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Aphex Model 204

Aural Exciter By Hugh Robjohns
Published February 2002

Aphex update their famous Aural Exciter and Big Bottom processes for the 21st century.

It seems hard to believe, but the Aphex Aural Exciter has been around for over a quarter of a century! First introduced in 1975, it has become a very common tool in the professional and home studio alike. The first generation Exciter could not be purchased, only hired, and it wasn't until the release of the Type 2 Exciter in 1981 that studios were able to purchase the system.

APHEX MODEL 204Two years later a simplified and cheaper version was developed — the Type B — to broaden the system's appeal to DJs, musicians and the smaller PA companies where budgets were too small to afford the Type 2. The Exciter circuitry has been continually developed and improved over the years with the low‑cost Type C launched in 1985, the professional Type 3 in 1989, and the Type C2 in 1992 — this last revision being quieter, more musical and easier to set up than previous low‑cost versions. Of more interest in the context of this review, the C2 also included the Big Bottom circuit for the first time. The Big Bottom can be thought of as a low‑frequency version of the Aural Exciter which is designed to enhance the bass frequencies in much the same way as the original Exciter enhanced the upper harmonics. (For more details on these processes, see 'The Aphex Technology' box.)

The latest in this surprisingly long list of products is the new dual‑channel Model 204, which incorporates the most recent generation of Aural Exciter circuitry as well as a new version of the bass enhancer, the Optical Big Bottom.

The Once Over

The Model 204 is an elegantly styled 1U rackmount box with integral power supply unit, and flexible I/O connections. It is a relatively shallow unit measuring only 180mm front to back, and it weighs about 2.75kg. The rear panel carries the ubiquitous IEC mains inlet with integral fuse holder and two sets of inputs and outputs for each channel. Electronically balanced XLRs and paralleled TRS quarter‑inch sockets are provided, and a slide switch associated with each channel configures its operating level between ‑10dBV and +4dBu.

The silver front panel is clearly divided into two channels, each with two sets of three silver control knobs, plus illuminated Process In/Out buttons and a Power switch on the left‑hand side. The Big Bottom section of each channel has controls for Tune, Drive (with adjacent LED) and Mix, while the Exciter section has controls labelled Tune, Harmonics and Mix.

Does My Bottom Look Big In This?

Looking at the Big Bottom section in a little more detail, the Tune control is only marked Min and Max (like all of the controls), but actually adjusts the low‑pass turnover between 50Hz and 190Hz, with 110Hz being at the 12 o'clock centre position. Adjusting this control allows the most appropriate range of frequencies to be enhanced by the processing. The Drive control sets the threshold for the dynamics processing and should be adjusted until the adjacent green LED flashes on bass peaks, indicating the onset of processing. Advancing the Drive control increases the amount of processing, and the LED reflects this quite well with its duration and brightness. The Mix control simply determines the amount of enhanced signal which is added back to the original, thereby determining the strength of the effect.

The Aural Exciter controls are equally as simple to understand and operate. The Tune control adjusts a high‑pass filter which controls the range over which enhancement will occur, and it spans 800Hz to 6kHz, with about 3kHz as the centre position. This determines the frequencies above which harmonics will be generated. The Harmonics control affects the quantity, number and nature of harmonics generated. The minimum position is generally considered to be ideal for subtle or complex sources — voices and complete mixes, for example — whereas, higher settings can be beneficial on individual instruments where a greater degree of edge and 'cut‑through' is required. The final control is Mix, which determines the amount of harmonics introduced at the outputs.

The most important controls on the entire machine are the illuminated push buttons which switch the processing in and out of circuit. This is not a hard bypass function — the signal is still routed through the main signal path of the Model 204, only the processing side‑chains are disabled. As with any form of signal processing, it is incredibly easy to fool the ear so that you end up adding more and more enhancement — brighter and louder is always perceived as better. By switching the side‑chain in and out, it is easy to check whether the settings really do provide the required enhancement and extra loudness, or just add confusion and spectral clutter. Personally, I treat the Aural Exciter's enhancement in the same way as reverberation — add enough to be audible and then back it off a few decibels. Too much can be very fatiguing and artificial, with an easily recognisable sonic signature.

Aural Thrill Seeking

The provision of both XLR and quarter‑inch jack connectors makes interfacing the Model 204 very straightforward, as does the ability to switch between ‑10dBV and +4dBu operating levels. The unit accommodates balanced or unbalanced operation quite happily, although there is a special warning note about not shorting the XLR signal pins to ground — common practice is to tie pins one and three together. You are advised to leave the unused 'leg' floating instead. It makes more sense to use the quarter‑inch connections for unbalanced operation anyway, so I'm sure this restriction will not present many problems in practice.

APHEX MODEL 204 rear panel.Model 204 rear panel connections.

The controls are very logical in function and use, and setting the unit up is simply a case of twiddling the controls until the desired sound is achieved. As I mentioned earlier, it is easy to get carried away and create the equivalent of the graphic EQ's 'smiley' curve, with lots of extra bass and sparkle, and this kind of thing becomes wearing extremely quickly. Fortunately, the Process In/Out buttons (sensibly placed to allow both channels to be dropped in and out together) make it easy to get a quick reality check.

Used on complete mixes, the Model 204 is a useful 'polisher'. It can restore life to flat, overprocessed or otherwise lacklustre balances with relative ease, as well as adding a pleasant warmth and fullness or weight to the bottom end when required. With all moderate settings the material can be made to sound substantially louder and fuller without any significant increase in the peak level. However, with extreme settings the level can rise quite dramatically, and all that extra LF energy might quickly cause a parting of the ways between cone and voice coil in less capable monitors!

The Exciter is much the same as the previous generations, albeit a tad quieter and smoother. Cranking the Harmonics control up too far produces a gritty, harsh sound — useful in some situations but certainly not all. During the review period I found the Harmonics control generally stayed below the halfway position, whether processing individual instruments or complete mixes. I also found the setting of the Tune control was critical in obtaining a natural and fatigue‑free sound.

The Big Bottom facility also needs to be used with great care, as the higher Drive settings introduce a substantial amount of LF compression, bringing subsonic rubbish into transparent audibility. If you are mixing on small monitors you have to be especially careful as, depending on the Tune setting, the lift may be confined to frequencies below the effective range of the speakers.

By far the most creative use of the Model 204 is on individual sources during mixing and, to a lesser extent, tracking. Although there is no substitute for careful microphone selection and placement, this Aphex unit can usefully repair the sound of badly miked instruments, and even enhance the sound of poor‑quality instruments! It's not just for fixing problems though, it is also a creative tool in its own right.

I found it worked well on a very wide range of sources — wind and brass instruments could be given a degree of bite to help cut through the mix without increasing their levels, female backing voices could be given a breathy quality which often worked well, and male voices could be made to sound much richer and fuller. Drums and percussion could also be brought to life. However, where processing instruments while recording is necessary, caution should be exercised as the effect can't easily be undone.

A throw‑away comment in the handbook made me try the Model 204 out on data reduced material and I found the unit extraordinarily effective in breathing fresh life into MP3 files and other heavily data‑reduced recordings. More usefully though, processing tracks through the Model 204 prior to encoding seemed to help them survive data reduction rather better. By adding a modest amount of harmonic enhancement before encoding, the material seemed to replay with a greater sense of space, stereo imaging and detail, instead of the two‑dimensional sound characteristic of the format. It doesn't restore uncompressed WAV quality, but it certainly improves matters.

All in all, this is an easy‑to‑use sonic tool which is extremely effective, restorative, and creative if used appropriately.

The Aphex Technology

Although the circuitry has evolved considerably since its first release, the basic concepts behind the Aural Exciter have remained the same throughout the product's long life and various incarnations. The basic idea is to recreate or restore high‑frequency harmonics in musical programme material — harmonics that are often lost as delicate signals pass through the various elements of the recording process. If these harmonics are lost or reduced in amplitude, the music will tend to sound dull and lifeless, lacking in clarity and detail. Unlike simple EQ, the Exciter doesn't change the balance of existing high‑frequency components, but simply recreates and then reintroduces the missing harmonics at a level determined by the operator.

The Exciter was first developed to combat the inherent losses of analogue recording and replay (and the attendant noise‑reduction systems) which were the major source of signal degradation in the '70s and '80s. The widespread adoption of digital recording over the last fifteen decades has improved the preservation of high‑frequency detail enormously, but the Aural Exciter remains a popular artistic tool, even if its corrective abilities are of less importance today. Having said that, one of the more common side effects of heavy data reduction is a loss of high‑frequency detail, and the Aural Exciter can be used very effectively to restore some of the 'life' to material recorded on, or transmitted via, data‑reduced systems.

The Aural Exciter uses various patented techniques to regenerate harmonics, at a level which is related to the level of the music. Along the way, the early designs have been refined significantly, and the current model uses a Transient Discriminate Harmonics Generator (TDHG) to create harmonics based on the spectral structure of signal transients, rather than any steady‑state components. The result is a far more predictable and natural‑sounding harmonic enhancement which is effective over a greater dynamic range than was achievable before.

The system works by splitting the input signal in two, one part being passed directly to the output while the other is processed in a side‑chain. This side‑chain comprises an adjustable high‑pass filter (to determine the range of frequencies from which harmonics are generated), and the TDHG circuit. The added harmonic elements are at a relatively low amplitude compared to the overall level of the music, and so do not increase the peak level significantly when mixed back in to the original signal at the output of the unit. However, these harmonics are perceived to add dramatically to the mid‑range and high‑frequency energy, as well as increasing the intelligibility and clarity of individual instruments and voices.

The Big Bottom system is superficially similar in approach, with a main signal path and a side‑chain. The side‑chain processing consists of an adjustable low‑pass filter followed by a phase‑shifting circuit and a dynamics processor. Essentially, the selected low‑frequency region is compressed and added back to the original, extending the resonance and sustain. However, because this side‑chain signal is phase‑shifted relative to the original, the addition does not result in a significant increase in peak level, but does increase the perceived energy and make the spectral balance seem deeper and more substantial. The latest development to this circuit has been the inclusion of an opto‑resistive device within the dynamics processor which has a temporal characteristic ideally suited to the compression of low frequencies.


  • Easy to use.
  • Works well on individual tracks and complete mixes.
  • The HF and LF processes complement each other well.
  • Has both corrective and creative possibilities.
  • Process In/Out switches allow instant reality check.


  • Can produce a lot of subsonic energy.
  • Fatiguing if pushed too hard.


The latest in a long line of Aural Exciters, the Model 204 benefits from all the incremental developments introduced by Aphex over the past 25 years. The inclusion of the Optical Big Bottom process extends the usefulness and creativity of the unit substantially.


£348 including VAT.