Hof Hofex Spectral Exciter II

A good psychoacoustic enhancer can take your sound higher than you thought possible, and though there are plenty of enhancers on the market, this new Austrian model proves that there's always room at the top. Paul White tries to control his excitement...

People are always asking me what's new and exciting, so I'm pleased to be able to tell you, hand‑on‑heart, that the HÖF Höfex Spectral Exciter II qualifies on both counts — it's new, and it's an exciter. The market is awash with budget enhancers, but the problem with some of the cheaper models is that, while they add top end and clarity, they can also make your mixes sound harsh or fatiguing. Additionally, they can enhance your background noise just as well as your music, which means that the hiss level goes up. HÖF are one of those companies that appear to look at the very best of what's already on the market, then try to do the job better: the aim of the Höfex is to produce a subjectively cleaner, more open sound, with improved stereo spread, while at the same time keeping undesirable artifacts such as noise and harshness to a minimum. In common with some other good enhancers, the Höfex also claims to be able to increase the subjective loudness of a piece of audio without significantly increasing its actual peak level.

Physically, the twin‑channel Höfex is a smart and stylish 1U‑high rackmount processor, featuring just four control knobs per channel and three buttons in the centre of the front panel. The inputs and outputs are on servo‑balanced XLR connectors and all the circuitry is analogue. (Incidentally, although HÖF have chosen to call their unit a Spectral Exciter, it's not related to the Aphex Aural Exciter, which is a registered trademark.)

Modus Operandi

Simple enhancers only treat the very high end of the spectrum, but over the past couple of years, it's become more common to see some sort of bass enhancement or bass EQ added, to help balance the overall sound. HÖF have built a system that incorporates four stages, termed: Glitter; HF Exciter with Spectrum; LF Exciter; and REX Room Excitement. As with most enhancers, the processing takes place in a side‑chain, after which the processed signal is added back to the original signal.

Glitter, as the name seems to imply, affects the top end of the spectrum. There is no real explanation of what this stage does — it may be a form of regular EQ somewhere between 12 and 16kHz, dynamic EQ or harmonic resynthesis — but whatever it is, it adds a subjective 'openness' to the high end.

The HF Exciter Processor apparently introduces phase shift and is described as generating "overtones", though it's not clear how these differ from harmonics. A high‑pass filter means that the process can be set to affect only signals above a specific frequency, variable from 500Hz to 8kHz.

To enhance the bass end there's the LF Exciter, which affects both the low bass and the lower mid‑range. HÖF are keen to keep their secrets to themselves, but I get the impression that dynamic equalisation plays a part in this process.

Finally, there's REX, a single button that brings in a stereo width enhancer and lights up a yellow LED. Again, I don't have any hard facts about how this process works, but as it claims full mono compatibility I can only assume that whatever is added is added equally, and in opposite phase, to both channels. Such explanations as there are in the manual relate to psychoacoustic principles — I found mentions of overtone enhancement, formant emphasis, phase shift, simulation of natural reflections, expansion of the impulse field, and an increased pressure due to sub‑frequency enhancement.

Controls

Though the internal workings of the Spectral Exciter are shrouded in secrecy, the control topography is relatively simple.

Spectrum is a high‑pass filter that determines which part of the spectrum will be fed to the HF Exciter overtone generator. It is continuously variable from 500Hz to 8kHz, but the most useful settings seem to be in the middle of the range. At high frequency settings the process is applied only to the upper regions of the audio spectrum, and so may not sound very obvious, but bringing down the frequency causes more of the spectrum to be processed. With some types of material, enhancing too far down the spectrum can start to produce an unpleasant sound (due to intermodulation) so it's normal to use very low Spectrum settings only for processing percussive sounds, for example, rather than complete mixes. The HF Exciter control then determines how much of the synthetic overtone signal is mixed back in with the original.

The LF Exciter increases low‑frequency energy, but without causing any significant increase in peak level, so I'm guessing that the process includes frequency‑selective compression. Turning the control clockwise adds more of the treated signal to the original, and the result is a more solid, deeper bass end.

REX is a simple 'on or off' effect for enhancing the 'width' of existing stereo material, so it can only be used when the Höfex is processing a stereo signal. Because the REX level is fixed, the only way to balance it with the enhancer's other effects is to use the level controls on the other sections. Listening in mono cancels out the REX signal without compromising the original audio.

The Bypass control isn't a hard bypass, but simply mutes the effect side‑chain, leaving the high‑quality balanced input and output amplifiers in circuit. A further refinement is the Effect Solo/Program switch, which mutes only the original signal, leaving just the effect side‑chain's contribution. This switch is useful for checking just what is being added to your signal, but it can also be used to kill the dry signal when you want to use the Höfex in a mixer's aux send/return loop. Normally, enhancers can only be connected via insert points, but this particular model can be used as a conventional effect. Of course, you need to pay attention to the pan positions of both the original sound and the excited sound when working in this way, and the processing delays inherent in digital mixers mean that the 'aux send' way of working is unlikely to achieve good results with such desks.

If you're familiar with the way in which many enhancers work, you may by now be wondering why there's no Drive control on the Spectral Exciter: HÖF have instead built in an ALC (Automatic Level Control) circuit that affects just the side‑chain input, so that the user has one less control to worry about. However, it's still possible to overload the circuitry if you really try, so overload LEDs are included for each channel.

The Sound?

As with any enhancer, the trick is knowing where to use the Spectral Exciter, when to use it, and — most importantly — when you're using too much of it. This particular device has a sensibly limited processing range so that, although you can still over‑use it, you can't take the process too much further than originally intended, as some boxes allow you to do. Being able to mute the dry signal enables you to solo the side‑chain's contribution, and listening to that in isolation tells you a lot about how the unit behaves.

As enhancers go, this is nowhere near the cheapest, but it's certainly one of the sweetest‑sounding and most intuitive models around.

Listening to the Glitter signal in isolation confirms my impression that it's rather like the 'air' EQ found on some units. The sonic impression is of a wide, low‑level boost centred between 12 and 16kHz, and it doesn't sound as though any distortion mechanism is being used to create extra harmonics, because there are none of the harsh, intermodulation products that you tend to hear when you solo a harmonic enhancer. I still can't tell if there are any dynamic elements to it, but this is a very nice control for emphasising frequencies that already exist in the mix, without making the sound too sharp.

Moving to the HF Exciter, this sounds to me more like a conventional harmonic generation system — if you lower the Spectrum frequency and turn up the level, you can definitely hear that characteristic harsh edge. Once the original signal has been added and the controls set to more moderate levels, the effect is much sweeter; this is the control to use when you want to build a top end for a signal that has very little going on in the top octave. For example, a dull‑sounding guitar or electric piano can be given a musically plausible top end that may never have existed in the original sound (if you tried to do this using EQ, there would be nothing there to boost). The HF Exciter section also works for adding breathiness to vocals recorded with a dynamic mic, though it's usually best to use HF Excite and Glitter in combination.

The LF Excite parameter works exceptionally well, adding weight and authority to the bass end without messing up the lower mid‑range. If you pile on a lot of effect, the output level does increase by a few dB, but you can make a significant difference to the perceived bass loudness without affecting the peak output level by more than a dB or so. Activating the REX control causes a noticeable widening of the stereo image, and the whole mix seems to move closer. The effect may simply be created by adding anti‑phase signals to the opposite channels, but it certainly works. However, I'd have preferred this to be adjustable rather than just on or off.

Summary

I've tried all types of enhancers over the last few years. Only a small number of models really stand out for me, and the Höfex definitely deserves to be numbered amongst them. While there are slightly more flexible enhancers around, the Höfex has the advantage of a beautifully simple and intuitive control system, plus a classy, musical sound. I also really like the fact that you need only three knobs to mix in the three types of enhancement. As enhancers go, this is nowhere near the cheapest, but it's certainly one of the sweetest sounding and most intuitive models around.