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SPL Rackpack

Modular Preamp & Processor System By Hugh Robjohns
Published June 2009

Sound Performance Lab have developed an enviable reputation for their hardware products, and now you can buy more for less with their modular rack system.

SPL Rackpack

With studio space at a premium, rack systems that hold compact modules make a lot of sense. SPL's 3U Rackpack is designed to accommodate up to eight of the company's all-analogue solid‑state or tube modules. Currently available for this system are the Preference Mic Preamp, the Premium Mic Preamp, the Transient Designer, the Full Ranger passive-coil graphic EQ, the Bass and Vox Rangers (using the same filter design), the Twin Tube and the Dynamaxx compressor. Other than the Preference Mic Preamp, all modules can optionally be equipped with I/O transformers from Lundahl, and all have dual outputs so that the signal can be split to two different destinations. (Where a transformer version is applicable, the output transformer can only go in the main output path). I'll be looking at the features and performance of the individual modules a little later, but first...


Power to the various modules is distributed via a ribbon cable in the bottom of the case, and there's a dummy module at one end housing the power switch, leaving eight free slots (unused ones are covered by blanking plates). While there's little remarkable about the rack itself, a lot of care and attention has gone into the external power supply (PSU), which is far removed from your average wall-wart, because it is around the size and weight of a house brick, and has been designed to deliver a very generous amount of extremely clean power, so that the low-noise capability of the modules is not compromised. This PSU connects to the main rack via a robust, locking connector and a heavy multicore cable.

The review unit came loaded with the full range of modules, starting off with both of the mic preamplifiers. As with all the other modules, these have thick metal front panels finished in satin silver, with a recessed black centre panel for the controls, giving the whole system a very homogenous and stylish appearance, regardless of which modules are installed or in what order.

Preference & Premium Preamps

Each module has the familiar SPL styling, with easy-to-operate (and surprisingly few!) controls.Each module has the familiar SPL styling, with easy-to-operate (and surprisingly few!) controls.

The Preference mic-preamp circuit is based around the SSM2019 chip in a DC servo configuration, which combines low noise and distortion with an impressively fast slew-rate, while virtually eliminating DC offsets. This circuit produces up to 72dB of gain, and its design places a minimum number of capacitors in the audio path, which in turn reduces phase shift and possible sources of distortion. Two additional op-amps function as a voltage differential sensor and a summing stage. Premium-quality components are used throughout all the modules, including MKP and Styroflex capacitors.

The Preference Mic Pre's retro-style, backlit moving-coil meter has two switchable modes to display average or peak (PPM) levels, with a -10dB button to give more meter headroom when working at hot signal levels. There are also two LEDs built into the meter: one showing signal present; and one that comes on 3dB before clipping. (All the modules in this series have these same warning LEDs). The preamp provides a very clean 48V phantom power supply, and there are push switches for polarity inversion, a 20dB pad, and a low-cut filter with a gentle 6dB/octave slope at 75Hz. Gain is adjusted via a chunky metal knob at the bottom of the module's front panel.

As I touched upon earlier, there's a single input and two balanced outputs, both on XLRs, and both capable of driving very long cable runs — so each module also acts as an effective signal splitter. The EIN is quoted as -129dB with a 0.047 percent THD + noise figure at 60dB of gain, and 0.0035 percent at 30dB of gain. The frequency response extends from 10Hz to 200kHz (-3dB).

Outwardly, the Premium Mic Pre looks identical to the Preference model, but it uses a Lundahl input transformer as standard, offering up to 80dB of gain. The transformer is where the extra voltage gain comes from, and this is followed by both a discrete differential amplifier stage and an instrumentation preamplifier stage.

Transformer-based audio circuits have a certain sound character that many engineers find more 'musical' than transformerless designs, but they also offer true electrical isolation, which can be beneficial from both safety and ground-loop perspectives, especially in mobile live-sound setups. An EIN figure of 128.3dB is quoted along with a THD + noise figure, at 60dB of gain, of 0.078 percent, and better than 10 times less than that at 30dB of gain. The frequency response is inevitably less than for the transformerless Preference, but still a more than adequate 10Hz to 68kHz (-3dB).

Ranger EQs

The Twin Tube Processor enables you to control tube saturation and harmonic enhancement using only two dials.The Twin Tube Processor enables you to control tube saturation and harmonic enhancement using only two dials.

The three Ranger EQ models look like graphic equalisers tipped on their sides, but they are in fact based around passive inductor/capacitor filters. The Full Ranger has frequency centres of 40Hz, 90Hz, 150Hz, 500Hz, 1.8kHz, 4.7kHz, 10kHz and 16kHz, and the cut and boost is controlled by a set of miniature sliders. Unlike a traditional graphic equaliser, where the filters are set one octave apart with identical response curves, this SPL design tailors the response of each filter band based on musical principles (rather than mathematical ones). Essentially, the filter curves get wider (lower Q-settings) the further you go up the audio spectrum.

The Bass and Vox Rangers are conceptually identical, but they have different filter frequencies and curve characteristics, optimised for bass and vocal applications, as the names suggest. The Bass Ranger has centre frequencies of 30, 65, 95, 170, 230, 500, 800 and 2000Hz, while the Vox Ranger is set at 220, 330, 420, 500, 800, 1600, 2800 and 4000Hz. The range varies slightly with the filter type, but is roughly ±12dB. The input, as well as the output stages, may be transformer balanced as an extra cost option, and each module has an output level-fader and bypass switch, as well as those simple but practical 'signal present' and overload LEDs. All of the faders are centre-detented, with red LED illumination in the fader knob. The frequency range is 10Hz to 30kHz (-3dB) with an A-weighted noise figure of -85dB.

Transient Designer

Next in line is the Transient Designer, the standard rackmount version of which we reviewed in SOS when it was first released. The Transient Designer's clever trick is that it provides separate control over the attack and release characteristics of a percussive sound using only two knobs, as the circuitry automatically adapts its threshold to the level of the incoming audio, so that the user doesn't need to worry about levels. In the centre positions, the two knobs do nothing, but moving in either direction allows the user to either enhance or suppress the attack and release envelope of the source sound. Using the attack control, you can, for example, back off the control when processing a kick drum, to get more of a 'bouncing beach ball' sound, or advance it to really bring out the initial slap. Similarly, adjusting the release can either make the drum sound very tightly damped, or it can bring up the decay for a really roomy sound. It works well on individual drum tracks or a complete kit mix, and it can also be effective on instruments such as bass guitar.

A third knob sets the output level, and there's also a bypass (On) button and a stereo link button. Stereo linking requires two adjacent modules to be internally connected using a small ribbon cable, and there are DIP switches on the circuit boards that also need to be set. This remains one of my favourite processors, and it has rescued an indifferent-sounding drum part on more than one occasion!

Twin Tube

All of the I/O is hosted on the modules themselves, so the rear of the rack unit is very straightforward. The channel numbers are usefully presented both ways up, so you can see what you're doing when peeping over the top.All of the I/O is hosted on the modules themselves, so the rear of the rack unit is very straightforward. The channel numbers are usefully presented both ways up, so you can see what you're doing when peeping over the top.

The Twin Tube processor's purpose in life is to create controlled tube saturation and harmonic enhancement, again using very few controls — just one knob to adjust the harmonic enhancement, and another to adjust the degree of tube saturation. A pair of buttons used in combination allows the harmonic enhancement to operate on frequencies centred at 10, 6, 3 or 2kHz, and there are separate 'On' buttons for the Harmonics and Saturation sections. The actual frequencies are, in fact, 9.8kHz, 6.6kHz, 2.8kHz and 1.9kHz, but the approximations have been printed to keep the panel tidy. Each section has its own tube circuit and, as you might expect, the tube saturation effect is created by driving the tube into a non-linear region of its operation.

A more sophisticated circuit comes into play for overtone/harmonic processing using an inductor/capacitor filter to modify both the overtones and their phase relationships with the source sound. The designers claim that the phase part of the circuit aligns the overtones, and works not so much by generating harmonics, as a traditional exciter does, but rather by selectively equalising the existing harmonic structure. It may be that this process relies on some of the principles pioneered in the SPL Vitalizer, although this isn't confirmed in the documentation. Using the harmonic enhancement can make a sound seem closer and more present, without necessarily making it louder.

As this is a tube module, it takes some time to warm up, so the 'On' buttons flash for a few minutes after powering up, to let you know when it is ready to use. The same signal and overload LEDs are provided as for the previous modules. An overall frequency response of 10Hz to 80kHz is specified for the Harmonics section, with an A-weighted noise figure of -87dB, while the saturation stage is quieter still, at -96dBu A-weighted, and with a 10Hz to 77kHz bandwidth. Distortion figures are not really relevant — because the module's job is to introduce controlled distortion!

Dynamaxx Compressor

The final module fitted to our review system was the Dynamaxx compressor, which is, again, an exercise in how much control the designer can put under a single knob, and is an update of the stand-alone design specifically created for the Rackpack system. The compressor, which has a soft-knee characteristic, has very sophisticated automatic attack and release circuitry, designed to dynamically match the time constants to the type of material being processed. It's also designed to prevent high levels of compression compromising high‑frequency detail. The gain-control element is based on a pair of THAT2181 VCAs in a double VCA drive-mode configuration, to cancel distortion.

Increasing the Compression control setting increases the ratio, while simultaneously lowering the threshold, and a gain-reduction LED ladder meter displays the gain change very clearly. The maximum compression ratio is 3:1, and a make-up gain control may be used to restore any level lost through compression, while a further button brings in a separate limiter side-chain — again with a soft knee — that acts on the same VCA pair to provide a more progressive style of limiting than the usual hard-knee type. Signal/overload LEDs are fitted at the top of the panel.

The 'On' button operates a hard bypass, and there's also an 'FX Com' button that flips to a fixed release time of 60ms, making it easier to create deliberate gain-pumping effects when required. Unusually, there's also a 'De Com' button. This converts the compressor into a type of subtle expander and is designed to help undo the effects of over-compression on pre-recorded material. It's worth noting that the limiter also plays a negative role in this mode if active, and actually gives a boost to transients. As with the Transient Designer, two units may be linked for stereo operation, providing that the necessary cable and DIP settings are attended to when the modules are installed. This module has a 10Hz to 200Hz frequency response (-3dB) and an A-weighted noise figure of -93dB when the make-up gain is set to zero. The main difference between the Rackpack Dynamaxx and the rack module that's been around for years is that the rack module also has a gate section, which is omitted here.

Sound Performance?

Both mic preamps are very clean and quiet, although the Premium model has a slightly more assertive sound that's probably due to subtle non-linearities and phase-shifts in the input transformer. I compared the Preference module on voice with my Universal Audio Solo 110 solid-stage preamp and found that it came very close. By contrast, the Premium had a smoother, slightly coloured top end, and it seemed to be adding a bit of weight to the lower vocal frequencies — which is pretty much what I'd expected. Both are seriously good preamps, so it's really down to picking the most appropriate for a given recording situation.

The Ranger EQs are particularly effective and musical, but I have a couple of ergonomic reservations regarding the panel markings. Unless you're looking at them straight on, the sliders obscure the frequency markings, which are, in any case, small and difficult to see in normal studio lighting, because of the illuminated fader sliders. That aside, the EQs have a wonderfully analogue sound, and you can add quite a lot of boost when needed without wrecking the sound. The Vox and Bass versions are particularly useful in tackling the main frequency bands of those two sources, and save lots of time hunting around with a parametric EQ. Even the top boost sounds warm, and there's none of that glassy, gritty edge that some solid-state active EQs impart when you add more than a dB or so of boost. I also compared the low end of the EQ with some of my better plug‑ins and found that the SPL modules seemed somehow more solid and believable, with no tendency to make the low end messy or boomy.

The Transient Designer works just like its rackmount counterparts, and is simply unbeatable for polishing drum sounds, although it can also work wonders on other types of percussive or plucked sounds. If your drums sound too dry, turning up the release will both lengthen the decay and bring up the room ambience, but if virtual gaffa tape is what you need to damp down ringing toms, you just turn it the other way. It's the same with the Attack knob — go left to soften the attack or right to make it more spiky and aggressive. Because of that auto threshold feature, it works regardless of the level of the drum hits that you're processing. I won't get into this too deeply, as we've covered the Transient Designer itself before in plenty of detail. There are just two knobs and it's brilliant!

The Twin Tube is an interesting device because it really behaves like two processors rolled into one. The harmonic enhancer adds brightness around and above the frequency of the filter, which is set using the four permutations of the four frequency buttons. This works very effectively, and sounds more natural than some harmonic-generating devices, but it still works differently from conventional equalisers — at least subjectively. It's really a matter of bringing out and enhancing what's already there, and I'm sure that the tube itself adds a touch of organic flavour to the proceedings.

The lower part of the front panel controls the tube saturation effect, and this is far less subtle unless used very sparingly. At lower settings it adds warmth, but go much further and the distortion that it introduces gets more obvious, becoming seriously crunchy at higher settings. Heavier distortion isn't only for electric guitar, however: it can work wonders on drums, some bass synth sounds and possibly death metal vocals. (On 'normal' vocals you really have to use it sparingly.) Still, the ability to introduce very obvious saturation extends the versatility of this module far beyond that of a vocal warmer!

Finally, the Dynamaxx compressor proved every bit as easy to use as the manual had promised. You can really lay on the gain reduction, and unless you select the fixed release-time mode, it resists all attempts to sound pumpy or dull. If you want dramatic pumping to use as a special effect, you just switch in the FX Com button, hit the limiter button, and crank up the compression! The Dynamaxx makes a great vocal leveller when used in its normal mode, and it adds a touch of warmth at the same time, but if you want to use it as a de-compressor or expander, it's probably best to err on the side of caution — because otherwise loud sounds get really loud, especially if the limiter is switched in. You also have to remember that all the controls seem to work in reverse in expander mode, even the make-up gain. Use it with care, though, and it really can restore some useful dynamic range to an over-compressed track.


Other than some issues concerning front-panel legend visibility, the Rackpack modules work brilliantly, and where features have been lost to make them fit the space, this doesn't really affect usability to any significant degree. The sound quality is up to SPL's usual high standard, too, while their innovative use of technology puts some very complex processing behind a surprisingly small number of physical controls. Also, as I completed this review, SPL announced a version of the Rackpack that can host the ubiquitous API 500-series modules as well: a mouthwatering prospect indeed!

The Rackpack system isn't cheap: the empty rack and PSU cost as much as a decent computer. But there's no denying the quality of what these guys build. SPL have a unique approach to product design, due in no small part to the innovative thinking of chief designer Wolfgang Neumann — and they deserve to do well with the RackPack, especially in smaller studios that still prefer analogue mixing.  


While there are other excellent rack systems out there, such as SSL's X-Rack, the API 'Lunchbox' 500-series, and others from Audient and Tonelux, few offer alternatives to the more innovative SPL products like the Transient Designer and Twin Tube. The only real alternative that offers these was recently announced by SPL themselves: their Rackpack 500 is a hybrid of the API and SPL systems. It can host four Rackpack modules, as well as four from the API 500-series. If your budget is lower, check out SPL's range of plug-ins, reviewed in SOS May 2009.

Pricing Information

Rackpack Frame £732$1269

Preference Mic Preamp £260$459

Premium Mic Preamp £524$899

Dynamaxx £418$535

Twin Tube Processor £418$669

Transient Designer £281$459

Full Ranger £418$669

Bass Ranger £418$669

Vox Ranger £418$669

All prices include VAT.


  • Unique SPL approach to signal processing.
  • Excellent sound quality.
  • Compact.
  • Simple to operate.


  • Fairly costly.
  • Panel legending difficult to read under normal studio lighting conditions.


The Rackpack provides a compact and practical way of housing and powering up to eight of SPL's most useful preamps and processors. It isn't cheap — but then more attention has gone into the design of the power supply than some companies put into the entire product!


See pricing information box.

The Audio Professionals +44 (0)1923 693770.

See pricing information box.

SPL USA +1 866 477 5872.