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PAR Session Trial: Premium 2-Channel Compressor/Limiters

Featuring API, dbx, GML, Manley and Solid State Logic

The application of dynamic-range compression has become a sophisticated and most challenging technical art at nearly every level of professional audio production. As a result, our modern compression tools are notably more complex (and, as an unfortunate downside, most vulnerable to misuse and abuse) than those within any other gear category.

With that in mind (and for quite possibly the most challenging installment of PAR’s comparative gear evaluation series to date), I was careful in selecting a Catalyst Recording recording/mixing session for these five premium 2-channel compressor/limiters. I ultimately chose an experienced client with whom I am very familiar — Southern rockers Evelynn Rose — to record a song, utilizing our compressors to their best abilities and the song’s artistic purpose. With drums, bass, electric guitar, acoustic guitar and lead vocal, we had the straightforward tracks needed to focus on carefully selected dynamic control.

The Contenders
These five compressor/limiters are, in alphabetical order:

1 API 2500 Stereo Compressor
Though our only single-rack-space unit (1U), the 2500 is hardly shy on features; it’s literally packed with them. Its single set of stereo controls select stepped variable Ratio (up to infinity:one) and Attack. Hard, Medium and Soft knees are switchable, as well as Loud, Medium and Normal “Thrust” modes (Thrust is API’s patented method of high-pass filtering the sidechain, letting more bottom end through without “pumping” the compression). There are old and new modes — Feedback or Forward, respectively — and unity gain or a switchable makeup gain circuit is available on the output. Sidechain inputs, variable stereo linking (from zero to 100 percent), numerous linking filters and a VU pair complete the 2500. List price: $2,995.

2 dbx 160SL Blue Series Compressor/Limiter
Like the 2500, the 160SL is also fully loaded, but with distinctly different parameters. Continuously variable Threshold, Ratio (up to infinity:one), Attack and Release are present, plus a variable peak limiter with Peak Stop Plus (which actually rounds off — not chops off — sharp peaks). The 160SL allows dual mono operation, has sidechain I/O, two big VUs and, of course, dbx’s patented Over Easy knee. List price: $4,995.

3 George Massenburg Labs (GML) Model 8900 Dynamic Range Controller
The 2U GML 8900 offers yet another unique set of parameters that control perceived loudness, not simply voltage. Continuously variable Threshold and Ratio (up to infinity) are present, as well as a Soft ratio setting that sounds unlike any of the other ratio settings. In addition, Timing (with grouped attack and release times), Release Hysteresis (for independently controlling release characteristics of the Slow RMS detector and reducing distortion), Crest and Peak factors are all continuously variable. Stereo linking and a sidechain input are available. List price: $6,150 plus $650 for the required GML 9015 power supply.

4 Manley Variable Mu Stereo Limiter/Compressor
Big and beautiful, the Vari-Mu’s large knobs and attractive bluish VUs control and display continuously variable input gain, threshold and output gain with stepped controls for Attack, continuously variable Release and a switchable high-pass filter for the sidechain (for preserving bottom end, much like API’s Thrust). Ratio parameter is not offered, but the unit can be switched between compression and limiting modes. List price: $4,400.

5 Solid State Logic XLogic X-Rack Stereo Bus Compressor Module
This SSL unit was the only non-19-inch rack unit contender, though the exact same components are commercially available in a 1U size (the XLogic G Series Compressor). You have likely heard this compressor before, many, many times: it is the master section mix comp from the ubiquitous G Series console. Built for SSL’s X-Rack modular system of vertically mounted, plug-and-play analog outboard processors, the SSL offered the most conventional control choices, not to mention the smallest knobs of the lot. Continuously variable Threshold and makeup gain are offered, with stepped Attack, Release and Ratio (up to infinity). Metering is a single, small VU showing gain reduction only. The X-Rack Stereo Bus Compressor can be housed in the Mynx (a two-module slot box, as it was for this review), or the eight-module X-Rack (with SSL’s Total Recall). List price: $2,259 plus either the Mynx ($499) or the X-Rack ($999). If you prefer to simply go 1U, the XLogic G Series Compressor is $3,595.

The Session

(Top to bottom): Solid State Logic XLogic X-Rack Stereo Bus Compressor Module (in a Mynx chassis); GML Model 8900 Dynamic Range Controller; API 2500 Stereo Compressor; dbx 160SL Blue Series Compressor/Limiter; Manley Variable Mu Limiter/Compressor. Photo: Rhon Parker

We first laid our drum track to a click (with scratch instrument tracks); in doing so, we did not utilize any compressors, opting to use some parallel compression on a stereo drum bus during our mix. However, we did lay down bass, electric guitars, acoustic guitar and vocals with these five compressor/limiters. We made efforts to try a taste of each unit’s offerings on each source: low ratios, higher ratios, various attack and release settings and the multiple mode choices as well. Yes, this was exhaustive testing, loaded with options, more than a little daunting in scope and subject to the tastes of picky artists/engineers. It was just like a real session, only more complicated!

On Bass Guitar

Bassist Joshua Wade has a fantastic Fender Jazz bass (with Badass bridge and sweet electronics) that gets a nice “pingy” sound (a la modern pop/punk) when picked. Josh asked me to maintain that defined ping and get some additional bottom to boot. With a Countryman Type 10 active DI via my Manley TNT tube mic pre and an sE Electronics Voodoo VR-2 active ribbon mic (via the TNT’s solid-state side) on the bass amp cabinet, we had a clean and punchy front end.

Josh used the API 2500 for his tracks, where I found a 70 percent stereo linkage “glued” the overall sound the best. I was a little surprised to find myself using the soft knee, 3:1, the new mode (which was much more aggressive), and slow release of one second. The Thrust circuit on Loud offered a bottom end the other units didn’t provide, not to mention that nice, piano-like ping. (Audio webclip #1) As I added makeup gain, we noticed a little noise floor — OK for this bass track.

The Manley Vari-Mu did very nice things here, too — rather smooth and pleasant with a sound that would’ve been the top pick if a more traditional bass sound was desired. The SSL Stereo Bus Comp offered a bit of excitement and sheen, while the GML offered a great overall fullness without being obvious or apparent (maybe even not apparent enough). Finally, the dbx seemed too grabby on the pick transients, even as I fiddled with attack time and knee.

On Acoustic Guitar

Vocalist Joseph William plays a nice, bright Taylor acoustic that records really well without the use of its onboard electronics; two good mics, and he’s good to go. I used my new acoustic guitar mic favorites while I still had them — the JZ Microphones V47 and V67 [separately reviewed in this same issue of PAR — Ed.] — with amplification from my AMS-Neve 4081 mic pre.

This test’s verdict wasn’t unanimous (unlike it was for bass guitar); we truly struggled with many viable choices. Finally, the Manley Vari-Mu got the gig with an attractive musicality and refined pleasantness that is nearly beyond words; what it does is distinct and desirable. Surprisingly, I preferred the limiting mode (for its firmer control and tight reaction at the knee) and the band agreed once we slowed the attack a little for pick transients (and slowed the release, too). (Audio webclip #2)

The GML nearly got the gig with a presentation that was largely invisible unless forced otherwise. The API and the SSL both did nice things that I could use without restraint. The dbx offered a nice, tempered frequency balance but I had trouble controlling the loudest peaks, wishing for more time to tweak.

On Electric Guitar

With Chris Nigra’s twangy Fender Telecaster through my old-school Marshall JCM 800 (miked with a Shure SM57 and the JZ V67, both through the AMS-Neve 4081), we already had the sound we wanted. Skeptically, we forged ahead and, frankly, most of the units were too grabby or too colored to do anything that truly improved on our initial tone.

We did find the API 2500 to our liking for what seemed like extra bite and a touch of attitude imparted on our “brown sound” tracks. Good ol’ 3:1 ratio with a medium knee, medium Thrust and now the Old mode setting (more “chewy” and interesting) provided steadier levels and no loss of crunch. I had to use the 2500’s makeup gain and, once again, we heard some noise while using it — OK for this loud pop song, but probably not OK for delicate acoustic work or classical recording. (Audio webclips #3 and #4)

On Lead Vocals

We chose the JZ V47 for Joseph William’s gruff lead vocal, coupled with the Manley TNT’s tube mic pre for a “present yet smooth” sound that slightly understated Joe’s gruffness for this pop-ish track.

In this application, all the compressors were certainly more than capable, and individual personal tastes varied our opinions. I wanted to go with the dbx 160SL really bad for its nicely open attenuation and a very pleasant frequency response, but the guys pushed harder for the SSL, thus I was outnumbered. I didn’t fret, as 3:1 with a pretty quick attack and auto release provided 8 dB of reduction on peaks, without any nastiness or lack of excitement, plus a touch of some that now-classic SSL color. (Audio webclip #5)

The GML did a fine job, but was perhaps not colored enough to please this rock crowd. The API showed good attitude, but its noise floor concerned me. The Manley sounded very good for my tastes — nicely smoothed and polished — and we liked both its limiting and compression, but just not for this singer on this track.

(For what it’s worth, I was still looking for more containment and a little extra attitude on the lead vocal in the final mix, so I went with that old “secret weapon,” the dbx 160x. You know, the old-school, 1U with the coolest double row of pumping LEDs in the biz.)

On the Drum Bus

In the mix, I grouped all 12 drum tracks to a stereo subgroup on my Soundcraft Ghost, put the squeeze on them, then mixed these contained drums with the normal ones, at approximately 60:40, dynamic to compressed.

The GML was our unanimous favorite here with nearly transparent compression (like, “Is this thing on?”) after I labored over its many options for this app. Not a bit of shrillness, with peaks slightly poking through, yet restrained and consistent, plus a big bottom, the GML gets the nod for its allowance of depth and respect of kick drum. I liked its soft ratio with constant compression and slower time constants for some aggressive density that works very well in parallel.

The API also performed well here, with its classic rock ‘n’ roll bite and eminently useful Thrust feature, the latter of which is right at home with drums and whole mixes.

The Manley was also particularly desirable here, as it did very mix-worthy things in both compression and limiting modes. The dbx was excellent in its own way; oddly enough, I liked this very clean compressor best when pushed hard, limited and mangled into a more aggressive, dirty, rock sound that I couldn’t quite use here. The SSL was also pretty cool in its own way, imparting its very familiar signature, for lack of more original words.

On Final Mix

The many LEDs and VUs of our premium compressor/limiter collection. Photo: Rhon Parker

I spent some time with each dynamics controller on this final mix, running through all the options until my choices were clear. Let it be known: This was not an easy decision, as I found each compressor was capable of improving my mix with some effort.

The API sounded fantastic on this rocking mix, but I never could settle on parameters. The combination of Soft knee/New mode/2:1/Loud Thrust/medium release was fantastic; Hard knee/Old mode worked every bit as well, so I couldn’t decide. How I wish I had such “problems” with all my mixes.

The dbx 160SL again proved to be a little sensitive to maladjustment on my part, but I found the secret combination for mix success. Some 2:1/slow attack/medium release with peak stopper definitely got things denser and nicely compacted. With avoidance of the auto envelope and careful peak stop threshold, I had thickness and density rivaled only by the GML (which also has RMS and peak control).

The GML with its multiple characters and deep control proved to be the trickiest controller of the lot. I almost entered sonic Nirvana with the soft ratio, but found that a 2:1 ratio was better behaved with less pumping. With high threshold/slow timing/the fast crest factor set low/the peak crest factor set high, there was very firm control with excellent transparency.

The Manley Vari-Mu offered the most finished, polite sound of our competitors, with a tendency the band said sounded like “classic rock.” It’s true: with compression mode/medium release/slowish attack/HPF in the sidechain, the Vari-Mu heralded a more analog time period with a smooth, pleasant character.

But ultimately, the SSL got the gig amongst this field of heavyweights. Despite lacking some of the finer controls of the others, requiring gentle tweaking of its tiny knobs, it not only contained levels it also added a subtle sheen, excitement and familiarity (you know, that classic SSL sound we grew up with!) No secrets here — 2:1, moderate attack and release times got the results we were looking for. (Audio webclips #6A and #6B)


Performing differently in most every way, with various features and sonic characteristics, these compressor/limiters are nearly equal in both quality and flexibility. Each is a truly world-class dynamics controller, fully qualified for pro-grade work, and all are way more useful than your typical budget compressor.

Forced to pick favorites, I will give the number-one spot to the API 2500, but only with reservations. Our song was rocking, and API is known to rock very well; it’s no surprise the band gave it top marks. I am floored by the flexibility of this unit and find the Thrust circuit as well as New/Old modes to be dealmakers for those seeking plentiful options in one box.

For my number-two pick, I can’t decide between the surgical precision and advanced control options of the GML 8900 or the sweet musicality and class of the Manley Vari-Mu. These two units are nearly sonic opposites, but their build quality and sonic flexibility make them leaders at specific tasks. For distortion-free clarity, I’ve never heard anything cleaner than the GML; for smooth, gentle, “classic-ness,” I’ve never heard anything sweeter than the Vari-Mu.

I found both the SSL XLogic Stereo Bus Comp and the dbx 160SL to offer extremely desirable sounds in nearly opposing manners. What I like about the dbx (other than fantastic build quality and its very well-conceived design) are its inherent flexibilities and lack of sonic character. With control options at every turn, both compression and peak limiting and largely neutral color, the 160SL is widely capable and flexible.

That said, the SSL is no slouch. It may be comparably simple and not as inspiring/ exciting as the other processors, but what’s under the hood is most powerful — not to mention most preferred — to many ears. The SSL isn’t as widely versatile as the others either (it is designed for “mixes only,” after all), but once it’s in the mix, the whole room smiles in peaceful harmony.

In retrospect, I must state that this Session Trial was a tremendous handful. It could have easily warranted another full day of testing; I honestly still can’t help but wonder if I missed a tasty sonic choice. However, I learned a lot in this evaluation, and I hope the audio webclips referenced here can help you navigate the incredibly subjective waters of premium compression/ limiter options. To be sure, our choices would not all jibe with yours, but that’s the beauty of dynamic-range compression: It’s critical work, but more art than science, and no two engineers will approach it exactly the same way or ever reach exactly the same results.

Rob Tavaglione owns and operates Catalyst Recording in Charlotte, NC.