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Hughes & Kettner Tube Rotosphere

Leslie Speaker Simulator

Our friendly SOS spin doctor welcomes you once again to what has, over recent issues, almost become a regular feature devoted to Hammonds and Leslie cabinets — or rather the growing number of new products designed to emulate them. This time round, the patient set to receive a full medical is the Hughes & Kettner (or H&K) Tube Rotosphere. H&K are better known for their highly respected range of guitar amplification products; the Rotosphere may well be their first product deliberately designed with both guitars and keyboards in mind.

The Rotosphere is a floor‑standing Leslie cabinet simulator, similar to Korg's G4 (reviewed SOS Sept '94.) Whereas the G4 utilises digital wizardry to provide the rotary simulation and the valve overdrive effect, the Rotosphere surprisingly boasts analogue signal processing and a real valve — the Tube in its title. The chrome‑enamelled, tank‑like metal casing oozes machismo, and houses just a few controls — a mere three knobs and three footswitches. Could this imply a frustrating lack of user control, or is it a shining example of well thought‑out design based on the 'less is more' principle?

The unit certainly seems built to withstand the most energetic stomping from an engineers' boot. Its only Achilles heel is the front‑panel perspex window that reveals the valve glowing inside, but as long as you're not wearing stilettos or running spikes, it should survive. The three knobs are also recessed to avoid accidental damage.

There are no fancy edit menus on this machine — no parameters, no patches. The three knobs provide input level/tube drive, output level, and lower/upper rotor balance. That's it. An LED by the input knob lights to indicate the onset of 'tube saturation' (overdrive), and a pair of flashing red/green LEDs show the current rotor speed, ie. fast, slow or stopped. The three footswitches are for bypass (with LED indicator), fast/slow (with LED indicator) and breaker (or rotor stop). This last switch is non‑latching — in other words, the rotors stop only while you keep the switch depressed, and as soon as you take your foot away, the rotor movement recommences. On the back panel, we find mono/stereo inputs and outputs, the guitar/keyboard selector switch, and a jack for a remote bypass/speed footswitch. The unit is powered by its own unique external power supply, which (the manual states quite firmly) is rated to cope with the large current requirements of the valve. So no using that spare Sound Canvas wall‑wart, or both it and the Rotosphere may end up in the stratosphere....

The Doctor Will See You Now...

After a brief warm‑up period (during which time the tube saturation LED remains lit) the Rotosphere is ready to boogie. For testing, I prepared a selection of sounds that I would be most likely to submit to the Leslie treatment: Hammond XB2 tones, and a varied selection of samples including guitars (clean and dirty), Wurlitzer EP200 and (a long‑time favourite of mine) the old RMI Electrapiano in organ mode. These were all routed to a group send on the desk (with the channel L‑R routings muted to remove the original sound), thence to the Rotosphere, and then back to the desk on a pair of channels panned hard left and right.

There are a handful of reasonably convincing Leslie simulations on the market, but this one really does pick up on the details that other units miss out.

Well, the sound of the Rotosphere is, quite simply, outstanding. There are a handful of reasonably convincing Leslie simulations on the market, but this one really does pick up on the details that other units miss out. All the sounds tested exhibited the tonal characteristics you would expect to hear emanating from a real Leslie — in particular the cyclic frequency boosting/damping. If I might indulge in some cringeworthy printed onomatopoeia to illustrate this point, a major feature that typifies a Leslie is the way it goes 'yowreeyowreeyow...' The Rotosphere demonstrates just how dramatically different a rotating speaker sounds from a chorus effect. There is even a distinct cabinet ambience or colouration present, even when the rotors are stationary. The depth and intensity of the effect seems spot on at either the slow or fast speed setting, and the acceleration/deceleration times, whilst being just a tad faster than you might expect if you know original Leslies, are well within the realms of reality.

Grunge Factor

The tube overdrive acquits itself very well generally. Ironically, it was perhaps least convincing using Hammond sounds, although only marginally so. Somehow, the distortion that a Leslie preamp confers to a Hammond is, to my ears, smoother and rounder than that of the Rotosphere, but that is not to say the latter does not sound pleasant — it does. The guitar sounds seemed very responsive to a touch of tube distortion, notably electric 12‑string (did someone just put on a Beatles CD?). In fact, this was also true of the other sounds I auditioned — the RMI organ sounds and Wurlitzer piano sounded the business too.

Umms And Ahhs

There is, happily, very little to criticise about the Rotosphere. The points that come to mind are as follows:

  • Concerning the non‑latching breaker switch; I can see no reason why this should not be a latching one. At present, you must remain rooted to the spot if you want to use the stationary rotor sound for anything more than a few seconds. Although the rotors may have stopped, the effect of standing on the pedal is not the same as a bypass, as the tube and cabinet tone colour remain operational, making the stationary sound perfectly usable in its own right.
  • On the review unit, the LED which is supposed to show the fast/slow status wasn't working — although this is actually a redundant indicator anyway, as you can see the rotor speed LEDs blinking on and off at the appropriate speed.
  • Lastly, it is a shame there is no built‑in MIDI facility to remotely control the speed, breaker and bypass settings, especially as the unit was intentionally built with keyboard players in mind; although of course having even this rudimentary MIDI implementation would have added to the cost. The manual suggests that the Speed/Bypass remote socket can be used in conjunction with a MIDI switcher — if there is a simple and inexpensive dedicated box on the market that fits the bill, I would be grateful for any information.


The Rotosphere sounds extraordinarily convincing. Although I don't personally subscribe to the anti‑digital league (in fact I'm eternally grateful for the wondrous toys that digital technology has made possible), the Rotosphere could surely be one of analogue's prize witnesses on the stand if the case ever came to trial.

The final factor to consider is one of cost. Considering the Dynacord CLS222 reached the rather alarming price of £760 in the final days of its existence (1992/1993), and Korg's G4 went for £299 when new (1994), the Rotosphere represents excellent value at a mere £249. Admittedly, you can pick up a new G4 for around £100 now that it has reached the end of its production run; the decision as to which unit to go for is likely to be based on budget versus the aesthetic requirements of the listener. It's also curious to note that all three of the above‑mentioned units, despite being three of the best Leslie simulation devices manufactured so far, have been denied assimilation into the MIDI collective. Thesis, anyone?

Time To Compare

The Dynacord CLS222 (long since discontinued, reviewed SOS December '92) is the one other unit that I felt came closest to capturing the tone and spirit of a Leslie, so comparison became inevitable for this review. The Rotosphere and the CLS have a great deal in common — most notably, the signal processing of both is entirely analogue. Could this be more than mere coincidence? I can almost see the knowing nods out there in Readerland... Both units are also endowed with minimal controls, and this design philosophy is understandable on units like these, which have been created to carry out just one specific task. Assuming you had a full complement of variable parameters, the ideal sound would still reside within a fairly narrow range of those variables. So why not design the unit to sound as good as it could possibly be at the outset? If it does the job sufficiently well, you won't want to alter anything. This was the design approach with the CLS222, and it seems to have been applied on the Rotosphere as well. A similar point was raised during the review of the Oberheim OB3<sup>2</sup> organ (see SOS September '97), on which the lack of detailed patch editing is vindicated by the highly accurate nature of the basic sound.

Overall, I'd venture to say that the Rotosphere is an improvement on the CLS222, for a number of reasons. Here comes the scientific bit — pay attention! The stereo imaging, whilst not as exaggerated as on the CLS, is more 'real world', and the cabinet colouration is more distinct. The Rotosphere also has a slightly brighter top end, and a wider notch in between the upper and lower rotors' frequencies (see below). The rotational waveshape appears to be more of a 'scalloped' triangle wave as opposed to the CLS222's sinusoidal waveform. The scalloped triangle wave more clearly conveys the impression of the upper horn whirling past a closely positioned mic than a sine wave, making the cyclic frequency motion more apparent. The Rotosphere also has the benefit of the built‑in tube drive. The two parameters featured on the CLS222 that are missing from the Rotosphere are fully‑variable rotation speed, and stereo width. In the case of the former point, H&K seem to have got the two available speeds set pretty much dead right — and as for the latter point, you can attend to the stereo width just as easily with the pan controls of your mixing desk.

One final important factor shared by the two units which contributes greatly to an authentic tone is the frequency separation between the upper and lower rotors. Most of the digital emulations I've so far encountered suffer from a considerable frequency overlap between the rotors (see Figures 1 and 2). In actual fact, to most resemble a real Leslie, the two rotors should have very little in common tonally, otherwise their shared frequencies can result in unrealistic phase cancellations (the Rotosphere achieves this, by using analogue band‑separation filters). The overlap exhibited by the digital simulations is not a problem inherent in digital filters — it is perfectly possible to generate digital band separation filters with 'brick wall' response curves. I would guess that the Rotosphere's designers probably just understand the nature of real Leslie speakers better than their competitors.


  • Amazingly detailed Leslie simulation.
  • Accurate rotor separation.
  • Built to survive a game of Jumanji.
  • Attractively priced.


  • No MIDI speed control.
  • Non‑latching breaker switch.


The Rotoverb really seems to capture the sound and spirit of a Leslie in great detail. The tube, while not being 100% true to a Leslie preamp, is nevertheless perfectly capable of producing good results in the majority of applications. The addition of MIDI would have been desirable, but is perhaps less likely to bother guitarists than keyboard players. A very desirable piece of kit for any rotophiles.