AxeFX, Kemper, And Digital Amp Emulation

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An article by Rammikin



Put simply, digital amp emulators are designed to achieve the same tonal quality as a tube amp. You can also use them to emulate solid state amps, but tube amps represent the holy grail of tone for guitarists, so emulation of those is generally the goal of emulation. Digital emulators have a number of advantages over tube amps, so it can be quite advantageous to use one instead of a tube amp. For example:

- Flexibility. An amp emulator can often achieve the tone of a number of amps, so it's like having an arsenal of many amps in one package.
- Volume. Since it doesn't rely on a cabinet and speaker to shape the tone, you can use a digital emulator in environments where you have to keep the volume low, and can even use headphones.
- Size. A digital emulator can be much smaller and weigh much less than a tube amp, making it ideal for carrying around.
- Heat. No tubes means very little heat compared to a tube amp.
- Reliability. Digital components tend to be tougher than tube amp components and stand up better to the wear and tear of transport and gigging.
- Reproducible. Once you have a sound dialed in, a digital emulator can give you the same tone day after day. The sound of a tube amp that relies on mic'ing a cabinet can be notoriously changeable from day to day.

Given these advantages, attempts to build digital emulators have been underway for many years. The problem is: a tube amp is surprisingly complicated and precisely emulating it is quite challenging. About 15 years ago, Line 6 had a bit of a breakthrough with the Pod. This represented the first digital emulator that achieved widespread success. It sounded great and was quite popular, but it still fell short of a high fidelity emulation of a tube amp.

For a few years afterwards, efforts to improve the fidelity of digital emulation languished. It was widely recognized that to significantly improve the emulation would require a device that was much more expensive than a Pod, which sold for a couple of hundred dollars. But there simply weren't many guitarists who would be willing to pay a lot more than that for a better emulator, so that discouraged companies from trying.



About 10 years ago, Fractal Audio emerged with a new digital amp emulator called the AxeFX. This represented a significant leap forward in amp emulation, but at a cost. With a price of about $2000, it was expensive, so only a few guitarists were interested in buying one. Those who did however, were rewarded with a device that came quite close to achieving the sound of a tube amp. The AxeFX has been revised over the years until today it is extremely difficult to tell the difference between an AxeFX and a real tube amp.


Kemper profiling amp
Kemper profiling amp

A couple of years after the AxeFX appeared, Chris Kemper, the creator of the Virus synth, created a company for the purpose of producing the Kemper Profiling Amp. The Kemper is significantly different than the AxeFX, and I'll discuss the differences below, but it was similar in that it aimed for that same high level of amp emulation quality as the AxeFX, and the same (roughly $2000) price point.

These were still expensive units, but they were both catching the attention of professional users and so, about 5 years ago, AxeFX's and Kemper's started showing up in the rigs of high profile users, both in the studio and on stage. Groups like Metallica, Periphery, and the Deftones were using digital emulators for the first time. These were guitarists for whom price was no object. They just wanted great tone and they were getting it from an AxeFX or a Kemper.

Modeling vs. profiling

Because the AxeFX and Kemper aim for the same thing: high end emulation of tube amps, this leads to inevitable comparisons. It's worth noting however, that they differ in the way they go about achieving emulation. The AxeFX is a modeler while the Kemper is a profiler.

Modeling in this context generally means the components of the amp are each digitally modeled. When you put the digital models of these components together, it's like assembling the components of a real amp. Hook them up together and, if the component models are good, you'll get a good emulation of a tube amp. That's the approach Fractal Audio took with the AxeFX. It contains accurate models of hundreds of popular tube amps.

Kemper, instead of using modeling to try to mimic the sound of specific amps before the unit leaves the factory, chose to use profiling. With profiling, instead of trying to virtually assemble digital components, you simply attach it to a real amp and it performs an analysis of the amp. The result of that analysis, called a profile, is then used to emulate that amp. Unlike modeling, you don't care what's inside the amp, just what comes out of the microphone when you feed it a test signal.

What is the advantage of profiling? Instead of relying on the manufacturer to build a fixed list of models that device is capable of emulating, you can plug a profiler into any amp in the world. Once you capture that amp into a profile, you can share the profile with others. The list of amps you can emulate is almost limitless. In practice, very few people actually do profiling. Most simply tap into the huge collection of profiles created by others.

Given that limitless ability to emulate amps, why would anyone prefer modeling to profiling? It turns out that modeling has some advantages over profiling. For example, a profile is a static snapshot of how the amp settings were adjusted at the time the profile was captured. You can tweak it a bit afterwards, but basically you're relying on the creator of the profile to dial in what they think the sweet spot is for the settings of that amp and the recording technique such as the mic and the placement of that mic. A model on the other hand is a recreation of the amp, controls and all. So the controls of a modeler will respond the same way as the original amp. You want a Bogner emulation that acts like the real thing where the presence control interacts with the gain? That can only be done with a modeler, not a profiler. I'll post a video that demonstrates this below.

Second, a profiler can emulate the sound of any amp in the world. But it's possible to consider a modeler to be even more flexible than that since you can emulate amps that don't exist. Want to experiment with triode plate frequencies or bias excursions that don't exist in any existing tube amps? You can do that with a modeler since it contains models of components that can be adjusted. A profiler is limited to profiling amps that exist in the real world.

AxeFX vs. Kemper

While they use different approaches, both the AxeFX and Kemper aspire to the same goal: accurate emulation of a tube amp. So, which is better? The bottom line of the profiler vs. modeler debate is that both achieve accurate emulation equally well. Neither is inherently a better way to achieve realistic tube tone from a digital emulator. That means you can get excellent tube amp sounds from both the AxeFX and the Kemper. You'll see both in the rigs of many guitarists for whom price is no object. There is no clear winner on the tone issue. They both sound close enough to a real tube amp that there is no way you can tell from a youtube video which sounds better, so be skeptical of anyone who tells you they hear a difference in quality. Trust me, if you hear a difference in a youtube video, that's because they were tweaked differently, not because one is inherently better than the other.

That's not to say there aren't important differences though:

Impulse Responses. I've used the word "amp" to refer to the combination of an amplifier and a speaker cabinet. In practice both the amplifier and the cabinet make significant contributions to tone. The model of a cabinet can be captured in something called an Impulse Response. There is a robust third party market of Impulse Responses to be used with modelers like the AxeFX. You can load an Impulse Response you got from a third party into your AxeFX and pair it with a factory amplifier model. In other words, the range of cabinets you can load into an AxeFX is limitless, in the same way that profiling is limitless. It is possible to load Impulse Responses into a Kemper but it's not the same. Because a profile contains a combination of the amplifier and cabinet, it's impossible to precisely subtract out just the cabinet so it can be replaced by a third party Impulse Response. In practice it works pretty well, but it's just an approximation of true Impulse Response loading.

Other features. So far I've talked about features that directly contribute to the sound, but there are other features to be considered when comparing an AxeFX to a Kemper. It's safe to say effects, signal routing, dual amps, and remote control from pedals are superior in the AxeFX. The AxeFX has USB audio in and out but the Kemper does not, and the AxeFX has an editor app you run on your computer to help with patch editing while the Kemper does not. I would say the Kemper is generally easier to use since that usually just consists of loading a profile. The AxeFX invites more fiddling with controls to dial in a sound and that can be intimidating.

Tone Matching. In addition to Impulse Responses, you can use the Tone Matching feature of the AxeFX to capture the sound of an amp. It's similar to tone matching in Ozone. While it can achieve similar results as profiling, it's not as automatic since you must manually choose an amplifier from the AxeFX before performing the match. It also has an advantage over profiling in that it doesn't require the original amp. You just need a representative sample of a guitar being played through the original amp to match it.

Revisions. Fractal Audio is continually updating the AxeFX. First the hardware. There was the Standard model, then the Ultra, then the AxeFX II, the AxeFX II XL, etc. and multiple generations of each. And the firmware. The firmware is continually getting updates. This can be great. They are always adding new amp models. A release a while back greatly improved the drive pedals. And the modeling is constantly being improved, although in recent years the incremental improvements have diminished as the emulation has already reached a pretty high level. Frequent updates can be a double edged sword though. Firmware revisions are often incompatible, so the sound of your favorite presets can change when you update the firmware. More problematic is: this makes it difficult to maintain a library of presets for users to share. Unless the creator of a preset used the same version of the firmware you're currently using, you may not be able to use that preset. Contrast this with the Kemper where there have been very few changes in five years. It's frustrating for Kemper users to see no significant improvements, but that means you can load any profile ever made into your Kemper and it will sound the way the profile creator intended. This makes for a robust Kemper profile sharing community that has no equal for AxeFX users.

Distribution. You can only buy an AxeFX directly from Fractal Audio. However, you can walk into a retail store near you and buy a Kemper, which is a lot more convenient. This has tended to stifle Fractal Audio's popularity, but this probably helps keep their prices down. When you look at the features, the Kemper should be much less expensive than the AxeFX, but instead there isn't that much of a difference in price.


Even with the success the AxeFX and Kemper have enjoyed in recent years, they are still small players in the guitar processor marketplace. There simply aren't that many guitarists willing to spend the kind of money these devices demand.

About two years ago, Line6, the maker of the Pod which was the breakthrough in modeling all those years ago, introduced the Helix. The Helix is definitely a contender that can compete with the AxeFX and Kemper and should be considered if you're in the market for a high end amp emulator, but it is priced about $1500 for either a floor unit or a rackmount unit, so it comes in under the price of the AxeFX and Kemper. About the same time that was introduced, Fractal Audio came out with the AX8 which has the same basic amp and cabinet sounds as the AxeFX in a floor unit but with less processing power for an even lower price than the Helix. Not to be outdone, Line6 recently introduced the Helix LT for a lower price than the AX8.


Digital emulation is a pretty deep topic, and I don't intend to cover everything here, just the highlights that pertain to comparing an AxeFX to a Kemper. And I've oversimplified in some places for the sake of brevity. In summary, these are all excellent options. There is no loser. You just have to look at the features and prices and make your own judgement about what works best for you. One thing is certain: this is a great time to be a guitarist!

Original thread in our boards