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Korg G4

Digital Rotary Speaker Simulator By Gordon Reid
Published September 1994

Convincingly reproducing the effects of rotating horns and speakers in software inside a 1U rackmounting box is a challenge to the designer. Bringing it in at under £300 is an achievement. Gordon Reid examines the latest incarnation of a keyboard effect that's come full circle...

Korg have come a long way in the last few years, not only developing into the dominant force in professional '90s synthesizers and workstations, but also expanding their product range for guitarists and bass players. One recent example of this is the recently launched 'ToneWorks' series of processors, built in Taiwan under a Korg license. While not 'true' Korg products (Korg USA wants to differentiate between the company's mainstream products and its guitar‑oriented sidelines), we in the UK already know the first of these products as the Korg G3 effects processor. Housed in a simple plastic moulding, the G3 features four pedal switches plus a number of control knobs... standard fare for our stringy friends, but not the kind of unit one would expect to find offered to keyboard players. Yet the G4 Leslie Simulator lives in exactly the same case (although the plastic is a different colour) and offers the same configuration of knobs and footswitches. Only the functions (and the screen printing) have changed. So, in common with the G3, the G4 features stereo inputs and outputs (with twin 48kHz 64x oversampling delta‑sigma analogue‑to‑digital converters (ADCs), and twin 18‑bit 8x oversampling DACs), a stereo headphone socket, and a 9v DC input from an external psu (although there is no option to run the unit from batteries). There is also a Leslie rotation 'speed' input, which we'll discuss below.

The G4 provides all the facilities of a valve‑amplified Leslie: overdrive, distortion, tonal modification, independent rotor speeds, and braking. These are controlled by four pedals: Bypass, Drive, Stop, and Fast/Slow; plus eight knobs: input volume, output volume, rotor speed/acceleration, horn speed/acceleration, rotor/horn balance, speaker‑to‑microphone distance, bass and treble microphone separation, and amount of overdrive. The first two knobs are trivial — set the input level so that the input level LED flashes only at peak playing levels, and set the output level to get the best from your amplifier or mixer. It's the other six which are of real interest...

The Leslie Effects

There are three factors affecting the bass sound of a Leslie — the rotor's slow (chorus) speed, its high (vibrato) speed, and the rate of acceleration and deceleration between the two. Korg have applied some simple physics to link the three in an algorithm they call 'IPE' — the Integrated Parameter Editing system — claiming that, because the factors are associative, a single algorithm can set them to optimum combinations each time you adjust the speed. Consequently, if you set the chorus speed to a 'high' value then the tremolo speed is also 'high', and the acceleration and deceleration are high (ie. it doesn't take long to change speed). Conversely, low rotation speeds result in less rapid acceleration and deceleration. So, although the knob is labelled 'speed', the IPE system isn't setting the speed at all — it's increasing and decreasing the power applied to the rotor which makes it rotate in the first place. In use this is fine, and apart from one or two occasions when I might have preferred a high 'slow' rotation speed with more rapid acceleration, I found Korg's combinations quite acceptable. The control is continuously variable between its extreme settings, the lowest of which causes the rotor to be 'stationary'.

From Dave Brubeck to Booker T, Keith Emerson to Jon Lord, the G4 grunts and growls its way through all manner of playing styles with a class totally belying its humble price tag.

The treble horn control is also labelled 'speed', but it too affects acceleration and deceleration. However, unlike the rotor control, when the speed is high the acceleration is low, and when the speed is low the acceleration is high. In this case, the IPE isn't setting the speed or the power — it's increasing and decreasing the mass of the horn assembly. Korg's combinations are again quite acceptable. There is an LED associated with the horn speed, and this flashes in time with the horn rotation, also letting you know whether the rotation is fast (red) or slow (green). And, as a bonus, if the horn stops rotating, the LED shows the speed of the bass rotor. Neat.

A third knob adjusts the relative volumes of the treble horn and the bass rotor, and this demonstrates how well Korg's developers have analysed the Leslie sound. A real Leslie speaker has a 'hole' in its frequency response, such that not only is there no overlap in the frequencies output by the horn and the rotor, there is a narrow band almost totally missing from the speaker's output. Without this hole, Leslie simulations do not sound convincing. Rotating the Balance control from one extreme to the other demonstrates that the G4 correctly imitates this: some middle frequencies are missing, and the sounds output from the horn and the rotor are quite different, lying, as they should, on either side of this hole. The treble horn alone gives a nasty scratching sound ,and the bass rotor alone is just a woolly muffle. Perfect!

Distance And Separation

Now you may think that emulating the Leslie speaker is the end of the story, but it's not. Unless you're standing on‑stage, you'll never hear a Leslie except through a PA or hi‑fi system. There are techniques for miking up and recording Leslies that can contribute enormously to the sound, and any engineer worth the title knows how to get the best result for the type of music being played. Fortunately, for a given set of microphones, there are only two factors that influence the sound: the distances of the mics from the cabinet itself, and the angle between them.

Only two microphones are used to capture the output from a Leslie: one mounted at the level of the treble horn, the other at the level of the bass rotor. If these microphones are very close to the speaker cabinet there is a significant difference between the sound picked up when the rotor (or horn) is facing a microphone, and that picked up when the rotor (or horn) is facing away. Consequently, close positioning produces the deep, swirling effect beloved by so many organists. Moving the microphones progressively further from the cabinet diminishes the difference between the extremes of the sound, especially when room ambience becomes a significant part of the signal. The G4 emulates a wide range of distances from, at one extreme, very close positioning, through to the less dramatic effect associated with greater microphone distance.

However, it would be unusual to place the two microphones directly above and below each other — the result would be very 'monophonic'. Separating them by an angle between 0º and 90º adds a dramatic stereophonic effect which enhances the modulations created by the Leslie itself. The G4 allows you to control the separation angle and set it anywhere between these limits, yet again showing just how well Korg's boffins have done.

The Overdrive

Despite all the above, there's yet another essential element in the classic organ sound: grit, bite, guts. Distortion occurs throughout the sound generation path of an old organ — dirty tonewheels, ageing pre‑amps, anguished Leslies and, of course, fistfuls of valves adding their characteristic buzz — so no Leslie simulator can be complete without an on‑board overdrive.

Digital overdrives have been improving rapidly over the last two or three years. Early digital effects units had a nasty, fizzy, annoying character that guitarists would spend hundreds of pounds avoiding. Thankfully, these have been replaced by much higher quality imitations such as the Roland GS6 and, more recently, Korg's own A1, which features a very passable valve overdrive effect. Korg have applied their experience with the A‑series to the G4 which, at low drive settings, adds warmth where most outboard units merely add rasp, and, at high settings, screams. From Dave Brubeck to Booker T, Keith Emerson to Jon Lord, the G4 grunts and growls its way through all manner of playing styles with a class totally belying its humble price tag.

Other Facilities

Yet this still isn't the end of the goodies tucked away inside the diminutive G4. For example, Korg have also analysed the tonal characteristics of stationary Leslie speakers and hidden a speaker simulator in the software. Regretfully this doesn't have its own control, but is instead toggled using a combination of the 'speed' and 'bypass' switches. This isn't too much of a problem — after all, if you're playing direct to a desk you'll want the simulator on all the time, and if you're playing 'live', directing the output of the G4 to an amplifier and cabinet, you'll almost always want the simulator off. The tonal change imparted by the simulator is refreshingly subtle. There is some rounding off of high frequencies (quite right too) and a number of cabinet‑like resonances are added in the mid‑range. More brownie points.

The G4 instantly turns any vaguely organ‑like patch into a gritty, dusty old C3.

There is also a 'bypass' function, a 'full‑stop' pedal, and a 'speed' input which allows you to connect either a simple latch‑type switch or a standard volume control pedal to the back of the G4. A latch enables you to switch between fast and slow rotation speeds even if you can't reach the G4 itself, but the real fun begins with the volume pedal, which dynamically controls the rotation speed, allowing you to determine not just fast and slow speeds, but any shade between. There's a memory too, so if you have a precise setup you wish to retain (maybe synchronising the rotation speeds with a sequence, or some such effect) you can store this.

Any Criticisms?

It would have been nice to see independent controls for the rates of the chorus and tremolo rotation speeds, and accelerations separated from speeds. However, Korg have chosen the IPE relationships well, so you'll rarely feel the urge to alter them. As Korg's senior Customer Support Engineer told me, "we've chosen the parameters such that, no matter how poorly you adjust the G4, you can't fail to get a sensible and usable result". Now you might occasionally want a stupid result, but you still can't fault Korg's choice and range of settings. If you can get the sound you want from a well miked‑up Leslie cabinet, you can probably get it from the G4.

Perhaps the only notable omission is that of MIDI — there isn't any. So, while the G4 is perfect for live use, you can't sequence it, and speed changes have to be affected manually (or footually). This isn't a huge problem, but it does mean that no sequence can be exactly reproduced. Whether this adds a human element to your work, or makes it impossible for you to reproduce last night's 'perfect' take (which you forgot to record) is probably just a matter of attitude.

In Use

There have been many attempts to copy the genuine organ sound — most notably in the '70s by Korg themselves, Roland and Crumar — and some of these keyboards are still highly sought after. Most desirable of the mock Hammonds is the Korg BX3 which, in most players' views, comes closer than any other to the sound of the original. But, although the BX3 is no half‑ton un‑split monstrosity requiring four roadies and a reinforced stage, it's no lightweight either. Worse, while the tone generation is good, the Leslie effect is only passable, and the overdrive is naff by modern standards. Consequently, if you've wanted that genuine 'C3 played through a Leslie 122' sound, you've had to play a Hammond C3 through a Leslie 122. Until now.

If you can get the sound you want from a well miked‑up Leslie cabinet, you can probably get it from the G4.

I use a Roland Juno 60 to produce my live organ sounds. This has more to do with cost and convenience than quality but, nonetheless, the Juno has been putting in sterling service for nigh‑on 10 years. So what could be more natural than to play it through the G4? The results are staggering: the G4 instantly turns any vaguely organ‑like patch into a gritty, dusty old C3. In fact, any stationary sound benefits from the movement imparted by the G4, and using it with a guitar also yields some impressive results.


Because of the sound quality of the G4, there are only two devices to which it can sensibly be compared: the rackmount Dynacord CLS222 and DLS223, both of which also come with floor pedals to control the major Leslie functions. The CLS222 has, perhaps, the best rotor imitation of the three, edging out the Korg by the narrowest of margins, but it has no overdrive, no MIDI, and no distance and separation controls (instead offering mono, stereo, and super‑stereo options). The DLS223 has all of these, but is less intuitive and less convenient than the Korg, the overdrive is not as convincing and (worst of all) there is an overlap in the crossover between the treble horn and the bass rotor, making it far less realistic. I feel the only equipment combination that can beat the G4 sonically is the combination of a genuine valve pre‑amplifier (such as the Energy Technology VKP1) and the CLS222. But since the CLS222 is now unobtainable (and even if you could find one, the combination of the two would cost over £1000) it's pretty clear that Korg have an absolute winner on their hands.

I'm sorry to lapse into reviewer's cliché mode, but it has to be done: having played a Juno 60 and BX3 through the review model of the G4 I decided that, MIDI or no MIDI, I was going to buy the first one imported into the UK. And I did. What more can I say?

A Little Leslie History

Just about every keyboard player is acquainted, in one way or another, with the Hammond organ. Indeed, back in the '50s and '60s it was almost impossible to be a jazz or rock player without being asked to play one of these monsters. But even the classic Hammonds, the B3 and C3, produce a rather dry and static sound if played through a conventional speaker system.

In 1937 Don Leslie bought one of the first Hammond organs expecting it to sound like a pipe organ. Unfortunately, whilst it had sounded excellent in the showroom, it was a disappointment when he got it home. Considering ways to overcome this, Leslie decided that the missing element was the natural movement that occurs when the sounds of hundreds of individual pipes, often many feet apart, interact to create a sum that is greater than its parts. He experimented with many configurations of rotating and static speakers, one of the most bizarre of which was a Heath‑Robinson affair with 14 speakers mounted inside a rotating drum. He tried different speeds and phase arrangements, connecting and disconnecting speakers until, almost by accident, he found that the most pleasing effect was obtained using a single speaker. The experiments continued: he replaced the rotating speakers with static speakers directed at rotating horn assemblies and so, with a few further refinements, the Leslie Speaker was born.

Yet the public almost never saw — or heard — the Leslie. The Hammond company hated the idea, and came close to buying the rights to the new design just so that they could squash it. It was only Don Leslie's persistence and commitment that saw the product launched. To counter this, Hammond released a so‑called 'Leslie‑proof' organ, that wouldn't power up without its own speaker connected! Nevertheless, since Leslie sold his first speaker in 1940, hundreds of thousands have been manufactured and the distinctive cabinet has become a fixture within many keyboard rigs, as well as being used by many guitarists.

There are many models of Leslie, but most of them create their sound by passing the output of two stationary speakers through two revolving rotors. The bass frequencies (those below 800Hz) play downwards into a single rotating drum, while a compression driver produces the treble frequencies and directs them upwards into what looks like two rotating horns (although one of these is a dummy, provided only to stop the whole assembly from shaking itself to bits). Each rotor assembly has two rotation speeds — slow for chorus effects, and fast for tremolo. The result is a sound that combines pitch shifting (the Doppler effect) with phase shifting, timbral modulation, and amplitude modulation. The classic Leslies, the 122 and 147, are twin rotor units with integral valve amplifiers, thus adding yet another complex element to the sound; the 'warmth' (or, if over‑driven, distortion) characteristic of valves.

In the '60s, a number of manufacturers copied the Leslie design, and you will now find many (inferior) alternative rotating speakers in the second‑hand shops. The '70s saw analogue electronics utilised in an attempt to re‑create the Leslie sound. A number of companies tried to use chorus circuits (and Tony Banks, for one, replaced his on‑stage Leslie with a Boss CE1 Chorus pedal) but the effect, and the resulting sound, was quite different. [I have often said that the only analogue way to simulate the effect of a Leslie speaker is to stick a horn in one ear, your finger in the other, and then spin around as fast as you can. Unfortunately, if you're playing to an audience, it's a bit tricky to get them to accelerate and decelerate correctly, and it's almost impossible to get them to synchronise themselves with your playing.]

About 10 years ago digital effects units were introduced, and some of the best of these approached the sound of a Leslie by passing the signal through a chorused pitch modulation algorithm, thus taking account of the Doppler effect caused by the rotation, but still overlooking the other consequences of the rotors. And it wasn't until fairly recently that these units began to offer independent pitch modulation for the bass and treble frequencies.

Yet only one company can really claim to have succeeded in emulating the Leslie. The Dynacord CLS222 stands head and shoulders above all other devices, and a recording of an organ passed through a CLS222 is all but indistinguishable from a recording of the same organ passed though the original speaker. Unfortunately, the Dynacord doesn't include an overdrive, is now deleted, cost about £600 when it could be obtained, and second‑hand units never appear for sale in the Reader Ads. It is into this breach that Korg have stepped, launching a digital Leslie simulator, with overdrive, for less than £300. It could be a winner...


  • Great Leslie simulation.
  • Light, robust, and very simple to use.
  • Superb value for money.


  • No MIDI.
  • Korg asking me to return the review unit.


The G4 fills a hole in the market more thoroughly than any number of Mars bars. What it does, it does better than, and at a fraction of the price of the competition. If you want the sound of a Leslie, try the G4 first.