Serpent Audio SB4001 Review Serpent Audio SB4001 Review
The original SSL uses quad VCA chips per channel in parallel, plus two in sidechain. It also had DBX 202 VCA chips, the best... Serpent Audio SB4001 Review 4.5

Snake Charmer

It was a crisp, clear day in Nashville at this year’s annual Vintage King Gear Fest. To me, events like this one are arguably more exciting than Christmas, and I would gladly take rows upon rows of the latest and greatest gear over awkward Christmas parties with estranged relatives I haven’t seen since 2004. Booths lined up all the way down the block, with manufacturers from all over the country presenting their shiny (or faux aged) new products to the ever-growing Nashville music industry. I took some time at each booth to demo everything to the best of my ability with a small stage and PA only a short 30 or so

yards away, with the legendary Dave Pensado speaking on basic audio technique loud enough for every studio in the area to hear. I decided to retreat into the quaint, but masterfully equipped Vintage King showroom. As I arrived at the signal processor section I am oh so familiar with. I started making a mental inventory in my head. Tube-driven opto compression… check. 1176 style FET compression, check. Pultec style tube equalizers… check and check. It wasn’t until I got to the 500 series display and a tasty selection of VCA compressors that it hit me. My studio was lacking a great SSL style bus compressor.

History Lesson

So why was I so set on an SSL style buss compressor? For starters, the use of SSL 4000 series consoles is one thing that Nine Inch Nails, Peter Gabriel (now part owner of SSL), Green Day, U2, and Coldplay have in common. It is Chris Lord Alge’s stated mixing console of choice (the celebrated engineer behind a few of the above examples) and even Deadmau5 regularly uses the $4,329 SSL G-Series bus compressor reissue on his tracks from within his decked out modular synth sanctuary. What do most of us have in common? We don’t own SSL large format consoles, and don’t want to dish out almost $4,400 for a single stereo compressor, what a bummer.

Of all of the revered features this console has to offer, from its incredibly musical but surgical EQ, to its ungodly flexible channel strip dynamics section (including my personal favorite snare gate which will silence the bleed from even the sloppiest drum recordings), the most famous is likely the quad bus compressor mentioned above, and many of you are probably quite familiar with its legacy. And when I heard it for the first time, it made me proud to be part British –  just enough to cancel out the tan from also being 50 percent Italian.

Voltage Control

The bus compressor found in the SSL G-Series consoles is a VCA (voltage controlled amplifier) design; using 4 VCA chips in parallel per channel, which I’ll touch on this later in this review. VCA is the same type of compression found in legendary compressors such as the original DBX 160 and 165, API 2500, and Neve 33609… all of which can be found behind some of the greatest recordings of all time. As for applications, SSL G-Series compression is a go to for drum busses, coaxing out larger than life and incredibly punchy drum sounds that can take a generic groove and give it more energy than a tweaked out rave kid on a Saturday night. It also lives on many acclaimed producers and engineers mix buses… with the term “glue” often used, as it has a way of making all of the tracks within a song feel like a more cohesive package. Some even use it for strings and guitars due to its ability to be extremely fast and transparent if you need it to be.



So, it’s 2017 and SSL G-Series consoles haven’t been made in decades but conveniently, there are dozens of choices – both hardware and software.  It can get a little dizzying when there are more options for this style of compression than the not insignificant number of years you’ve been alive. So many great plugin options such as the SSL Series collection from Waves, SSL Duende, Cytomic’s The Glue, UAD’s integration, and these are just the tip of the iceberg. It’s almost as though to even get a job at one of these companies, you are tasked to create some kind of super-cloned SSL plugin. Then, you have the hardware varieties if you’re trying to squeeze out those last few percentage points of authenticity, such as Smart Research’s ‘C2’ and ‘C1’ (Alan Smart formerly worked at SSL during the G Series era), TK Audio’s BC501, SSL’s personal reissues, IGS Audio’s ‘S Type’, the Alta Moda Hippo, the Bettermaker C502V, and what ended up being my personal favorite, Serpent Audio’s SB4001. I’m going to start with a bold statement. “I like the SB4001 more than I like the original SSL G-Series Bus compressor, sue me.” Let me tell you why though. 

Mike at Serpent Audio is well known for capturing the indefinable ‘essence’ of classic units better than even the original companies can ven do in their own reissues, due to years of experience and, what I can only assume, is some unearthly, voodoo magic. He takes the designs, modernizes and improves upon them, then finds a way to release them at a price that is obtainable for the average semi-professional or home project studio. For the SB4001 he took many features that were typically reserved for plugins, and incorporated them into the hardware itself. Like many, I’m a big fan of parallel compression. I use the hell out of it on drums, vocals, bass, electric guitar, and even the mix bus, depending upon the project. But phase can be such a pain in the digital world when using parallel tracks, so Mike was kind enough to put a BLEND (wet/dry) knob directly on the front panel, so that you can blend the compressed signal with the dry signal to your liking without any headache. When given the option, I almost always let at least 10-20% of the dry, uncompressed signal through to keep things natural sounding and, on the SB4001 this is a breeze to do. There’s also a HARD-BYPASS button to quickly and painlessly audition what the unit is imparting on the source material.


If you want to give the signal some color, hit the “GRIND” button to produce some additional THD, though it is much subtler than I expected it to be. Often times it will give you that little something special on the drum or mix buss of a more aggressive project, but it’s far from an Earth shattering amount of harmonic distortion and it sometimes leaves me desiring more. Luckily, we live in a day and age where even converters can be bought with built in transformers, so there’s plenty of coloration to go around. It’s sure a stone’s throw away from the days when engineers couldn’t rip the tubes and transformers out of their gear fast enough, which is a shame, because when I went to buy a vintage input transformer for my LA2A, I realized that these days, you’ll be lucky to find one for anything close to $500 on eBay.

Ok, so tons of SSL-style units, especially in 500 series format have the wet/dry feature and a “THD” style button… so what makes the Serpent unique? For starters, seemingly bomb proof construction and a supremely ergonomic design. Everything you’d want stepped is stepped with ultra sturdy, grade A potentiometers, and everything you’d want variable is variable, i.e. blend, threshold, the variable release function, and gain.

Release Me

Yep, you read that right. This little unit has a VARIABLE RELEASE MODE, so not only can you use the stepped feature like the original unit, but you can switch to VARIABLE RELEASE MODE and dial in the perfect release setting for the tempo of the song. This is especially useful for pop, electronic, hip-hop, and certain rock tracks in which you can use subtle pumping from the compressor to add movement to the song, if you’re into that kind of thing. Even tweaking the settings to record audio examples for this article, I spent way longer than I’d like to admit, tweaking the release ti – there are just that many possibilities.

AUTO RELEASE MODE 1 is the classic SSL setting from the desk, which consists of a program-dependant release that is fast for sources such as transients, pick attack, etc, and slows down if the signal contains slower source material and sections. This is many engineers’ go to release setting on the mix bus, including my own (often times) and is essentially a smart “set and forget” type setting that works on a wide variety of sources. It’s an absolute God-send when you don’t have two hours to fiddle with parameters. I was shocked at how well it handled vocals on a singer-songwriter project I was working on last week, paired with a Cloud Microphones JRS-34 and RTZ 9762 Dual Combo preamp.

My LA-2A was making the JRS-34 (a modern RCA 44 interpretation, with real deal NOS RCA ribbon and the whole 9 yards) thick in the low mids to the point where I was worried about the clarity of the vocal in context with the rest of the mix. The vocals were incredibly dynamic though, and I knew I wanted compression on the way in, so I tried the Serpent. 4:1 ratio, 15mS attack, 90% wet with A1 release. The vocals were not only smoothed out dynamically, but also situated nicely in the front of the mix whilst all of the singer’s clarity and nuance remained intact in a beautifully modern way. Again, there were no audible compression artifacts or side effects whatsoever. For more pop-based music, I can see myself shaving off even more in terms of gain reduction around -6 to -10dB, and turning it down to around 60-70% wet.  

A2 (AUTO RELEASE MODE 2) on the other hand, is essentially “fast mode”. It’s still program dependant but is a quicker and more dynamic auto release mode that has characteristics unique to the Serpent Audio SB4001. For faster tempos, this mode is hard to beat on drums/drum machines and mix bus duties. Rock, double time hip-hop, pop, and electronic disciplines such as DnB and drumstep are a match made in signal processor heaven (Which is a real place by the way residing right next to dog heaven), and hops out of the way before any over-compression occurs.

New Technology

The original SSL uses quad VCA chips per channel in parallel, plus two in sidechain. It also had DBX 202 VCA chips, the best VCAs of their time – but times have changed. Fixing the flaws of the DBX 202, the Serpent uses 2181 VCA chips made by THAT Corporation, which is run by the same guys responsible for creating the original DBX 202s. Talk about authentic!

The benefits of a quad VCA design is that each corresponding VCA chip corrects the previous VCA chip’s errors, allowing you to get extremely transparent compression if you so choose, and many times this is what I’m after. It’s like tasking two close friends to feed and walk your dog whilst you’re out of town, knowing that if one falls through, you still have a backup to get the job done. The SB4001 was one of the only units I could find that used this method, except it uses a dual THAT Corporation VCA in the sidechain opposed to two separate VCAs, which is only a relatively minor difference in comparison to the circuitry of the competitors’ SSL style compressors.

All of this is important because it gives you increased versatility to use it on all types of applications outside of the typical mix bus, drum, and maybe electric guitar uses. Options that I certainly don’t mind come in the form of features and tonal possibilities available from a piece of gear that I have already purchased, and the design of this unit gives you that in spades. In this case THAT 2181 brilliantly captures the vibe of the original, but gives it the possibility of being even cleaner if you so wish, due to a significantly reduced noise floor and lower distortion at higher levels.

Not only that, but I found I could make it even more aggressive than its inspiration – if that is what I was going for. It also has a very impressive stated frequency response of 20Hz-20Khz, +- 0.11dB, meaning that it extends well past our threshold of hearing, in both directions, and puts many dedicated mastering compressors to shame in that regard.


The SB4001 is a game changer on the drum buss or mix buss. I put it to use, so that the kick drum or other low end parts are not overworking the compressor, helping to keep pumping and over-compression to a minimum. I immediately scrapped every model that didn’t have this feature during my search, since I find this to be an invaluable tool while I mix. But the SB4001 also has a Boost and Slope sidechain filter option.

Boost was perfect for this dense mix where we wanted to smooth the attack and high mids of the piano without crushing the transients, and almost acted as a very broad multiband compressor. It produced no audible compression artifacts what so ever when set to a 2:1 ratio, 10mS attack, and auto release 1. Even when I cranked the unit to the point I was afraid I was going to break the VU meter, trying to push the compressor to create artifacts, it wasn’t having it and the gain reduction stayed very transparent and useable.

Boost was my friend on the piano recording, and proves that Mike at Serpent Audio really thought outside of the box to up the versatility this little unit can provide us. What Boost does is produce a 1KHz high shelf boost in addition to a very gradual 60Hz roll off in the sidechain, so that it is more sensitive and compresses more signal from 1KHz and up, and less from 60Hz down. Slope on the contrary creates a sloped filter that has a 1KHz crossover point, which makes the compressor less sensitive to the frequencies at the far ends of the spectrum, many times balancing out an overly present or harsh source, such as electric guitar. I have heard full sized compressors costing much more crap out after 8dB or even 6dB of reduction, so this is really a testament to the SB4001’s design. Would I ever actually use these settings? Probably not, but it really showed what this little guy was capable of. 

I settled at about 4dB of reduction with it set to roughly 85-90% wet. Yep, auto release 1 was not a typo, there is more than one auto release mode on this unit, and it is brilliant.

“I have heard full sized compressors costing much more crap out after 8dB or even 6dB of reduction, so this is really a testament to the SB4001’s design.”


The SB4001 has the typical parameters such as attack ranging from 0.1mS to 50mS, with a total of 12 stepped attack options (0.1mS, 0.3mS, 1mS, 3mS, 5mS, 10mS, 15mS, 20mS, 25mS, 30mS, 40mS, and 50mS) and a stepped release of .03Sec, .06Sec, and 1.2Sec in addition to the auto modes and variable mode, which ranges from 0.1Sec to 1.2Sec (+-10%). It also takes the somewhat restricting and outdated three ratios of the original unit (2:1, 4:1, and 10:1) and doubles them to add the addition of a 1.5:1, 6:1, and 8:1, which opens up a whole new world of possibilities.

I can’t tell you how many times I’ve been working on a track with an SSL plug-in and thought, “shit, 2:1 is too much for this mix”, or “dammit, 4:1 isn’t enough for this drum bus, but 8:1 is a little much”. I really gravitated towards 6:1 for typical drum compression, and then loved the familiar 8:1 for gnarly drum room crushing, where you beat the shit out of the room mics until every nuance is brought out and sounds like it’s lifted straight from a Tame Impala record. 1.5:1 was excellent for sparser and less aggressive styles on the mix bus where only a hint of gain reduction was called for. This allows the unit to be brought into play on mixes where you’d typically steer clear of SSL style compression.

Side Chain It

One other well thought out feature I didn’t utilize ‘till recently was the key input sidechain feature. As if all of the other built-in side chain features weren’t enough, you can plug in an EQ or interface/converter output to the front panel of the unit using a standard TRS connector. This cranks the versatility up to 11 (there is no knob for this), and you can then harness the Serpent to do everything from duck a synth bass with the kick drum for that classic pumping effect, or even use it as an infinitely tweakable de-esser.

One particular singer-songwriter who was very dynamic, was also quite sibilant. Being a soprano, that sibilance really cut through the mix. I specifically chose to use a ribbon microphone for this reason, and the JRS-34 eliminated the majority of her problems, though there were a few phrases where the dreaded sibilance fought its way through. Using my Elysia xfilter, I was able to find the problematic frequencies (around 3.1KHz for this particular singer) plug it into the key input of the SB4001, and boom, top-notch de-esser.

From there, I was able to adjust attack and release settings to taste, and it worked wonders. How many hardware de-essers have a blend knob? Definitely a cool feature in this situation also, since I was able to beat the sibilance back down below troublesome levels, and then let through just enough dry signal that the “ess” and “tst” were still annunciated clearly. Whilst I wouldn’t get the SB4001 specifically for the ducking or de-essing capabilities, it’s definitely a great sidebar to all the other applications this unit excels at.

“This is one of the most transparent vocal compressors I’ve ever used, which caught me completely off guard. I’m talking borderline fader-ride level clean, where it can sound more like automation than compression.”


This is one of the most transparent vocal compressors I’ve ever used, which caught me completely off guard. I’m talking borderline fader-ride level clean, where it can sound more like automation than compression. As someone who abuses the hell out of the automation features in my DAWs to the point where it will likely someday give me Carpal Tunnel, this was music to my ears. Not only that, but I was shocked how incredible it sounded on a stereo piano track I am producing for a country rock album. What helped make this possible were the incredibly useful sidechain filter options. The first three allow you to tell the compressor not to compress anything under 60, 90, or 120Hz.  

Value Judgement

All of this quality and versatility is obtainable for roughly a quarter the price of the official SSL 1U rack mounted reissue, or roughly half the price of the 500 series variation. Though it is priced similarly to other boutique offerings in this category, it is more jam-packed with features than I previously thought was possible in a doublewide, 500 series offering. Mike seems to be proficient in ergonomic designs, as he’s soon releasing his Splice MKII compressor (which is both a REV A and REV D 1176 in a 2U package) in a single space 500 series module, further pushing the limits of what 500 series can accomplish.

Even on extremely loud sources, never once did I experience any audible clipping, which says a lot about the available headroom of this unit (+21 DBU) and the stated dynamic range of >119 dB.  The SB4001 can be brilliantly transparent, even more so than its inspiration, but never in a sterile nor boring way. It’s clean like a John Hardy, Millennia, or Forssell preamp is clean. Iit stays musical and never sounds boring. A tell-tale sign of a great compressor to me is the gain reduction itself being transparent, with the unit offering some kind of enhancement from the rest of the signal path. The SB4001 achieves this with flying colors, and can perform extreme levels of gain reduction while leaving the original signal intact.

Whatever style producer/engineer you happen to be, the Serpent Audio SB4001 can be put to great use in your setup. While I’m still a little surprised by the subtlety of the GRIND button, there are so many saturation and distortion options nowadays that it’s a small gripe with an otherwise outstanding unit. I would have also loved the ability to monitor the input and output gain as the VU constantly shows gain reduction (quite accurately), but with proper gain staging and attention to your DAW’s meters, it is nothing to worry about.

Audio Examples


SSL bus compression has been around since 1987, and it’s not going anywhere in a hurry. No fads or gimmicks here, just high quality components and seemingly endless versatility and features at a reasonable price, all crammed into a doublewide 500 series unit. That it looks gorgeous in a 500 series chassis certainly doesn’t hurt either.

I’ve tried several other SSL style bus compressors since I purchased the SB4001 several months ago, and not once have I desired more than Serpent Audio has provided me. Certainly not enough to consider adding a second SSL flavored compressor anytime soon, as great as many of them sound. A friend of mine who owned a competing unit has since sold it to pick up an SB4001, which speaks volumes as well.

Price and Availability

This unit is available for $1,250 USD or roughly 1,170€, but Mike runs sales on his entire product line a couple times a year (including right now) shaving off a couple hundred USD/Euros, bringing the grand total down to $999USD/936€. click HERE to find a dealer near you

SSL style compression done perfectly, with a slew of incredibly thoughtful added features, some of which unique to this particular unit. Added versatility and applications that go well beyond what you would expect from a compressor of this type. This unit sticks out from an oversaturated crowd of similar units, all without the SSL price tag. I have heard some comment that the front panel is “crowded”, but you try to squeeze that many features onto a doublewide 500 series unit and get back to me.
  • Cleaner OR more aggressive than the original
  • Darn near every feature you could ask for
  • Incredible bang for your buck
  • Some may find Grind a bit subtle
  • VU only meters gain reduction

5 of 5

5 of 5

4 of 5

5 of 5

5 of 5

Michael Frasinelli

Studio Owner, Audio Engineer, and Producer based in Nashville, TN, mentored by and working closely with Grammy Award winning Producer, Engineer, and Musician Randy Kohrs. Highly active analog gear habit, with a plugin hobby on the side. DIY audio enthusiast with an interest in all things circuitry.


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