You are here

SPL Optimizer

Many musicians are familiar with the SPL Vitalizer. Now the company hope to win a space in the racks of studios world‑wide with their new Optimizer. Dave Lockwood finds some unique features behind the company's unconventional approach to equaliser design.

SPL rose from relative obscurity to cult status with their Vitalizer, and although that unit behaves in some ways rather like an enhancer, the secret of its sound is as much tied up with its designers' philosophy on equalisation as with conventional aural enhancer techniques. It is too early to tell whether the Optimizer will take off in the same way, but like the Vitalizer, it has a very engaging sound, can be used to good effect on most types of material, and has just enough features to arouse your curiosity.

The Vitalizer was billed as a 'make everything sound better' box, but the Optimizer is presented as a specialised parametric with added value features. The circuitry is based around a type of state‑variable filter, so that in addition to the familiar parametric functions, it can also provide band‑pass, high‑pass, low‑pass and notch filtering modes. The parametric mode is the one most likely to be used for general equalisation, while the other modes are available for more creative effects. The notch filter is also useful for taking out troublesome frequencies such as mains hum.

According to the designers, the Optimizer relies heavily on psychoacoustic principles — or how the human hearing system perceives sound. SPL claim that, in addition to its subjective tonal attributes, the Optimizer has a noise level approaching the theoretical minimum for analogue circuit design — and it's worth remembering that the performance of analogue circuitry in this respect is still considerably better than that achieved by 16‑bit linear digital systems.

Few things about SPL products are conventional, and even though the filters are based on the established state‑variable principle, they incorporate proprietary feedback systems to control the circuit's phase response. Furthermore, instead of going for a constant Q equaliser design, SPL have gone for non‑constant (or proportional) Q filters, in the belief that these sound more musical. One practical advantage of the proportional Q design is that the signal level doesn't change significantly when the Q setting is altered.


Housed in a 2U rackmount case, the Optimizer is based around four identical filter stages, which may be set for 4‑band mono, dual 2‑band mono, or 2‑band stereo operation via a front panel switch. Both electronically‑balanced XLR and unbalanced jacks are provided for signal connections. The operating level may be switched from +4dBu to ‑10dBv to maintain optimum headroom and noise performance in both professional and semi‑pro environments. There is also a ground lift switch to help avoid ground loop problems. A peak LED is fitted to each EQ section, and flashes at approximately 3dB below clipping. An output gain control is used to compensate for any gain change resulting from the EQ cut/boost settings.

The four filters are all identical, and can be set anywhere between 10Hz and 23kHz in four switched frequency ranges. This allows finer control than would be possible if a single knob was used to cover the entire spectrum. The bandwidth of the circuitry extends much higher than the top filter frequency, and the designer obviously subscribes to the Rupert Neve/Malcolm Toft school of thought, which suggests that an audio bandwidth in excess of 40kHz is necessary. Although nobody is quite sure why this should make things better, some people think that it helps minimise undesirable phase shifts, which, even when relatively small, can affect the subjective sound quality of an audio device.

In Quad mode, all four filters are connected in series, although the outputs from each pair of filters still remain individually accessible, which may be useful on the odd occasion when you're trying to create special effects requiring two differently EQ'd versions of the same signal.

The Filters

Each of the filters can be switched to parametric, notch, band‑pass, high‑pass or low‑pass operation, with a choice of roll‑off characteristic in all modes other than Parametric. In the Notch filter mode, where the notch depth is preset at 60dB, the user has control over frequency, bandwidth, and the steepness of the sides of the notch.

As you might expect from an analogue processor, the majority of the controls each have their own dedicated function, although the parametric Cut/Boost control becomes a Gentle/Steep control in the notch, band‑pass, high‑pass and low‑pass modes. If this is set fully anticlockwise, a gentle roll‑off is created; if you turn the control fully clockwise, a more steep‑sided response results. The subjective difference between choosing a gentle or steep filter characteristic is quite noticeable; Steep sounds very positive and business‑like, while Gentle, as its name implies, is far more subtle and very musically flattering. Because of the way the equaliser circuitry is designed, setting the control to its centre position in Steep/Gentle mode results in no signal at all, presumably because complete cancellation is taking place, but there are some useful settings to be had between the centre position and the two extremes, as long as you are prepared to put up with the gain reducing as you approach the centre 'null' position.

In Use

When used as a conventional parametric equaliser, the Optimizer's performance is much as you would expect of a quality equaliser, providing an adequate cut/boost range of plus or minus 12dB. Bass sounds can be lifted or given punch without clouding the image or making the mid‑range muddy, and the mids and highs can be sharpened up without causing harshness. The words sweet and musical come to mind, but with an underlying impression of solidarity and power.

When used for stereo processing (in Dual mode), equalisers 1 and 2 work as a pair, as do equalisers 3 and 4, whilst in mono, all four equaliser sections are linked in series. Consequently, the Optimizer only provides two EQ bands per channel when used in stereo, although this is generally enough, especially on complete mixes that just need a bit of sweetening to finish them off. Indeed, so effective is the EQ in the upper reaches of the spectrum that it is able to apparently lift out detail in much the same way as an aural enhancer, but without adding noise or artificial harmonics to the signal. It is almost as though it is able to pull the mix into better focus, even when only tiny amounts of EQ are used.


Although expensive for a 4‑band equaliser, the Optimizer is a unique and versatile product, with a very attractive sound character. It is suitable for routine equalisation, post‑production sweetening and the creation of special effects, and although the alternative filter modes are slightly quirky in operation, the unit is, by and large, easy to set up. My only real criticism is that the overall bypass control does not work when the unit is unbalanced, although from a technical viewpoint, I can appreciate that it was difficult to arrange it otherwise.

My overall impression of the Optimizer is that it does offer something that isn't provided by the competition, and so, like the Vitalizer, it will find its own niche in the marketplace. It would be a mistake to dismiss the Optimizer as 'just another equalizer', as the range of its EQ parameters prevent it from suffering from the 'single good sound' limitation of some of the established 'classic' EQs. Rather, it should be regarded as a useful additional equalisation tool, with a high degree of reach and focus, allied to precision and flexibility.

Special Effects

Each of the Optimizer's filter sections can be set up independently and tuned anywhere in the audio spectrum, making it possible to construct unusual filter combinations that conventional EQs can't even approximate. Using these additional filter modes, it is possible to create very obviously filtered sounds, and some of these are useful in processing electronic keyboards, or for producing 'far‑away', vocal sounds. The notches may also be used to create artificial stereo effects from a mono input, by simply treating the right and left channels with notches set at different frequencies. However, be aware that this isn't always mono compatible.

Brief Specification

  • CCMR: ‑77dB
  • NOISE (A Wtd): ‑93dB (EQ in, set flat)
  • FILTER MODES: Parametric, Notch, Band‑Pass, High‑Pass, Low‑Pass
  • Q: 1.5 to 0.2kHz
  • FREQUENCY RANGE: 10Hz to 2.4kHz, 16Hz to 3.3kHz, 34Hz to 7.1kHz and 112Hz to 23kHz
  • NOTCH: Provides more than 60dB of attenuation. Q varies the notch bandwidth and Roll‑Off sets a steep or gentle roll‑off characteristic
  • OUTPUT GAIN: ‑80dB to +5dB


  • Very flexible — not limited to a single sound character.
  • Wild enough to be used for special EQ effects (many of which are impossible using a standard EQ).


  • Slightly quirky control system (may not be entirely intuitive).
  • Relatively high price.


Provided that you can afford it, this is the box to buy if you want an EQ that will do almost anything.