Review: Sound On Sound Jan '/99
By Paul White

KILLER QURE
SPL Qure Parametric Equaliser

Paul White gets down to some serious equalising with SPL's most Qurious product to date.

No product from SPL is quite what it seems, so I wasn't in the least bit surprised to find that their new parametric equaliser includes a mysterious extra control called Qure. What's more, I was even less surprised when I read the handbook description of how it worked (twice) and came away little the wiser.

I can tell you that the Qure is a 2U deep, two-channel, hybrid tube analogue equaliser designed for applications such as mastering, though it also makes a fine general-purpose studio equaliser. With a choice of both balanced jack and XLR inputs and outputs, each channel comprises three bands of fully parametric EQ, variable-frequency shelving high- and low-pass filters, plus the enigmatic Qure control. Each EQ band, the high/low shelving filters and Qure can be switched in or out of circuit independently, while Qure may also be switched between two frequency ranges. With its gold front panel, mesh valve windows and stylish design, the Qure looks great, but how does it differ from the competition?

Background
As any experienced studio engineer will tell you, no two equalisers sound quite the same, and SPL claim to have done a lot of research on what exactly constitutes a good-sounding design. Their parametric filters are based around a refinement of the well-known state-variable filter, and have a proportional bandwidth characteristic rather than the more common constant Q. Proportional Q circuitry reduces the amplitude of a signal as the bandwidth is widened so as to maintain a more even perceived level during adjustment.

Though the Qure unit is by no means an all-valve design, a 12AX7 tube stage is used in the mid-range parametric filter with the aim of preserving detail and warmth, while at the high end, LC filters (those employing coils as well as capacitors) are used to preserve transparency. At the low end, solid-state circuitry is used, ostensibly to keep the bass frequencies solid and stable. This takes the valve versus solid-state argument to a new level as SPL are, in effect, saying that solid-state circuitry works best at the low end while valves work best in the mid range.

It seems the designers have gone to enormous lengths to provide a clean signal path, including the use of single op-amps rather than doubles or quads. There's also a rigorous grounding and screening system that places each filter section on its own circuit board, and a lot of effort has gone into providing a clean power supply, including a separate rail for the bypass relays. The input stage is based around SPL's own 'Super-Balancing' hybrids for maximum common-mode interference rejection. As standard, the input and output stages are transformerless, but Lundahl input and output transformers may be specified as an option. The noise performance is stated as -99dB (A-weighted) with a dynamic range of 113dB.

The EQ Controls
Each channel of the Qure is equipped with both input and output level controls, though there seems to be no form of level metering. Perhaps this isn't necessary, as even when applying quite a lot of EQ to a hot signal, I didn't notice any signs of clipping, but metering is still useful when optimising gain structures. Together, the parametric filters cover the range 15Hz to 21kHz with a reasonable degree of overlap and the manual explains that these filters, including their control law, take into account the way sound is perceived by humans rather than relying entirely on textbook calculations. This is a part of SPL's general design philosophy and one that has certainly helped their previous designs sound different to the competition.

The mid-band is designed with a 'non-reciprocal' response, which means the filter has different characteristics in the boost position to the cut position. This arrangement allows the mid control to function as a notch control in cut mode (high Q setting), providing up to 36dB of cut, compared with the 15dB cut range of the high and low sections. The available amount of boost is 15dB in all bands.

The two shelving filters have similar characteristics to those used in a gate's side chain, and because they have variable frequencies, they're very effective in 'bracketing' a section of the spectrum. The HF Cut filter is designed with a second order (12dB/octave) Butterworth response and may be adjusted from 2.5kHz to 20kHz. The LF Cut filter has a similar response and may be tuned from 40Hz to 200Hz. Such filters are particularly useful for constraining sounds that would otherwise spill into adjacent parts of the audio spectrum, and also for removing HF harshness from individual sounds without affecting the overall tonal balance too severely.

Behind the enigmatic Qure control is a new SPL process that uses an inductor-based circuit to manipulate and enhance the upper harmonics of the material being treated. A 12AX7 valve stage provides the necessary valve coloration, and engaging the Qure switch places an inductor/capacitor/resistor network in the valve's output path. The outcome is described in fairly loose terms, but it seems the effect is dynamic (changes with the intensity of the input signal), and some modification of the harmonic structure of the sound takes place. The aim is to create a silkier top end that helps 'cure' the harshness of digital sound, while giving a better sense of presence to vocals. In addition to the Qure On switch, there's also a Shift button that switches its focus from the 2-6kHz part of the spectrum to the 440Hz-2kHz section.

The parametric equaliser has a separate hard bypass switch for each of the three bands, and there's a further bypass switch for the HF/LF shelving filters. All the rotary controls have a stepped feel to them, and a Master control switches the whole EQ in and out of circuit using a hard bypass relay. If the power fails or there's a PSU fault, the relay automatically bypasses the Qure.

In Use
After switching the Qure on, there's a wait of around 45 seconds as the PSU slowly builds up the valve voltage so as to reduce valve wear -- the manual suggests that this approach can extend the valve's working life by up to four times. After 45 seconds, the Warm Up LED changes to Ready and the unit is ready to use.

The front panel is clearly laid out with sensibly-sized knobs, though the legending is fairly small and may be difficult to see in some situations. However, this isn't too much of a problem as it soon becomes apparent where everything is. Even with no EQ (but with the Master EQ button set to On), there's a very subtle change in perceived tonality because the signal passes through the valve output stage. I found it easiest to engage the regular parametric bands first and then bring in the Qure control last, though switching in Qure does bring about a small level increase that can make it more difficult to judge its true effect.

Tonally, the Qure is a smooth-sounding equaliser that's very forgiving, even when you need to apply a lot of EQ. The bass end remains firm while the high control doesn't generate the harshness of some cheaper designs. Having the variable shelving filters is also a real help, and in many instances you can get the results you need using just these on their own -- but switch in the three parametric bands and you can tackle almost anything.

The Qure control is the icing on the cake and I suspect it evolved from some of the principles used in SPL's Vitalizer enhancer. It's clearly some form of enhancer, and reading the manual confirms that it juggles frequency response, phase and harmonic distortion in order to do what it does. As enhancers go, it's very smooth-sounding, yet able to add air and separation to a mix at the turn of a single control. It is particularly useful for bringing vocal tracks to the front of a mix, but it's also good for overall mix sweetening. Like the Vitalizer, it seems to have a clarifying effect on the mid range, not just the top end.

Ergonomically, the panel layout is straightforward, though I found it slightly odd having the cut/boost controls on the bottom row. As these are the controls most often accessed, I felt they might have been easier to manage on the top row.

Summary
It seems so logical to combine EQ and enhancement that I'm surprised it isn't done more often. In the Qure, you have all the tonal shaping tools you need in an easy-to-use form, and I'm pleased to see that the immensely useful tunable shelving filters have been included. The mix of technologies used to optimise the individual EQ bands gives this equaliser a classy, smooth sound, but that's not to say it can't assert itself when a sound really needs pulling around to make it fit. Such sophistication doesn't come cheap, but a good equaliser is a welcome ally, whether you're mixing or mastering.

One claim which SPL make for the Qure is that it can help take the harsh digital edge off sound. I don't really want to get into the 'analogue versus digital' argument here, but it's true that some budget digital gear can sound a little hard and unyielding. I tried the Qure in this capacity with an album I was mastering, where most of the recording and mixing stages had been digital. By using the EQ section to pull down the more strident presence frequencies a little, adding the merest hint of low boost, then bringing in a touch of Qure, the mix was noticeably sweetened without making it sound processed.

For the benefit of those who not only skip straight to the conclusion section of a review, but also to the last line of the conclusion, the Qure combines the best elements of a first-class equaliser with the enhancement qualities of SPL's Vitalizer.

Find out more about the SPL Qure