Aphex Model 204 Aural Exciter

The original Aphex Aural Exciter, first offered in 1975, came without the Big Bottom circuit, which was added in 1992. Later revisions of the Aphex Aural

The original Aphex Aural Exciter, first offered in 1975, came without the Big Bottom circuit, which was added in 1992. Later revisions of the Aphex Aural Exciter included the Model 104 Type C and Type C2 units. Aphex has now released the Model 204 Aural Exciter and Optical Big Bottom, yet another refinement of the original unit.

For those not familiar with these proprietary Aphex processes, the Aural Exciter enhances clarity and intelligibility by adding phase shift and musically related synthesized harmonics to audio signals. The Big Bottom circuit combines a lowpass filter and dynamics processor to compress and delay incoming low-frequency information. The process is reverse amplitude dependent, meaning that more is applied as the input level drops and less as the signal gets hotter. Together the dynamics processor and time delay create sustained bass frequencies that are perceived as being louder yet don't noticeably increase peak output. According to Aphex, the sleek Model 204 updates the Aural Exciter and Big Bottom processor blocks with improved circuitry, including an optical gain-control element for the Big Bottom compressor.


Like its most recent predecessor, the single-rackspace Aphex Model 204 provides two identical channels, each with independent controls for its two processing sections. Understand that the Big Bottom and Aural Exciter circuits are wired in parallel to the dry audio signal, thus preserving the phase, spectral content, and dynamics of the original signal. The processed signals are summed with the dry signal at the 204's output jacks.

Three continuously variable rotary knobs — labeled Drive, Tune, and Mix — control the Big Bottom process per channel. Drive adjusts the amount of signal sent to the optical compressor, and an associated LED lights to alert you when the compressor's threshold is exceeded. Tune sets the corner frequency, from 49 to 190 Hz, for a lowpass filter that lies just before the compressor in the Big Bottom's circuit path. The user controls the range of the bass frequencies that the Big Bottom compresses by tweaking the Tune knob. The Mix knob determines how much of the Big Bottom's compressed bass-band signal is added back in with the dry signal.

Three other continuously variable rotary knobs — labeled Tune, Harmonics, and Mix — control the 204's Aural Exciter circuitry per channel. Tune sets the corner frequency for a highpass filter that ranges from 800 Hz to 6 kHz. All frequencies higher than the Tune knob's setting are patched through to a harmonics generator. The Harmonics knob, which controls the number of harmonics added to the highpass filter's output signal, adds higher partials as you turn the knob clockwise. The Mix knob adjusts how much of this harmonics-rich signal is added back in with the dry signal.

Two backlit Process In/Out switches allow for global bypass on each channel; rather than provide shunts between the input and output jacks, these simply defeat the processing for each channel so only the original signal flows through the box. Although you don't get dedicated bypass switches for the separate processes, which would be helpful for A/B comparisons, you can effectively bypass the Aural Exciter or Optical Big Bottom circuit on either channel simply by setting the processor's Mix knob hard left (off). The 204's power switch and associated LED are conveniently located on the front panel.

The 204's rear panel provides XLR and ¼-inch TRS input and output jacks for each channel (see Fig. 1). Although both types of jacks are balanced, they also accept unbalanced lines (albeit with the concomitant 6 dB loss in signal level). You can connect the 204 to your system through mixer insert jacks or by patching the unit in series with other gear.

Each channel sports a rear-panel switch that lets you select between -10 dBV and +4 dBu operating levels. The Model 204 also boasts an internal power supply and a detachable, three-prong AC cord, which connects to the IEC power receptacle on the rear panel.


Learning to use the 204 is easy, thanks in part to the clearly written and comprehensive owner's manual. You need the manual to fully grok the 204's controls. It illustrates, for example, Tune-knob settings (for the Aural Exciter and Big Bottom functions) corresponding to seven corner frequencies. The Tune knobs on the unit, however, are marked Min and Max at the extremes with hash marks between. Clearly, the controls are meant to be set by ear, but frequency-specific marks would be helpful.

Some may wish for input- and output-level controls, but those aren't necessary on the 204 and would add degrading amplification stages to a pristine signal path. The Big Bottom's Drive control provides enough range to trigger the optical compressor, even with fairly weak input signals. If you need more gain sent to the 204, connect the unit to your mixer's insert jacks and boost the mixer channel's trim controls. Moreover, because the 204's processing causes little or no increase in signal level, at least with moderate Mix-knob settings, output level controls are largely unnecessary.

One feature I missed was a stereo-linking function, which would have allowed more accurate processing of stereo program material. But that is a minor lament — matching channel settings is simple enough, and the knob positions are easy to see, even in low light.


Using the Big Bottom and Aural Exciter processes made it a cinch to dial in a beefy bottom and snappy click to kick-drum tracks. I did, however, wish the Big Bottom's Tune control could be adjusted to a setting below 49 Hz — often on kick I want to boost only the low lows, around 30 Hz, without affecting frequencies above, say, 60 Hz.

The Aural Exciter also worked wonders on a snare-drum track. Although I had miked the drum's top head only, after processing with the Aural Exciter, it sounded like I had also used a bottom-head mic — the processed sound had much more sizzle and punch. The sole drawback was that the process, no matter how I tuned it, also hyped the hi-hat bleed in the snare mic, making the hats sound a bit harsh and too cutting. That is a good example of a case in which a bandpass filter for the Tune function would have allowed better results.

The Aural Exciter added clarity and restored nuance to electric-bass-guitar tracks. By cranking the Big Bottom's Drive control and turning down the Mix knob, I achieved increased sustain and fairly consistent levels on the track. It wasn't as good as what can be had using a conventional split-band compressor or even a broadband compressor such as the Aphex Expressor, but it was usable nonetheless. Once again, though, I wished that I could lower the Big Bottom's corner frequency to add thunder without any hint of boominess.

The Aural Exciter also lent clarity to and increased the intelligibility of vocal tracks and added wonderful sparkle to acoustic-guitar tracks. But it was on drum-overhead mics that the Aural Exciter sounded truly phenomenal. The cymbals grew richly complex from the added harmonics, and a touch of the Big Bottom gave some nice room tone to the kick drum. Drum overheads turned out to be my favorite application for the 204.

As a stereo-mix processor strapped across my Yamaha 02R's stereo-bus outputs, the 204 gave added punch and zing to 2-track mixes. The unit had no trouble handling the +26 dBu levels the 02R's analog outputs dished out. In this application, it was usually best to tune the Aural Exciter's highpass filter to its highest setting and the Big Bottom's lowpass to its lowest frequency. Still, it wasn't always possible to get the effect I wanted without also affecting frequencies at the far ends of the spectrum that I preferred to leave untouched.

For another test, I used the 204 to enhance an old 16-bit mix that was lacking in extreme highs and deep lows. After setting the controls pretty much as described in the aforementioned stereo-bus application, I was amazed by how punchy and airy the processed recording sounded. This was no substitute for a great mix, but the experiment clearly demonstrated the 204's usefulness for remastering applications.

The 204 was useful in preprocessing stereo mixes before printing them to cassettes. You're probably asking, “Who needs those anymore?” But recently, a major producer (whom, for privacy, I won't name) requested that demos be submitted to him on “old-fashioned cassette tapes.” Compared with unprocessed cassette dubs, those preprocessed with the 204 exhibited improved clarity, intelligibility, and punch, as though a cloudy veil had been lifted from the sound.


The Aphex Model 204 Aural Exciter and Optical Big Bottom is a unique, useful, affordable processor. For most applications, I was more impressed with the Aural Exciter function than with the Big Bottom. The Aural Exciter does what no other processor can. Especially on percussive (plucked or struck) instruments that produce inherently complex overtones, the Aural Exciter adds an airiness and richness that's hard to beat.

The Big Bottom, too, can yield great results in some applications — most notably processing drum overheads and remastering recordings that are deficient in bass frequencies — but in general, I prefer the flexibility afforded by a quality split-band compressor or an equalizer and a compressor chained in series. Then again, that signal chain costs considerably more than the 204.

My main reservation about the Model 204 is its lack of precision control: rather than lowpass and highpass filters, bandpass controls or external sidechain inputs would have allowed more exacting control over what frequencies get processed by the Big Bottom and Aural Exciter blocks. But again, such features would add considerable expense to the modestly priced 204.

One last caveat: the 204's wide-ranging controls make it easy to overprocess signals, resulting in boomy lows and harsh highs. Used judiciously, though, the Model 204 can greatly enhance individual tracks and full mixes. The 204 isn't just a valuable tool; at under $400 for two channels, it's a bargain.

Michael Cooperis anEMcontributing editor and owner of Michael Cooper Recording, located in beautiful Sisters, Oregon.


Model 204
spectral enhancer


PROS: Excellent for percussive instruments, remastering applications, and preconditioning cassette mixes. Affordable. Easy to use. Plug-and-play operation with balanced or unbalanced lines. Operating levels independently switchable per channel. High bandwidth and headroom.

CONS: Processes lack independent bypasses. Corner frequencies not indicated on Tune controls. No sidechain inputs. No stereo-link function. High- and lowpass filters can force compromised settings that affect frequencies adjacent to intended area of treatment.


Aphex Systems, Ltd.
tel. (818) 767-2929
e-mail sales@aphex.com
Web www.aphex.com

Model 204 Specifications Inputs(2) transformerless, active-balanced XLR; (2) transformerless, active-balanced ¼" TRSOutputs(2) transformerless, active-balanced XLR; (2) transformerless, active-balanced ¼" TRSMaximum Input Level+27 dBu (+4 dBu operating level); +12.5 dBV (-10 dBV operating level)Maximum Output Level+25 dBu (+4 dBu operating level); +12.5 dBV (-10 dBV operating level)Operating Level+4 dBu or -10 dBV (switchable per channel)Frequency Response10 Hz-38 kHz (+0.5 dB)Dynamic Range120 dBHum and Noise-93 dBu (unweighted, 22 Hz-22 kHz)Crosstalk-79 dB (10 Hz-22 kHz)Total Harmonic Distortion0.0003% (10 Hz-22 kHz @ max. output)Dimensions1U × 8.25" (D)Weight6 lb.