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Fractal Audio Axe-FX Ultra

Guitar Amp & Effect Simulator
Published June 2009
By Paul White

Fractal Audio's Axe‑FX is the guitar processor with '80s looks and 21st‑century power!

Fractal Audio Axe-FX Ultra

This is the first product I've tried from Fractal Audio Systems, and it's one of a pair of guitar amp and effect simulators built by the company. The Axe‑FX Ultra and standard Axe‑FX are similar but the former, reviewed here, has a faster processor and more memory, which equates to the ability to run more effect models at the same time. The Ultra also includes additional effect algorithms, enabling it to create more abstract guitar sounds, including an arpeggiator, synth effects, vocoder, looper, multi‑band compressor, ring modulator, quad chorus, diffuser, resonators, crossovers, and more sophisticated delay and pitch‑based effects. My first impression is that if the designers of the Lexicon PCM80, Peavey's ReValver and the Line 6 Pod X3 got together with no budget restrictions, this is somewhere close to what they'd come up with.

Like the standard version, the Axe‑FX Ultra is packaged in a 2U steel case and is powered directly from the mains. Its black front‑panel design is somewhat utilitarian, but the rear view reveals a comprehensive range of I/O options to meet both stage and studio requirements. There's a reasonably large display window, which shows both values and graphical control panels, and navigation is accomplished by the now familiar matrix of dedicated buttons, cursor and page buttons and a data wheel. Power amp, microphone and cabinet simulations may be enabled or disabled on a per‑patch basis, so the user can set up some patches for live performance and others for studio use.

Rapid Response

The Axe‑FX's cabinet emulations are based on Impulse Responses, or IRs, taken from real speaker and mic setups, a technique that generally produces very accurate results, and Fractal Audio say that their tube amp algorithms model aspects of the original circuit down to the component level, reproducing the dynamic way in which the frequency response of the amp being modelled varies under playing conditions. Power‑amp damping, rectifier sag and transformer characteristics are all modelled and can be adjusted by the user. They've gone into this level of detail for over 50 amplifier types teamed with 39 speakers, 10 microphone types and a host of stomp‑boxes and rack effects. However, the designers also point out that many of their modelled effect devices are designed for optimum playability rather than as copies of specific devices — some users will view this as an improvement, while others may feel differently! There are also sophisticated features such as intelligent harmony generation and the ability to swap tone stacks between amp models.

Fractal Audio Systems are also keen to point out that they take the technical sound quality of the unit very seriously. They use 24‑bit Cirrus Logic converters, and their analogue front-end and output‑stage circuitry employs no electrolytic capacitors in the signal path. Analog Devices op‑amps are used in the analogue sections and a noise floor of better than ‑105 dB is quoted. The maximum analogue output level from the unit is around +18 dBu. The Axe‑FX Ultra's dual‑core floating‑point processor gives the unit enough power to run two different rigs at the same time, as some software plug‑ins now also allow, and is claimed to have enough horsepower to give some of today's desktop computers a run for their money.

On The Button

The process of editing Axe‑FX patches will be familiar to anyone who's used a typical rackmount effects unit. The value dial is used to adjust parameters, while Enter confirms an activity. Exit cancels the current step, the four cursor buttons allow you to move around inside a page or screen, and the page buttons move forwards or backwards through the available pages. Tabs at the top of the edit windows show you which page you are on, and the Layout button is used to place processing blocks into the grid and to access the routing menu.

The Control button accesses the menu for the internal controllers, including a tempo setting, two LFOs, two ADSR envelopes and an envelope follower. Individual effects can be bypassed, and there are dedicated buttons to access the Global settings page, tuner display, I/O configuration and Utility menu.

The Recall button lets you select presets using the value dial (and they load almost immediately), while Store and Bypass need no further explanation — though it should be noted that storing a patch overwrites one of the factory patches, which can either be the current one or any other of your choice. A tempo setting is saved with the preset, but a new one may be tapped in at any time using the Tempo button. Parameters can also be assigned to MIDI controllers for real‑time control, making live performance rather easier if you add a suitable MIDI floor controller.

In addition to the buttons, there are also status LEDs below the display. The one labelled 'Edited' lights up if you change a preset and haven't yet stored the changes, 'MIDI' lights if MIDI data is being received, and clip LEDS warn of excessive levels at the outputs. Internal clipping is unlikely because of the floating‑point architecture, but it is possible to overload the 24‑bit output converters, in which case the internal mixer gain or individual block gains should be adjusted downwards.

The Matrix

As with most such products, the user has the option of deciding how deeply to edit. Up to 12 simultaneous effects can be set up in series or parallel, with a high level of editability, so you could simply create one virtual pedalboard setup for live use, storing different control and bypass settings as presets.

That, however, would be to miss out on some neat routing options. The user interface's routing section is based on a 4 x 12 grid, where effect and amp blocks are drawn from an inventory of available items and then set up in the order in which you'd expect their hardware counterparts to be used. Some items are available in multiples, but once you've drawn all the available instances out of the 'store cupboard', that's as many as you can have. Once an effect has been slotted into the grid, it can be patched left‑to‑right or to any other adjacent effect to create parallel signal paths. Each effects block has the same routing options, in that the input sums up to four stereo inputs from the four rows and the output is in stereo. An output mixer then combines the outputs from the four columns into a stereo output.

Blocks can be edited separately, a process aided by the easy‑to‑understand graphical interface, which uses tabs where multiple pages are involved. Only four of the 12 columns are visible at any one time, so the display can be scrolled across to 'look at' the section you're working on, and unused locations can be linked across. Real‑time monitoring of controlled parameters is also supported, so if you have a gain control that ramps up after being triggered by a guitar note, for example, you can see the on‑screen knob turn as you play. This is no gimmick, as it makes setting up dynamic effects much easier.

Choosing a new amp is done via the Type knob in the Amp edit page, which scrolls through the available options once Amp 1 or Amp 2 has been inserted into the grid. When editing amps, you get the obvious gain, EQ and volume controls on the first page, some slightly unusual ones on the second page (rectifier sag, cabinet thump and other esoterica), then on the third page there are advanced functions that let you change the tone stacks, transformer characteristics and so on. A fourth page addresses the mix parameters — level, balance and bypass mode. A similar paradigm applies to effects, where some adjustments are graphical and others in the forms of lists of parameters and values.

In Use

The first thing any self‑respecting guitarist does when confronted with a product like this is to explore the presets. There are over 350 here, and they show off the creative potential of the machine, both for emulating recognisable amp types and for generating ethereal sounds that would normally take a rack of sophisticated processors and a lot of patience. Many of the weirder sounds make use of slow-attack envelopes, pitch‑shifting and shimmery modulation. I've heard similar things from Native Instruments' Guitar Rig and from my own Roland VG99, but the Axe‑FX Ultra does them really well, and with plenty of potential for variety.

The straight guitar amp sounds are pretty impressive: while most modelling boxes can manage filthy rock sounds quite adequately, this one also delivers well on those cleaner sounds or slightly overdriven sounds that clean up as you back off the volume control. There's a definite sense of something solid behind the sound, and those IR-generated cabinet responses really fill out the low end. Of course, you're unlikely to find a preset sound that just happens to work perfectly with your own guitar, so as with a real amp, you'll probably need to adjust the gain and EQ at the very least, and even if setting sounds up from scratch is too daunting, modifying the ones that you're given isn't difficult.

At the time of writing, Fractal Labs are in the final stages of developing both a plug-in version of the Axe-FX, called Axe-PC, and a fully fledged graphical editor that will allow you to tweak Axe-FX patches from an attached PC. This will make a big difference to the user-friendliness of the system, as many guitar players are reluctant to engage with anything that looks in any way technically daunting. Even though the on‑screen editing is fairly intuitive, the sheer number of variables and routing options makes the experience more like setting up a synth patch than using a guitar amp, especially as there are no familiar amp controls on the front panel.


For some users, the Axe‑FX Ultra might seem like overkill, especially if they don't want to use the more abstract sound capability of the unit, but for those who need as much creative freedom as possible, it's probably the only hardware solution that comes close to matching software for its sheer versatility. Furthermore, unlike software, which has to be written to conserve CPU power and allow multiple plug‑ins to be run at the same time, the Axe‑FX Ultra can have all its DSP for its own use, which is pretty much the same as running one very sophisticated software plug‑in that drains the power of the entire computer. There's little to dislike about the Axe‑FX Ultra's capabilities, so if anything is going to put people off, it's that the user interface feels rather dated, while the physical presentation doesn't scream 'Got to have it!' either. As I touched upon earlier, the control paradigm is straightforward enough, but because of the number of options and adjustments, a well thought‑out graphical editor with drag‑and‑drop capability will be a valuable addition. I'd also have liked a headphone output and some front‑panel amp controls for gain and tone settings.

Sound is a subjective thing, of course, but I thought the Axe‑FX Ultra captured the punch and dynamics of a real amp being miked in a studio rather well. The array of effects is impressive, and most sound really musical, though inevitably the pitch‑shifter sounds a little bit grainy. Some of the sounds that can be conjured up by, for example, feeding delay or reverb from a reversed version of the sound fed through a pitch‑shifter and/or a series of resonant filters rival synth sounds in their complexity, and this is an area where the Axe‑FX Ultra scores highly against the competition. However, if you simply want to wig out on metal overdrive with a bit of delay, there are less costly options!  


The closest software alternatives are probably Peavey's ReValver on the amp-modelling side and NI's Guitar Rig when it comes to effects and routing. On the hardware front, the Roland VG99 (which requires a special pickup) and the Line 6 Pod X3 Pro probably come closest.

At The Back

Although there's only a solitary instrument input on the right of the front panel, the rear panel of the Axe‑FX Ultra is a pretty busy place. Input 1 is used when employing the unit as an effects processor, and there is also a choice of a digital input and output on both phono (S/PDIF) and XLR connectors. When you're using the digital I/O, the input can accept 24‑bit/48kHz data and Input 1 is disabled. When the rear‑panel analogue input is in use, the front‑panel jack is overridden. Input 2 and Output 2 can be used as an effects loop for externally connected devices, or as an auxiliary output for monitoring, and may also be used as I/O for the effects loop 'block' when placed in the grid with its (on‑screen) input left unconnected.

Fractal Audio Axe-FX UltraThere are two sets of outputs, one unbalanced and one balanced, with a ground‑lift switch to lift the screen of the cable from the chassis ground, helping to avoid ground loops. The IEC mains inlet is also on the rear panel, though the power switch is sensibly located on the front panel. One obvious omission, though, is a headphone output, which I would have found useful when editing patches.

To facilitate real‑time parameter control, the unit also has MIDI In, Out and Thru sockets: the MIDI In is on a seven‑pin DIN rather than the usual five‑pin version, enabling phantom powering, for those pedals that support it, on pins 6 and 7. Two expression pedal inputs are provided, on jacks, and these may also be used with footswitches.

Buying An Axe‑FX

The Axe‑FX and Axe‑FX Ultra are currently available in Europe only by mail order from distributors G66. Obviously, this makes it hard to try before you buy, so they offer a no‑quibble, 15‑day, money‑back guarantee.

Published June 2009