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Universal Audio Waterfall Rotary Speaker

Leslie 147 Emulation Plug-in By Hugh Robjohns
Published May 2023

Universal Audio Waterfall Rotary Speaker

This isn’t the first virtual Leslie speaker plug‑in — but it might just be the best one yet!

As countless organists around the world will gladly discuss at considerable length, simulating a real Leslie speaker is just about the most challenging technological feat known to humanity. Okay, so that might be a slight exaggeration, but it’s certainly true that a real Leslie speaker generates a fiendishly complex, yet instantly recognisable form of constantly varying modulation.

Invented by Don Leslie in the late 1930s, the eponymous speaker cabinets were intended originally to enhance the sound of Laurens Hammond’s first console organs, making them sound more like the real church and theatre pipe organs that Hammond was trying to emulate. Although the Hammond company wasn’t interested in using Leslie’s originally designs, his speakers quickly became a must‑have accessory for Hammond organ players, and went on to define a sound that we all know and love today. Moreover, through the 1960s Leslie speakers became very popular effects for electric guitars, electric pianos, and even vocals.

There have been well over 50 different models of Leslie speaker over the years, with very different cabinet sizes and shapes, different amplifier designs, different speaker configurations, and even different speed options. However, the ‘classic’ Leslie arrangement involves a large bass speaker firing downwards into a rotating drum with an angled baffle, which throws the sound horizontally into the room. The high end is projected via a compression driver firing upwards into a rotating horn (there is only one working horn — the second is a dummy for balance only!). Early Leslie speakers offered only the fast Tremolo speed option (or stopped), but later popular models all provide slow (Chorale) and fast (Tremolo) modes, and some also included a stationary option (Brake), too.

The rotating bass rotor and treble horn create both cyclical amplitude and pitch (Doppler) modulations, but the cyclical effect is surprisingly intricate due to myriad internal cabinet reflections. Complicating things even further, the bass rotor spins in the opposite direction to the treble horn, and the lightweight Bakelite horn speeds up and slows down much more quickly during speed changes than the relatively heavy plywood bass rotor — another complex artefact which plays a critical role in the characteristic Leslie sound.

Early analogue Leslie simulators were, it must be said, profoundly unrealistic. However, with the availability of ever‑increasing amounts of digital processing power more modern Leslie emulations have become remarkably realistic. For example, the digital emulations built into Suzuki‑Hammond’s last few series of digital tonewheel organs have actually been remarkably good, and Neo Instruments’ Ventilator series of effects pedals are widely acknowledged as setting the standard for compact hardware Leslie emulators.

However, for such a phenomenally complicated form of modulation, software plug‑ins can afford even greater digital processing resources and consequently it is in this form that the most realistic emulations can be found. Several virtual Hammond organ VSTi plug‑ins now have really impressive Leslie simulators and, joining an already impressive cohort, we now welcome a brand‑new offering from Universal Audio in the form of a plug‑in called Waterfall Rotary Speaker. This new plug‑in is derived directly from UA’s new Waterfall B3 Organ VSTi plug‑in but, whereas that Leslie simulator can only be used as a part of the whole organ instrument, the Waterfall Rotary plug‑in is a stand‑alone effect which can be used with myriad instrument sources, not just organ.

To the undoubted chagrin of UAD fanatics everywhere, this new Waterfall Rotary Speaker plug‑in is currently only available in the Native format, for all major Mac and PC platform DAWs and secured via an iLok account. (A 14‑day free trial is available.) The Native plug‑in is compatible with VST3, AU, AAX64 and LUNA host applications, and is also available within the UA Spark subscription portfolio. Unfortunately, though, it is not compatible with any of UAD’s hardware accelerators or any of the Apollo interfaces — although I imagine UA must be considering developing a SHARC‑compatible version for the hardware accelerators, since it would undoubtedly be a very popular and powerful effect in the Apollo console, for example.

While modelling the cabinet, speakers and mic techniques are critically important, there’s more to the full Leslie sound story, and UA have meticulously modelled the unique characteristics of the valve amp employed in the Leslie 147 (and 122)...

Waterfall Rotary Overview

Under the hood, Waterfall Rotary is built on a phenomenally detailed model of a Leslie 147 cabinet in near‑perfect condition (see 'What Is A Leslie 147?' box). The plug‑in window shows a beautifully detailed photo‑realistic rear‑quarter view of a Leslie speaker setup on a carpet in a virtual studio. There’s an array of mics on stands positioned around the cabinet, with the top and bottom cover panels removed to reveal the rotating horn and drum, along with the valve amp chassis tucked alongside the bass rotor. These superb graphics show the horn and rotor turning at different speeds and in the correct contra directions, too! We shouldn’t be swayed by pretty graphics, of course, but the attention to detail here is genuinely impressive and almost mesmerising in much the same way as a real Leslie.

Various user configuration options are arrayed in two control panels, one above and one below the graphical representation. The top control section features classic chicken‑head rotary controls for setting the input volume and drive level. (Clicking the identifying labels below the volume and drive controls recalls their default settings.) There are also separate trimmers for adjusting the nominal fast and slow rotation speeds of the horn and drum individually, along with their respective acceleration/deceleration rates.

To the right‑hand side, two vertical slide switches change the rotation speed (between fast, brake, and slow modes), and turn the whole Leslie effect on/off. Adding immeasurably to the sense of realism, switching to the slow speed (from fast) generates a realistically audible relay click and belt slip, which is a nice realism omitted by most emulations.

Camouflaged into the bottom left‑hand corner of this top strip of controls is a configuration ‘cog’ which opens a drop‑down box with options to adjust the amount of mechanical noise, and assign MIDI control of the speed change function to a mod wheel or sustain pedal. The sustain pedal can be set for hold or toggle actions, and while the pedal can only switch between fast and slow modes, the mod wheel selects slow Chorale for CC values 1‑53 and the fast Tremolo speed for values 74‑127. The 54‑73 range activates the Brake mode.

The mechanical noise level can be set from off through to the level captured in the room (100 percent); there’s no setting beyond 100 so it can’t be exaggerated.The mechanical noise level can be set from off through to the level captured in the room (100 percent); there’s no setting beyond 100 so it can’t be exaggerated.Returning to the immense level of detail embodied within this Leslie emulation, the mechanical noise generation includes not only the speed change relay’s click (which even sounds different when going from fast to slow than it does from slow to fast), but also a blend of wind noise from the treble horn, some bass rotor rumble, and even some belt slippage when switching from fast to slow. The only small digression from reality is that the relay click volume doesn’t noticeably change between the two bass mic positions when it would in real life (more on that below).

The mechanical noise amount slider is configured to give what UA claim is an authentic noise level when set to 100 percent (I wish my own 122RV were that quiet!) and when set at 0 there is no mechanical noise at all. It’s superbly realistic, though it might also have been nice to be able to exaggerate the mechanical noises a little for unconventional musical applications. Nonetheless, even without playing sound through the Leslie, it really sounds just like the real machine standing in the studio, primed for action — that’s a really big plus as far as I’m concerned.

In the bottom control panel section, a variety of options are provided for ‘miking’ the virtual cabinet in different ways to generate a variety of distinct sound characters. This variable miking idea is a fairly common feature of many Leslie simulators and the ‘recorded’ sounds obviously varies enormously depending on how and where it is miked. Indeed, different musical genres have largely settled on specific miking arrangements as a key part of their sound.

A rotary control here adjusts the balance between the treble and bass mics, allowing the user to emphasis the low end or treble, as desired. A second knob sets the overall output volume, with an associated stereo bar‑graph level meter for guidance. The inclusion of this output volume knob allows the input to be intentionally overdriven without overloading the signal feeding the rest of the DAW signal path, thereby allowing some really ‘graunchy’ sounds to be created or, by backing off the input level and cranking up the output instead, a really clean ‘hi‑fi’ kind of sound can be attained instead.

Mic Position

Either side of the balance control are graphical representations indicating the selected horn and rotor virtual mics and mic positions. Not surprisingly, the selected mics and mic positions around the Leslie cabinet are all reflected perfectly in the photo‑realistic graphics.

I was slightly surprised, though, to find that the miking options in this emulation only allow the bass rotor to be captured in mono, with a single microphone positioned either straight on or at an angle from the amp side. It’s true that, historically, most recording engineers do and have captured the Leslie bass in mono (and sometimes without the bass rotor even moving) but, to my ears, stereo Leslie bass is an important part of the real‑life sound and quite a wondrous thing. After all, the bass rotor handles musical fundamentals up to around 800Hz (A5), which is a significant portion of most instruments’ ranges.

The modelled mics can be selected independently for the treble horn and bass rotor, and there are several mic placement options.The modelled mics can be selected independently for the treble horn and bass rotor, and there are several mic placement options.

When it comes to (mono) mic selection, though, UA have provided a number of coyly identified classic and popular capacitor microphone options, which we can assume are based on Neumann’s U47 FET, U67 and U87 mics, along with a Bock iFet7 and a dynamic option in the form of an Electro‑Voice RE20.

Universal Audio Waterfall Rotary Speaker

The treble horn miking encompasses similar but more versatile options, with emulated microphones including popular alternatives comprising AKG’s C414, Neumann’s KM84, U87 and U67, as well as dynamic models of Sennheiser’s MD421, Shure’s SM57, and even the delicious Coles 4038 ribbon mic. Three mic location options are available: a single central mono position, a stereo spaced pair positioned in the cabinet corners pointing in towards the horn spindle at 90 degrees to each other, or a wide stereo pair placed at the cabinet sides (180 degrees). Naturally, these two stereo arrangements give radically different flavours of stereo image movement.

While Waterfall is easy to configure, there are, of course, plenty of instrument and genre‑themed presets to get you started.While Waterfall is easy to configure, there are, of course, plenty of instrument and genre‑themed presets to get you started.There are no facilities to vary the virtual distance of the mics from the cabinet (which, in simple terms, alters the depth of modulation) but the provided options all sound delicious, natural, and definitely ‘album ready’.

While modelling the cabinet, speakers and mic techniques are critically important, there’s more to the full Leslie sound story, and UA have meticulously modelled the unique characteristics of the valve amp employed in the Leslie 147 (and 122) which contributes so much to the distinctive overall sound, especially when being slightly overdriven. Indeed, getting the uniquely non‑linear saturation and distortion characteristics of the Leslie amplifier right is a real challenge and achievement. But this is an area of modelling technology at which UAD have always excelled in my view — and something that many Leslie emulations haven’t quite mastered.

As a result, Waterfall Rotary Speaker is capable of considerable yet always believably realistic tonal variety, just by juggling the input volume, drive and output level controls (the volume and drive controls interact quite significantly, but again convincingly). It’s great fun to fiddle and experiment with different settings, of course, but to make life easier UA have provided a selection of presets creating bespoke configurations tailored specifically for different sources including vocals, guitars, basses and strings, as well as classic keyboards (organs, electric pianos, and more). Some presets are based on different musical genres and style, too (eg. Jazz, Rock’n’Roll, Pressure Cooker).

Stirring The Air

I’ve heard and used a lot of Leslie emulators over the years, but none really sound like a physical Leslie 122 or 147 in the room with me! That’s perhaps not surprising, since a two‑channel stereo recording can never deliver the visceral, three‑dimensional, 360‑degree enveloping impact from the unique combination of direct and room sound that a physical Leslie creates. However, the whole point of a plug‑in like Waterfall Rotary Speaker is that it’s trying to reproduce the precise sound of a miked‑up Leslie whirling away in a studio. So it’s within that context that we must judge the results, and I’m overjoyed to say that UA’s Waterfall Rotary Speaker plug‑in does a really, really, crackingly good job! It genuinely, honestly, sounds exactly like a real, skilfully miked‑up and tip‑top condition Leslie 147 to me — and it’s a sound I know intimately well from miking up my own Leslie in exactly the same ways, and with many of the mics that UA have emulated.

It genuinely, honestly, sounds exactly like a real, skilfully miked‑up and tip‑top condition Leslie 147 to me — and it’s a sound I know intimately well from miking up my own Leslie.

I think a large part of this success can be placed upon those meticulously recreated mechanical noises, which add so much realism to the complete sound (just like generator crosstalk, contact variations and key click play a vital role in authentic B3 emulations). Another true highlight is the progressive valve amp saturation, which progresses into increasingly aggressive distortion in such a natural and believable way, but without the underlying the fear I’d have in the real world of roasting the virtual Leslie’s compression driver! As I mentioned earlier, UA are absolute masters when it comes to modelling the subtle and complicated characteristics of non‑linear valve circuitry, and they’ve done a superbly brilliant job of recreating the 147/122 tube amplifier, adding responsive control of the amount and depth of overdrive distortion.

Further tonal variations can be achieved by selecting different virtual mics and mic positions, and I found those to behave absolutely exactly as I would expect in the real world. I mostly found myself using either the 4038s/U47 combination, or the C414s or KM84s on the top and RE20 on the bass (which is a common real‑world setup I use). As mentioned earlier, I would have liked an option for stereo miking on the bass rotor, but that’s certainly not a deal‑killer and mono bass unquestionably pervades most recorded Leslies anyway, so who am I to argue?

For organists seeking an outstanding Leslie emulation, the same Waterfall Rotary Speaker (but viewed from the opposite corner) is included as an integral element of UA’s new Waterfall B3 virtual instrument plug‑in. I’ve only given that a cursory examination but so far it seems every bit as good as the Leslie, making the combination very good value for money and well worth a full demo.

However, the Waterfall B3’s Leslie effect can only be used with the organ source — external sound sources can’t be run through the Leslie at all. So, guitarists and other musicians wanting to use the Leslie effect must instead use the separate Waterfall Rotary Speaker effect plug‑in.

It will come as no surprise that I already have several Leslie‑emulating plug‑ins available in my DAWs, but without doubt UA’s new Waterfall Rotary Speaker is the one I shall use exclusively from now on. It is simply the best I’ve experienced, as it sounds so convincingly real as a professionally miked‑up Leslie in every respect, and is creatively but predictably controllable. Really nice job UA!  

What Is A Leslie 147?

For anyone not fully conversant with Leslie speaker model numbers, the 147 was introduced in 1963 alongside the possibly better known (at least to Hammond aficionados) 122 cabinet. These two models share identical cabinets, measuring 736 x 1041 x 533mm (WHD), weighing around 68kg and divided internally into three horizontal chambers. The treble and bass drivers are mounted on the top and bottom panels respectively of the central chamber, with the 15‑inch bass speaker firing down through a hole into a spinning rotor in the bottom chamber. The plywood bass rotor features a sloped baffle which projects the low frequencies outwards horizontally into the room, with a fabric cover to reduce wind noise.

A one‑inch compression driver mounted under the top panel fires upwards through a hollow collar into a rotating horn in the upper chamber. Although this horn appears to have two trumpets, one is sealed off and is only present to provide dynamic balance. Original horns are fitted with baffles across the end to diffuse the sound, but these are often removed in an effort to produce a higher SPL.

Both the bass rotor and treble horns are rotated by belt drives from independent dual‑motor assemblies, and it’s because these separate motor assemblies are mounted in opposite orientations within the central chamber that the bass rotor and horns rotate in opposite directions. In both cases the belt drive is taken from a pulley on the fast motor spindle, while the slow motor’s spindle rests on a rubber‑coated flywheel attached to the other end of the fast motor’s spindle. The slow Chorale speed is always active by default, and a control signal of some sort has to be received to activate the fast Tremolo mode. A Brake function, which removes power from both motors, was available as an option for the 122/147, and became a standard fitting in some later Leslie speaker models.

A relatively simple 40 Watt push‑pull valve amp powers the bass driver and compression horn via a passive 800Hz crossover. A pair of large 6550 output valves drives a matching transformer, while a single 12AU7 serves as the input buffer and phase splitter. The front end of the amplifier is slightly different between the two models. The 147 was designed as a ‘universal’ model and requires an unbalanced (line‑level) input for compatibility with virtually every brand of organ (or other electrical music sources). In contrast, the 122 model requires a balanced input, provided as standard by Hammond console organs. The different input configurations are achieved with slightly different wiring arrangements of the 12AU7 buffer/phase splitter valve. The rest of the amp is identical.

Another small but important difference is in the speed selection circuitry. The 147 model’s fast motor speed is activated by applying mains voltage via a couple of the (Amphenol) input connector’s pins to activate a relay which re‑routes mains power from the slow to the fast motors. The 122 model differs by employing a second 12AU7 valve (only one triode half is used) configured to detect a 60V DC switching signal conveyed as a phantom voltage within the balanced audio connection. When the phantom voltage is present the valve activates the speed relay so that power is switched from the slow to the fast motors, just as in the 147.

UA Go Native!

Perpetual licences are now available for bundles of UA’s Native plug‑ins.Perpetual licences are now available for bundles of UA’s Native plug‑ins.

Announced alongside Waterfall Rotary Speaker was the availability in native format of a range of plug‑ins, including preamp, tape, outboard, effects and instrument emulations, which hitherto had been available only to owners of UA’s Apollo interfaces and UAD‑2 DSP accelerators. As well as being available individually, the plug‑ins can be purchased in one of three bundles: Creative Edition ($499), Mix Edition ($699), and Diamond Edition ($999), which contain 10, 23, and 28 UAD plug‑ins respectively. Each purchase will include the new UAD Native version for Mac and Windows, as well as Apollo real‑time and UAD‑2 versions, which run using the onboard DSP of UA hardware.

You can find out more at


  • The most astonishingly convincing Leslie emulation I’ve heard!
  • Horn/rotor speed and acceleration/deceleration can be adjusted over a wide range.
  • Leslie overdrive is spot on.
  • Characteristic mechanical noises are recreated perfectly.
  • Perfect ‘album‑ready’ mono or stereo virtual mic and placement options.
  • Mesmerising photo‑realistic graphics.
  • MIDI speed control options.
  • Compatible with Mac and PC DAWs.
  • No UA hardware required.


  • No bass rotor stereo miking options.
  • Mechanical noises cannot be exaggerated.
  • Not (yet) available for Apollo or UAD Accelerator platforms.


A superbly precise and astoundingly accurate emulation of a miked‑up classic Leslie 147 speaker, and available as a stand‑alone native plug‑in.


Waterfall Rotary Speaker and Waterfall B3 Organ (includes the same Leslie emulation): full price £199 each; discounted to £99 when going to press.

Waterfall Rotary Speaker and Waterfall B3 Organ VSTi (includes the same Leslie emulation): full price $199 each; discounted to $99 when going to press.