Review: SSL X-EQ & X-Comp

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Since releasing the original Duende (now called Duende Classic) FireWire DSP hardware with the C200/9000K series-inspired EQ & Dynamics Channel and Stereo Bus Compressor plug-ins included, SSL has been hard at work expanding the Duende line and developing add-on plug-ins for the DSP systems. In addition to the 19-inch rack Duende Classic ($1,875), SSL now offers the FireWire Duende Mini ($995) and the Duende PCIe card ($1,495). Not quite a year ago, the Duende add-on plug-in Drumstrip made its mark as an intricate transient shaper and gate for drums and percussion, after which requests for mastering-grade tools surfaced in online user forums. Apparently, SSL listened.

Each of X-EQ's 10 frequency bands have a color-coded ball that you can pull around in a grid to attenuate the frequency. A detailed spectral analysis underneath shows the results as you go.

The highly sophisticated X-EQ and X-Comp deliver superb audio quality, thanks in part to Duende's 40-bit floating-point engine. Designed with mastering in mind, X-EQ is a 10-band parametric EQ featuring selectable filter topologies and completely customizable EQ types per band. Likewise, X-Comp's unique frequency-splitting parallel compression technique isn't modeled on any specific hardware. Instead, it provides some really cool parameters allowing you to emulate of a wide range of compressor designs, from gentle dynamics control for mastering to brickwall effects and nearly everything in between.

Both plug-ins are included in the Duende 2.5 software/firmware update, which you must burn into your Duende. But to use the plug-ins you must unlock them directly from the GUI using a code you receive over e-mail once you purchase the plug-ins at SSL's Web Shop. Supporting Audio Units, VST and RTAS on both Mac and PC, the plugs are provided in mono and stereo versions and can operate at 44.1, 48, 88.2 and 96 kHz. A mono instance at 44.1/48 kHz occupies one Duende processing slot, stereo 44.1/48 kHz occupies two slots and stereo 88.2/96 kHz instances occupy four slots.


X-EQ's bands 1 and 10 are highpass and lowpass filters, respectively, with a choice of five different topologies or tonal characters each. Bands 2 and 9 are shelving EQs with a Q range that extends right up to 10.3 and a degree of undershoot/overshoot at higher Q settings for achieving a familiar vintage sound. The remaining six bands are all parametric bell EQs with a choice of nine EQ types, each. All shelving and bell bands can have their frequency set between 20 Hz and 20 kHz with ±20 dB of boost/cut. Bell bands can also be switched from the usual serial signal flow of a parametric EQ to the parallel signal flow of a graphic equalizer.

All bands have individual activation buttons and share a common parameter control section with Gain, Freq and Q knobs, along with filter shape selection. Alternatively, you can simply click-and-drag color-coded balls for each frequency band in the EQ curve display. The Analyze button superimposes a real-time FFT spectral frequency analysis onto the curve, showing exactly how you're affecting the signal.

Three-in-one input and output meters with gain controls ranging from -36 dB to +12 dB flank the EQ section. The widest meter is a responsive peak meter (stereo or mono) with a large red clip light that indicates when at least three consecutive samples pass that are higher than 0 dB. A thinner meter to the right shows RMS (average) level, useful in judging the perceived loudness of the signal. Having these side by side gives you a clear idea of the peak-to-RMS ratio, or crest factor, of the signal — a very pro touch. The third meter function, a “dynamic history” bracket, shows the recent dynamic range of the signal. The top of the bracket shows the most recent peak, while the bottom shows the level of the smallest peak. If the bracket is long, it indicates that there has been a large swing in the dynamic range.

Because Duende exhibits noticeable latency at higher host buffer settings, you can toggle a global bypass button within the plug-in without any interruption to monitoring.


At first glance, X-Comp looks like just another dynamics processor, only with a much snazzier interface. It is in fact a pseudo multiband device capable of some pretty cool tricks. For one, it features a symmetrical knee design offering precise control over the way that transients are handled around the threshold point (Knee 1) and, conversely, the way in which the compressor backs off again after achieving the ratio value (Knee 2). The overall Knee control is linear and defaults to hard compression at its 0 dB setting, but it goes to a softer compression as the control is increased toward a maximum of 40 dB. That effect shows up in real time on the large, center Compression Law display with two draggable nodes.

Threshold carries a range of -48 to +12 dB with linear response. SSL claims to have chosen -48 dB to allow for similarly low ratios that achieve subtle compression effects. The Ratio control therefore has a range from 1:1 to 50:1 and a logarithmic response, which gives finer resolution and makes the setting of low-ratio values easier. Attack can be set from 0.5 to 100 ms and Release from 1 to 2,000 ms. The Max GR control can limit the maximum gain reduction, similar to how the light-emitting sources of opto-compressors have only a finite maximum brightness. Adjustable from 20 to 60 dB, this provides the key to emulating a wide range of vintage compressor characteristics.

The unique Bleed Through window houses the controls behind X-Comp's frequency-splitting parallel compression technique and consists of a simple frequency graph (20 Hz to 20 kHz) and two adjustable filter crossover points, offering an effective way to “grab and move” the boundaries of frequencies to be compressed. By default, the entire frequency range is sent to the filter sidechain and processor equally. If you engage one or both of the “bleed through” filters, the affected range of frequencies (below cutoff for LF bleed, above cutoff for HF bleed) are no longer routed through the compressor but are instead passed unaffected and summed with the compressed signal at X-Comp's output.

The cutoff range for the highpass filter is from 30 to 300 Hz and 2 to 12 kHz for the lowpass filter. Because the bleed controls are adjustable in range from 0 to 100 percent, you can even proportion the amount of overall signal that gets filtered. Makeup gain ranges from -6 to +36 dB and always controls the level of the compressed signal only. It can therefore act as a secondary balance control of sorts, between the compressed and filtered signals. Having 6 dB of attenuation is extremely handy here because the “middle” compressed signal can often become quite loud and overpower the bleed through.

Clever metering has defined new SSL plug-ins. The I/O Diff meter in X-Comp, also known as an amplitude histogram, shows the relative occurrence of various amplitudes before and after processing. With a vertical scale representing the full range of all possible amplitudes (-□∞ to 0 dB), thin blue horizontal lines protrude left and right of the midline; their lengths show the number of times that a particular amplitude is occurring at input and output, respectively. That lets you see the shift in dynamic range that happens over time due to compression and quickly identifies signals with very large dynamic ranges that could use squashing. It can also be a terrific visual giveaway of that dreaded overcompression syndrome, illustrated when the blue lines are bunched up in the middle of the dynamic range, with little activity occurring above or below. Another novel approach, the GR History meter not only shows the compressor's real-time gain reduction (0 to -30 dB), but it also indicates how it has been fluctuating over the past second — handy for assessing your attack and release times. The main input and output level and metering sections are the same as on X-EQ.


I tested both plugs in every provided format, but largely on a Pro Tools|HD rig using the included FXpansion VST-to-RTAS wrapped version. Everything ran successfully on my Duende Classic, with not a single crash or performance hiccup. A handful of presets are installed, offering decent starting points for common mix tasks but stop short of including any specific third-party gear emulations.

X-EQ has an enormous dynamic range. Using proprietary filter algorithms, SSL has created one of the biggest sounding, yet cleanest and purest EQs in existence. You can drive it as hard as you like — particularly in parallel EQ mode — and it still sounds musical and smooth. There are no so-called “controlled distortions” or coloration fingerprints left by X-EQ.

SSL claims that with a properly designed, fully parametric analytic EQ such as this, every amplitude and phase characteristic of any other equalizer setup can be re-created. While that may be true, you'll need a really good ear and clear understanding of vintage EQs to faithfully simulate a particular model, which is where a “clones” folder in the presets might come in handy.

I experimented greatly with the various Bell filter types. Classical Symmetrical provides an almost constant Q characteristic, as found in various mixing consoles and outboard gear. Classical Asymmetrical features a much narrower cut, something you'd find in very early equalizers. Four types of Constant Q provide ample variations of boost and cut characteristics, including a modern setting where Q is always perfectly maintained independent of bell gain.

X-EQ was perfectly suited for any task I threw at it. I could surgically EQ loops to fit into arrangements, notch out frequencies spotted in the FFT analyzer on instrument tracks and polish tracks at the mastering stage. The five shelving filters (Critical, Bessel, Gaussian, Butterworth and Chebychev) gave me just the right roll-off rates for any noise or frequency I was trying to tame or remove. X-EQ can seemingly do it all.

X-Comp, though simpler, was another pure delight to use across a wide variety of material. On drum submixes and loops, allowing some of the very low bass frequencies past the compressor maintained the natural sub energy, while hammering the mids resulted in a bigger sound than I'd get from an ordinary compressor. Letting some high frequencies through on kick mics, snares and toms prevented the sharp beater or stick transients from being lost with fast attack times. The unique double-knee design made that even easier — and more flexible — as I could relax compression around peak transients and preserve the rest of the signal while still having great control over dynamics. You can easily drive this compressor hard for a sound that makes vocals scream, guitars smolder and drums literally explode. SSL has implemented first-order filters to ensure that phase cancellation does not occur at the summing stage. The frequency splitting sounds extremely transparent without the ringing often associated with higher-order cut filters.


As with everything SSL, I expected really big things of these plug-ins, and I got more than I bargained for from both. You can customize X-EQ's filter curves to those of virtually any equalizer on earth to simulate classic API, Neve or Pultec settings or mix and match bands to create your own monster. The spectrum analyzer is not only an incredible education tool for engineers learning the numbers behind what they're hearing, but it also helps veteran mixers visualize what they may think they're hearing. To make X-EQ the ultimate mastering EQ, I'd like to see the addition of mid-side (M/S) processing, a de-esser and the “image shifter” controls typically found on expensive mastering hardware.

X-Comp is nearly as flexible, but won't be my go-to mastering compressor primarily for its lack of true multiband compression and limiting, which I love about McDSP ML4000. I also feel SSL should have gone one step further with its notion of frequency region-based processing and allowed the user to divide dynamics into amplitude zones, similar to Roger Nichols Digital Dynam-izer.

While the asking prices are comparable to what you'd see on the TDM market, some might consider X-EQ and X-Comp expensive compared to other hardware-exclusive platforms, such as the Universal Audio UAD systems. Aside from the sonic benefits already mentioned, the most logical justification to help those people decide is that you're not only getting high-level mastering plug-ins but also an extremely configurable EQ for critical mixing, and one of the most musical track and bus compressors, capable of being as modern or vintage as you like. Either way, they fill a tremendous niche and make investing in a Duende more tempting than ever.


X-EQ > $599

X-COMP > $499

Pros: Powerful mastering-grade 10-band parametric EQ and frequency-dependent compression. Superb audio quality with equally brilliant GUIs that include real-time FFT analysis and amplitude histograms. Professional multimetering with dynamics history indicators.

Cons: Plug-ins exclusive to Duende — no native versions.


Mac: Duende DSP hardware; VST, Audio Units or RTAS host

PC: Duende DSP hardware; VST or RTAS host