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SPL EQ Magix

Mic Preamp/Equaliser By Paul White
Published May 1995

SPL have combined their Vitalizer circuitry with a powerful parametric/notch filter and added a low‑noise mic preamp to create an extraordinary audio toolbox. Paul White takes the lid off and looks inside.

SPL's Vitalizer is now well established as the thinking man's mix polisher, but the company also has considerable expertise in parametric EQ design, as confirmed by their excellent, if somewhat idiosyncratic, Optimizer. EQ Magix sees a marriage of those two concepts, packaged alongside a very high quality mic preamp, the aim being to provide a one box solution to mic or line‑level tone shaping.

As soon as you see an EQ Magix in the flesh, you know that you've come up against something a little bit different. The marbled front panel features laser‑etched graphics, and even the top‑cover ventilation holes spell out SPL. So, what exactly is inside the magix box?

Open The Box

The front end of the system is a high‑performance mic amp built around the highly specified SSM 2017 chip, which provides a mic gain range from 7dB to 65dB with a noise floor scarcely above the theoretical minimum. A 30dB pad in place of the more usual 20dB pad gives the front end sufficient range to handle line‑level signals, and 48V switchable phantom powering is provided, as well as a phase‑invert switch.

A buffered, unbalanced insert point is located between the mic amp and the following stage of parametric equalisation, which means that further processing can be added to the chain if necessary. It also provides a means to 'break out' the mic preamp, allowing the preamp and equaliser sections to be used for separate jobs. The parametric itself has an extraordinarily wide range covering 14kHz to 58kHz, and even that is little more than half of the available bandwidth of the audio path, which extends from 10Hz to 100kHz. While not suggesting that anyone can hear 'out‑of‑band' frequencies directly, research indicates that what you can't hear at the top end of the spectrum may well affect what you can hear lower down, so SPL are obviously keeping their options open. It's certainly true that modern digital noise‑shaping systems can pile up the out‑of‑band noise, so this unit may be one way of dealing with it.

The single‑band parametric stage may be configured either as a constant Q cut/boost equaliser or as a dedicated deep notch filter, and the frequency range is switchable so that the same control can cover either 14Hz to 3kHz or 280Hz to 58kHz. Up to 15dB of cut or boost is available in normal parametric mode, with the Q being variable from 0.6 to 3.6. For finer control, the Q Fine button halves the range of the Q control past the midway position of the control, so that the maximum available Q is 1.9. Below the halfway point, the control functions as before.

Flipping to notch mode retains the frequency setting of the filter but now puts in 50dB of cut. In this mode, the Q control is normally set fully clockwise, leaving the Cut/Boost control to vary the roll‑off of the filter from gentle to steep. The filter response may be narrowed further by increasing the Q setting, but this also has the effect of reducing the overall gain of the equaliser. When the parametric section isn't needed, or when setting up, it can be bypassed using the Active button.


Moving onto the Vitalizer section, this is very similar to the stand‑alone Vitalizer, except that it has sprouted a new button called Combine, which I'll come to shortly. Despite the myth and mystery surrounding its operation, the Vitalizer is essentially an equaliser based on interactive filters. At low frequencies, the circuit employs a novel feedback system so that when very low frequencies are boosted, lower mid frequencies are simultaneously attenuated to 'make room' for the increased level of bass without incurring boominess or muddiness. The Bass control has no effect in the centre position and produces different results depending on whether it is moved clockwise or anti‑clockwise. Move the Bass control to the left, and the sound takes on a deep, rounded tone, whereas moving it to the right creates a tight, punchy bass sound but with less of an 'in your boots' quality. This effect may be further modified using the Deep button, which widens the range of the bass filter (while maintaining the same centre frequency) and increases the amplitude by 4dB.

The important mid‑range is handled by the Mid‑Hi Tune control which may be swept between 1kHz and 20kHz and determines the frequency above which the signal will be enhanced. In practice, settings of between 2.5kHz and 5kHz seem to give the best results, and the effect is to add clarity and definition to mid and high frequency sounds as well as bringing up the level slightly to compensate for any increase in bass energy caused by using the Bass control. In broad terms, using both the Bass and Mid‑Hi controls recreates the familiar 'smile' frequency response curve often used to create the impression of increased loudness at low listening levels. The overall amount of Bass plus Mid‑Hi enhancement is controlled by the Process knob, and this, in turn, varies in effect depending on the input level to the Vitalizer section.

To take care of input level, a 10dB boost button is available on the input for situations where the signal coming from the Parametric section is too low to create the maximum effect, but a clip LED is also fitted, and if this starts to flash, the 10dB boost must be switched out or the input level to the unit reduced.

The final gloss is put on the sound using the Harmonics control, which works quite independently of the Process control. Rather than adding controlled distortion to the signal, the Harmonics section relies upon further filtering, the input of which is derived from a mix of the unprocessed Vitalizer input and the output from the Mid‑Hi tune control (high‑pass filter). Even so, the result still seems to be to add 'air' and definition to the sound, though arguably in a smoother way to some of the alternative harmonic enhancement systems.

Every facility provided by EQ Magix works perfectly and delivers the kind of tonal excellence that has come to be associated with SPL designs.

As mentioned, when the process level control is increased to add more high and low frequency enhancement, the mid‑range is simultaneously subjected to attenuation, but if the new Combine button is depressed, the Vitalizer works purely as an additive filter. Again, this section may be switched in or out of circuit with the Active button.

After leaving the Vitalizer section, the signal passes through the output stage, where it encounters another control which, in this case, may be switched to function as a level control or an active Pan Pot by using the Fader button. There's also a Mute button to kill the output, and PPM metering which monitors the two outputs in 3dB steps from ‑18dB to +9dB. The active pan pot option compensates for the 3dB level loss incurred when passive pan pots are used, but as this is essentially a single‑channel or mono processor, I can't honestly see why it was thought necessary to include it.

In Use

Because the EQ Magix has a mic preamp at the front end, you might be tempted to think of it as a mic amp with EQ, but I think there's a lot more to it than that, and my first real‑life test of the unit came at a recording session when the bass player's active bass broke down and he had to use the house bass, which was fitted with pretty old strings. With a DI box to feed the bass through the EQ Magix, I was able to use the parametric to put the 'zing' back into the sound, while using the Soft bass setting on the Vitalizer section to really fill in the low end. The result was a very acceptable bass sound from a barely acceptable bass! What's more, because there are relatively few controls, it only took a few moments to set up.

Being an enthusiastic Vitalizer user, the capabilities of the Vitalizer section came as no surprise at all to me, but those who haven't heard one before are always pleasantly surprised at the full, open, detailed sound that can be achieved without making the material seem over‑processed in any way. Used in conjunction with the single‑band parametric section, this offers all kinds of possibilities for creative as well as 'cosmetic' EQ, and having the notch option on the equaliser is very useful when you have to deal with hum problems or similar frequency‑specific signal pollution.

The mic amp behaved faultlessly, as did the pan/level output stage, though I must reiterate that I haven't thought of a single use for the panning option yet! However, being able to split the signal path using the insert point is very useful if you want to use the mic amp for one signal and the equalisers for another.


Every facility provided by EQ Magix works perfectly and delivers the kind of tonal excellence that has come to be associated with SPL designs. As an audio toolbox, EQ Magix has no obvious counterpart, though I'm never entirely sure about single‑channel processors, as I need stereo for much of the time. Similarly, EQ Magix doesn't quite provide a one‑box solution to vocal recording, because you invariably need to use a compressor while recording. On the other hand, used carefully, the EQ facilities can significantly enliven a lacklustre vocal part.

As an equaliser, the EQ Magix is made extremely powerful by the combination of parametric and Vitalizer, yet is far easier to set up than a conventional multiband EQ, and it's equally suited to use with instruments and sound effects as it is to vocals. Everything about the signal path oozes quality, even the apparently pointless pan pot, and I have to give SPL full marks for original cosmetic design — the front panel looks quite fantastic. In all, I'd give a qualified thumbs‑up for this unit, as it does everything it sets out to do very well indeed — and yes, it does sound kind of magic! For stereo processing, however, I'm hanging onto my Vitalizer.


  • Definitive SPL EQ sound.
  • Very high quality signal path.
  • Great cosmetic design.
  • Very intuitive to use once you've spent a few minutes getting the feel of the controls.


  • Single channel only.


A unique and highly flexible approach to equalisation.