Field Test: Solid State Logic XLogic Multichannel Compressor

May 1, 2004 12:00 PM, By K.K. Proffitt

A 5.1 Unit With That Old Familiar Sound


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Solid State Logic continues the modern tradition of large-console manufacturers by adding processor units to its inventory with the latest release of the XLogic series, which employs the Grammy-winning SuperAnalogue technology introduced with the SL 9000 for excellent bandwidth and dynamic range, as well as surface-mount technology for compact profiles.

XLogic modules include the XLogic Channel (with G and E Series “Twin-Curve” 4-band parametric EQ and optional 96kHz A/D converter), the XLogic G Series Stereo Compressor (a new version of an old favorite, with twin VCA design and auto-fade) and the XLogic 4-Channel Mic Amp (a smaller version of XL9000's Super-Pre with motorized control for mic gain that can be adjusted on the unit or via remote). It also includes the SL944 remote (which can control two of the XLogic Mic Amps) and the answer to DVD multichannel audio engineers' prayers: the XLogic Multichannel Compressor.


Unless you're one of those audio engineers who has steadfastly eschewed the center channel in favor of the retro “quad” version of surround, or one who refuses to recognize the frequent need for the LFE in professional multichannel productions, you'll immediately appreciate the flexibility and control offered by the XLogic Multichannel Compressor. It's the solution to the common multichannel mixing dilemma: how to maintain the bottom end while controlling the center channel, without losing the total surround soundstage.

The XLogic Multichannel Compressor owes much of its heritage to the traditional SSL mix bus compressor, but adds the XL9000's DC-coupled design, yielding better specs than older implementations. Looking under the hood reveals 56 THAT 2181LA VCAs with trims for in-circuit adjustment for low distortion and excellent feed-through performance, as well as LT1115 op amps (touted as being the lowest-noise op amp available) and LT1010 buffers to increase op amp output and reduce thermal feedback. There's also a turn-on relay to prevent audio thumps.

We measured 101 degrees with a probe over the vents of the unit in open air with a room temperature of 71 degrees and the unit sitting on half-inch spacers with no heat-producing gear under it. If you rack-mount this box, heed the mounting instructions to leave one rackspace above it for ventilation.


The compressor provides a clean, clearly visible layout with the traditional SSL square, lighted push buttons and the six-LED “Max” display, an alluring piece of eye candy that shows channel contribution status to the sidechain.

It can run in two modes — multichannel or dual-stereo — which allows it to function as separate 2- and 4-channel compressors. Dual-stereo mode reassigns controls so that the upper set governs left, center, right and LFE, and the lower set provides compression for the surround channels.

An illuminated gain reduction meter is lighted by a ghostly white LED. Users can switch between monitoring the main 5-channel sidechain to monitoring either the LFE or stereo B sidechain, depending on dual-stereo mode status. Although the controls are well-defined, the engineer should keep in mind which channels are contributing to the signal path and their allotted levels and behavior. Next to the meter are two compressor sidechain controls with In switches, and threshold, attack, ratio, release and makeup controls.

In multichannnel mode, the upper set controls the five full-bandwidth channels, while the lower set is assigned to the LFE channel. This is the traditional multichannel mix setup, where the LFE is often used for sound effects that can push to maximum headroom allowance in a split second.

If the surround channels seem to be the problem area (often encountered in live or upmixed situations in which the audience is primarily in the back), then you can switch to dual-stereo mode so that the lower sidechain controls work only with the surrounds.

The center of the front panel is the mode section, which comprises two switches: Sidechain Sum and the dual-stereo mode discussed above. Sidechain summation is used when one instrument or element is too prominent in all of the channels. When one channel has an element that is too prominent, XLogic's RMS detector works well to trigger compression. With a multichannel signal, however, it's best to sum the channels to finesse the overly loud element. Summing in the traditional sense falls prey to phase cancellation (especially in recordings of acoustic environments), so SSL engineered this aspect appropriately: They rectified the signals prior to summation.

The sidechain-adjust section dominates the right half of the front panel, and it's the most important and innovative area. The circular Max display with five bi-color LEDs in a pentagon around a center LED for the LFE is remarkably intuitive for indicating which channel is driving the compressor.

Four sidechain trims control how much of each channel or channel pair contribute to the sidechain. These include left/right, center, LFE and surround. Center and surround may be disconnected from the sidechain input by activating their respective iso switches, but they will still be controlled by the master level control.

The Link switch allows the LFE to merge with the main channels, incorporating it with the sidechain for the main compressor and switching the control of the LFE signal path to the main compressor. There's also a 12dB per octave, 120Hz filter switch and a 10dB pad switch to reduce the level of the LFE if the mix has been made to Dolby Digital specifications. It would be nice if the 120Hz filter could be expanded to include 80 Hz, as current Dolby recommendations for the LFE use in music mixing call for 80Hz lowpass to accommodate playback in consumer systems with bass management.

The far-right section has two controls with their respective In switches: master level and auto-fade. Master varies from ▸ to unity gain. Auto-fades can range from 1 to 60 seconds. Releasing the lighted auto-fade switch creates a fade-in, while engaging the switch creates a fade-out.


SSL arranged the XLR inputs and outputs in the standard ITU grouping: L, R, C, LFE, Ls and Rs — an excellent choice. There's also a big heat sink, but you'll still need that space above the unit if you place it in a rack. Next to the power switch and IEC cable input is the housing for a removable vertical PCB that can be configured for settings of 100V, 120V, 200V and 240V.


Working with the compressor brought back many fond memories of working with SSL consoles. I used to call the Quad compressor the “make-it-bigger button,” and I distinctly remember that compressor's characteristics.

With XLogic, however, the engineer should tweak very carefully, because it is easy to misuse the box and render a formerly spacious mix into a cramped space that sounds like a closet. I also found that with certain channel trim settings, I could put the machine into a state that caused radical changes on an acoustic guitar (recorded multichannel) by simply changing threshold from 1.5 to 2, while varying the attack throughout its range seemed to do relatively little.The answer to these problematic situations lay in the trim control section. The prudent engineer will want to focus immediate attention on exercising variations in trim control relationships.

Of course, a little compression goes a long way. For one session, I recorded a vocalist/guitarist in surround. Although the vocalist was closely miked, his guitar still bled into the vocal microphone. Because his vocals were also picked up by ambient microphones, I faced a mix with a very dominant center-channel vocal, a stereo front that was mostly guitar and surround channels that were nearly perfect.

I needed to make the vocals “sit” in the mix rather than dominate it. Again, the solution involved tweaking the channel trims. I let the center channel contribute most to the sidechain, with a bit of the left and right channels also going to the sidechain. I isolated the surrounds from the sidechain so that they were only affected by the main out and not the compressor. I also summed the sidechain so that the main contributor to the compressor was the vocal and not the guitar. Finally, I set the top compressor controls with a fairly slow attack and release and a very gentle ratio of 2:1.

I had to work with the trims quite a bit so that the compressor didn't squash the front ambience. At certain settings, the wall behind the guitarist seemed to leap forward, but after playing with various settings, I was able to make the guitarist sit comfortably in the center with his vocals focused but not overbearing. The ambience remained relatively untouched.

The XLogic Multichannel Compressor is elegant with logical yet complex routing, excellent components and, most of all, the classic SSL bus compressor's sonic signature. The unit gives the engineer ultimate flexibility and control over multichannel mixes in the analog domain.

Solid State Logic, 212/315-1111,

K.K. Proffitt is chief audio engineer of JamSync in Nashville.

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