Digitech RPM1

There have been Leslie simulators before, but none to exploit the sympathetic warmth of valves.

Made by IVL Laboratories (famous for their Intellifex guitar processors), the Digitech RPM1 is the latest in a burgeoning range of devices designed to emulate a rotary speaker, or Leslie, as the genre is better known. The additional attraction of the RPM1 is a built‑in 12AX7A vacuum tube (or valve, to us Brits), to provide the characteristic growling overdrive of the real thing. Previous units have either used a built‑in solid state/digital circuit to replicate the effect, or required the addition of an external unit to do the job — so an all‑in‑one valve‑based design is a logical concept.

The Looks

Externally, the RPM1 is a very ruggedly‑built 1U rack, with six knobs, a power switch, and three vents arranged in a 'V' shape, through which the glowing valve can be seen. The rear panel has a Euro mains socket with integral fuse, a jack for a continuous foot pedal, and a jack for the 3‑button FS300 footswitch which comes as standard. This controls speed, brake (rotor stop) and bypass. Two inputs are provided for mono or stereo instruments, and unusually, there are three output jacks. These jacks allow you to take a separate mono feed from the bass rotor, leaving the horn coming from its own outputs in stereo, thus enabling the user to EQ or otherwise process each constituent of the sound individually.

The six knobs function as follows: Firstly, input gain can be set (with reference to a peak LED) to obtain the optimum operating level. The Drive control, unsurprisingly, sets the amount of overdrive produced by the 12AX7A valve. Horn Speed regulates the overall speed of the upper horn; Rotor Acceleration governs the rise and fall time when changing speeds, and appears to also affect the lowest rate of the bass drum — perhaps not the most ideal arrangement. Spread continuously varies the angle of the 'mics' on the upper horn; fully clockwise puts them 180 degrees apart, for maximum stereo spread with two apparent sound sources, whilst fully anti‑clockwise effectively places them together (closely synchronising the phase of the left and right channel modulations) — more like a single sound source. Lastly, Balance sets the relative level of the bass and horn signals when using the stereo outs only. When using all three outs, the level of the bass output is unaffected by this control.

The Sound

Rotary speaker simulations vary considerably. Objectively, however, a successful simulation relies upon whether it is accurate (convincing) or not. I have to say that the RPM1 falls rather short of the ideal in various areas, not the least of which is the rotary simulation itself. Firstly, there seems to be little evidence of natural internal cabinet reflections, room reflections, or of any muted, unrotating sound that would exist in the real thing. It is these factors which provide the frequency sum and differences that should result in a rich, swirling sound. The overall impression given by the RPM1 is of unenclosed, rotating speakers in a totally anechoic room.

At the slow speed, the sound is disappointingly lacking in movement (especially in the case of the bass rotor), the modulations being somewhat shallow. Modulation intensity (especially amplitude mod) increases dramatically at the fast speed, but owing to the lack of reflected sound, it comes across simply as a fast vibrato, rather than the complex interactions one might expect.

The manner in which the stereo image is dealt with is curious. The bass rotor has a restricted stereo width which cannot be changed, even with the Spread control. In the case of both the horn and the bass, the pan/filter modulations seem to have little or no sync relationship to the pitch/amplitude mod. Even stranger, at the fast setting, the pan and filter mod speed of the bass rotor appears not to change at all, remaining at a slow rate. Stranger still, the horn's pan speed does pick up, but the filter sweeps stubbornly remain at the slow rate. Shouldn't these elements all operate in sync with each other?

The RPM1 appears to exhibit a concave, slightly logarithmic curve that causes the rotors to reach their top speed rather abruptly, coupled with the tendency to take just a little too long at the initial stages. Without researching into the principles of applying constant acceleration to a given mass, I'd expect the curve to be more of an 'S' shape, with a linear section in the middle. On switching from slow to fast, the horn's pitch mod depth increases almost instantaneously, long before the amp mod has had a chance to catch up. This has the effect of 'tugging' the sound downwards, and out of tune momentarily; a rather discomforting phenomenon.

With regard to tonal quality, the RPM1's designers have recognised the need for a fairly steep crossover, to separate the sound components. They also claim to have voiced the unit to reproduce the tonal characteristics of a real Leslie cabinet. In practice, the horn's frequency range is fairly accurate, while the bass rotor lacks any appreciable bottom end. Even with the lower drawbars of my Hammond fully on, there was no real warmth or energy present.

The manual promises that the overdrive will cause the sound to 'take on a warmer, or 'furry' tone'. However, the sound actually produced even at minimal drive levels is altogether too fizzy and crackly from the horn output, and blurry and indistinct from the bass output. This caused me to check that neither the RPM1 nor my desk inputs were overloading, which they were not. This is a great shame, and quite surprising, as there are a good many solid state and digital overdrive circuits I've encountered that are more effective.

Conclusion

It is strange, considering the high cost of this unit, that MIDI has been entirely forsaken. Surely there could have been MIDI control of the speed, if nothing else? When you look at the excellent‑sounding Korg G4, which offers virtually the same facilities at almost half the cost, it's extremely hard to justify Digitech's pricing. And no, I don't have a deal with Korg, or own a G4. It just seems a little excessive (continuing the comparison) to pay an extra £250 for a valve and one more output.