nice simulation of the classic 40W tube amp, warming the sound in clean settings, and beautifully muddying the mix at higher modes. It's not quite as good as a pair of 6550's through an output transformer, however, so don't expect miracles. I have retired my older tube pre-amp, even though its dual-12AX7 design is a little more flexible. Real Tube Twinkies can try substituting an 7025 for the 12AX7, which will round off the tone a bit, and punch up the midrange, too.

Cost and Portability

Like most Leslie simulators, the RPM-1's advantages include portability and lack of maintenance. Since the system is modular, my gear is more portable than any Leslie. I also don't have to hassle with microphones and stands. On the down side, you do need a sound system.

When considering the cost of the RPM-1, you need to factor in the entire amplification system. If you already own a keyboard sound system for your piano, synth, etc. the RPM-1 may be extremely attractive. If you need to buy an amp and speakers to use with the RPM-1, you should compare the RPM-1/amp/speaker combo to a new or used Leslie, and factor in your need for portability, then decide what's best for your situation. The unit retails for US $579. Street prices vary quite a bit, but a price of US $450 seems about right. I don't know if IVL markets the unit in Canada under another name; if so, its price may be more appealing to Canadian buyers.


After two weeks of ownership, I almost returned the RPM-1 to the dealer. Then I took time to play around with the unit, and I spent the next couple of weeks changing settings until I was really happy with the sound (while thoroughly trying the patience of the other band members!) Here are tips I learned along the way, which may help others:

The unit is aided by a bit of EQ-ing, on the subtle side. The high end goes up way beyond a classic B-3's 6K, so the top octave of some clones and synth samples may sound a bit shrill unless the high end is rolled off. I use 5.5K as my roll-off point with my Alesis S4, and kill the treble at 12dB/octave; I don't find it necessary on the V3. A 3dB boost at 3K seems to improve both units' ability to cut through a rock band mix, too. (It also imitates the response peak of most dynamic microphones.)

One of the unique aspects of a real two-rotor Leslie is that anything fed into it "sounds like a Hammond" -- Rhodes pianos, Farfisa organs, synths, etc. In my opinion, the RPM-1 cannot make that claim. It can't take a cheap imitation organ and close the sonic gap, nor can it quite imitate the growl of a guitar through a two-rotor Leslie. Obviously, an exact Leslie imitation, warts and all, is a huge undertaking, so the RPM-1's designers focused on the sound when coupled to a tibia-voiced organ and made certain tradeoffs, which probably kept the costs down. I found adjustments on the RPM-1 to be touchy when used with my Roland D-50 (with its good, but not great, Hammond sounds); nearly all of my earlier complaints -- stereo field problems, bassiness of the horn/rotor balance, muddy overdrive tone -- went away when I switched to Alesis S4, which has better Hammond samples, and the sound with the V-3 is superb. Obviously, the closer you get to a real Hammond, the better.

While the user's guide recommends that the Spread setting be set fully clockwise for maximum effect, I find this setting to have an unnaturally wide stereo field, unless the speakers in your sound setup are placed far (at least 8 feet) apart - a luxury many performing musicians cannot afford. If you prefer a mic'd Leslie sound, I recommend starting at the 1 o'clock position -- equivalent to two microphones at about 100 degree separation -- and making minor adjustments from there, based mostly on the Tremolo speed sound. If you like a "room" Leslie, a wider setting may be more your cup of tea.

As stated previously, the RPM-1 sounds best when used through a stereo sound system. Unfortunately, many PAs are run in mono mode. Unless the Spread control is set to the widest setting, this may provide a different effect through the PA than seen in the stage mix. If you find this to be a problem, try adding a bit of the Right channel feed to the PA mix, usually 15-20 dB below the main feed.


The RPM-1 is a great product for those players who want a high quality Leslie sound in a very portable package. As with any product, the RPM-1 has its good and bad points, but the unit offers excellent Leslie simulation at a reasonable price, and it weighs less than a Fake Book! If you're in the market for a Leslie or Leslie simulator, you should at least take a listen to this piece.

Copyright (c) 1996 by Bruce A. Wahler. Reprinting of this document for non-commercial purposes is freely allowed. Use at your own risk.

No laboratory animals were used during the evaluation ot this product!

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