Compression—we all need it, we all use it. For all stages of the recording process—tracking, mixing and mastering—it is an essential part, as well as an important key to the overall sound of the final product. Sometimes, though, the range and types of compression can be a bit difficult to battle through. So, we're hoping we can simplify that a little bit with this article and touch on basic concepts while looking at some of the most famous and in-demand compressors out there. We'll also look at some alternatives to the originals, both hardware and software. But first, a couple of basic concepts.

How Compressors Work

What, exactly, is compression, anyway? In the simplest terms, compression reduces or, perhaps more accurately, squeezes (hence the name) the dynamic range of a signal, making soft things louder and loud things softer. A compressor is great for keeping an instrument (or a mix) with a wide dynamic range from alternating between overloading and disappearing. Common controls you'll see are ratio, threshold, attack and release, all of which we'll be talking about as we cover the different classic units we've selected here. Note, however, that not every compressor has all of these controls, and some have all of these and more.

Finally, before we dive into examining these five iconic compressors, some of you may be thinking, "Why would I need a hardware (sometimes referred to as ‘outboard’) compressor when there's a perfectly good software version that came with my DAW?" It's a good question and certainly valid. But there is a reason—well, a couple of reasons—and we'll hit the less glamorous one first. While it's not as much of a problem as it used to be, a hot enough signal can still overload the inputs of your audio interface. If you've got an input signal that has sufficient dynamic range for the peaks to do that, you'll get digital clipping, which is not the type of musical distortion most musicians are looking for. It also can't be undone in post-processing. So, a hardware compressor can control those dynamics before you hit the inputs to your DAW too hard and discover you've added an unsalvageable amount of unwanted artifacts to a track bearing a great performance.

The second reason to use an outboard compressor is for when you do want distortion—that meaty grit that adds body and vibe to a track (or to a stereo bus). Especially with compressors like the Empirical Labs Distressor (about which more below), you can dial in just the right level of clipping to liven up a track, while keeping the signal going into the DAW at a nice, sedate level that won't cause trouble. So, you have the best of both worlds at your fingertips.

Enough with the philosophy, let's get to the gear.

The Compressors

The five compressors listed here, along with the software and hardware variants and descendants, are some of the most iconic hardware compressors ever developed. Taken as a group, they have been parts of some of the most celebrated recordings in musical history. While some of these designs date back to the 1950s and early 1960s, they have stood the test of time and are as great today as they were then.

Universal Audio 1176

Two of the compressors on this list come to us via Universal Audio. The first one we'll look at is the Universal Audio 1176. The UA 1176 and its spinoffs (which we'll touch on below) may be the most commonly found outboard compressors out here. The revolutionary feature of the 1176 was its sheer attack speed. Originally designed by Bill Putnam, Sr. to help AM radio stations avoid "splatter" (when a radio signal overmodulates and occupies more bandwidth than it is supposed to have), which required a very, very fast attack, made possible by a design based around an FET (Field Effect Transistor). The current version is based on the classic 1176LN design, which was a refinement of Putnam's original design done by former UREI engineer Brad Plunkett. Plunkett, coincidentally, was the engineer who had originally designed the wah-wah pedal for VOX. It is a small world, especially in the musical instrument industry.

Pictured: Universal Audio 1176LN Limiting Amplifier

The UA 1176 set the standard for modern compression controls, with input and output (makeup) gain, attack and release controls, and four available compression ratios—4:1, 8:1, 12:1 and 20:1. Its speed of response makes it exceptionally excellent on sources with strong transients that need taming, like cymbals, and has a sonic signature with a bit of "crunch." One unique, and unintended, feature of the 1176 is the "all-button" mode. At some point, some nameless engineer decided to see what happened if you pressed all four ratio-select buttons on the unit at once. The resulting sound became part of rock 'n' roll history. The overdriven sound with a compression ratio that fluctuates between 12:1 and 20:1, depending on the input signal, and unique attack and release response, became a big part of the sonic signature of many huge-selling rock records by Led Zeppelin, the Rolling Stones and others. The late, legendary engineer/producer Bruce Swedien famously used an 1176 on every single vocal recorded by Michael Jackson. Small wonder, then, that the 1176 sound has maintained its status as an icon.

“There’s a very specific reason why pieces of gear like the 1176 and LA-2A became ubiquitous,” says producer/engineer Glenn Rosenstein (Ziggy Marley, Patti LaBelle, U2).

Universal Audio has several additional options based around the 1176, notably the 6176 channel strip, which combines an 1176 with their legendary 610 tube mic preamp. They also feature a compressor circuit inspired by the classic 1176 in their Volt "76" model audio interfaces and carefully modeled versions in their UAD plug-in line, for use with their Apollo and UAD Satellite boxes and their native Spark plug-in collection.

UA 1176 Alternatives

With the popularity of the UA 1176, it's no surprise that other manufacturers and software designers have come up with their own hardware and software spins on the basic concept. Warm Audio with their WA76, and Black Lion Audio with the Bluey Modified Blue Stripe, are hardware choices that put you in that "76" ballpark at a more accessible price. And if you're more inclined toward keeping everything "in the box," the IK Multimedia Black 76, Arturia Comp FET-76 and Slate Digital FG-116 Blue Series FET Compressor plug-ins are all worthy of consideration.

Universal Audio LA-2A

Almost the flip side of the speed and crunch of the 1176 is UA's other iconic compressor, the LA-2A Limiting Amplifier. This tube-based, optical compressor uses a light source paired with a photoresistor to get the job done. There may be no compressor that is as gentle with vocals and other sources that respond best to a slower attack and release as the LA-2A. Originally designed by Jim Lawrence, a Jet Propulsion Lab engineer and a radio enthusiast, the key to the LA-2A's sound is the T4 optical sensor, which Lawrence was familiar with from his work in aerospace. Bill Putnam, Sr., was so taken with the design that he bought Lawrence's company, Teletronix, and originally sold the LA-2A under his pre-Universal Audio company, UREI. When UA brought back the LA-2A more than 20 years ago, they put a lot of time and effort into recreating the T4, which hadn't been manufactured for a couple of decades, to ensure the new version maintained that unique sonic signature.

Pictured: Universal Audio LA-2A Classic Leveling Amplifier

The LA-2A is one of the simplest compressors to use, having only an input volume and a gain reduction knob. But the smooth, lush sound of the compressor doesn't really need anything more. The LA-2A’s lush personality is particularly seductive on vocals, and it has been called into service for hitmaking vocalists from Beyoncé, Shakira and Alanis Morissette to Kurt Cobain and Jack White (whose performances on Icky Thump were sometimes compressed to add vibey distortion by producer Joe Chiccarelli).

For many music makers, the LA-2A love encompasses the original hardware model, as well as the many plug-in emulations currently available.

“Some of the first hardware I used for vocal processing were the Teletronix LA-2A, and the Manley VOXBOX and Massive Passive EQ,” says producer Just Blaze (Jay-Z, Snoop Dogg, Beyoncé). “The UAD versions of those are my go-to plug-ins.”

One unique feature of the LA-2A is that the compression is program-dependent. In other words, how quickly it attacks and releases depends on just what kind of signal it's getting. You'll notice that the release profile is quite relaxed and, the louder the original signal was and the longer it lasted, the longer it takes to completely release. While that's not something you'd want for drums, brass or any source that features sharp attacks and shorter sustain, for vocals—spoken or sung—and sources like bass, the compression is amazingly smooth and musical—almost liquid.

Like the 1176, UA makes a channel strip which combines the classic 610 mic pre with an LA-2A, as well as superbly programmed UAD plug-in versions for their Apollo hardware, satellite boxes and the UAD Spark plug-in collection.

LA-2A Alternatives

As with the 1176, there are a number of other products inspired by this classic optical compressor available, both tube and solid-state based. The Warm Audio WA-2A is a fine tube-based implementation with a couple of unique twists of its own. The Golden Age Project COMP-3A and Apogee's Opto-3A plug-in are inspired by the LA-3A, the solid-state successor to the original LA-2A. Once again, for the "in-the-box" aficionado, IK Multimedia's White 2A is a fine software emulation of the original.

API 2500+

Our next compressor is the API 2500+. While API may not be as familiar to the general public as some of the other manufacturers, they've been a top name in professional recording consoles for decades and developed the very first-ever automated fader control system for recording consoles in the early 1970s. The 2500+ is an exceptionally versatile stereo mix compressor that can also be used as dual mono compressors on separate program material. It's also the first of the compressors we're looking at that has a "knee" control. What does knee mean on a compressor? If you look at a representation of how a compressor reduces gain when it hits your set threshold, you can see the gain curve bends. The "knee" is basically how quickly that bend happens when the signal hits the threshold. Hard knee makes a very quick transition, soft knee a slower one. The softer the knee, the less energy gets removed from the transients of a signal as it passes the threshold. It's a very convenient feature if you've got signals that you don't want to instantly squash, but do need dynamic control. The 2500+ also features the patented "Thrust" control, which adds filtering to the sensing circuit resulting in more low-end punch and a more in-your-face final result.

Pictured: API 2500+ Stereo Bus Compressor

The API 2500+ is an excellent bus compressor for mixing or mastering, and it has some unique features like the ability to switch between traditional feed-backwards sensing, where the compressor looks at the output to determine how to apply the compression, and feed-forwards sensing, where the input levels feed the compression. These approaches react differently to different types of program material, and the ability to select which sounds more musical for a specific instance is a big plus. API also makes 500-series "lunchbox" form versions of the 2500, the 527A and the 529 Stereo, with similar features and sonic qualities, though not precisely the same performance.

API 2500+ Software Emulation

There are no hardware alternatives to the API 2500+, but Universal Audio has an exceptional software emulation as part of their UAD plug-in series.

Empirical Labs EL8 Distressor

The Empirical Labs EL8 Distressor, designed by former Eventide engineer Dave Derr, was one of the first compressor designs aimed at the world of digital recording. Using digital control of analog processes, the Distressor has a huge bag of tricks. For one, its compression knee changes with the selected compression ratio, with a soft knee on lower ratios, getting gradually harder the more control you need over transients. A high-pass filter in the Detector section lets you control that drum kit without losing the punch of the kick, and a mid-boost that's available pre-compression can help remove mid-range harshness from distorted guitars. Speaking of distortion, one of the things that many mix engineers love about the Distressor is its ability to add analog distortion to a signal, subtly or not. With selectable settings to add even-order or odd-order distortion, to emulate tube or tape saturation, respectively, and incredibly flexible linking capability, there's a reason why the Distressor was quickly elevated to the status of a must-have unit in many studios.

Pictured: Empirical Labs EL8 Distressor Audio Compressor

Empirical Labs also makes the EL8X, a Distressor that adds "British Mode," which features a similar effect to the 1176 "all-button" setting for a unique compression sound.

EL8 Distressor Software Alternative

Like the API, the Distressor has no hardware competition, but, once again, Universal Audio has an excellent plug-in version as part of its UAD series.

Solid State Logic The Bus+

Solid State Logic (SSL) made their reputation on the recording consoles they first developed in the 1970s. A big part of that reputation was based on the integrated bus compressor that was a part of those boards, and many engineers in the years since have relied on those compressors to get "the sound." The SSL The Bus+ is a two-channel compressor combined with an analog dynamic EQ that builds on that legacy. A digitally controlled analog compressor, the Bus+ replicates the sonic personality of several different generations of SSL bus compressor, from the grunge of the 4000 Series boards to the clean, smooth punch of the 9000 Series. It would require a lengthy article on its own to really dig into the wide range of features here, but a standout is the addition of an analog dynamic EQ. This EQ acts like a compressor below a set threshold and an expander above it, enabling subtle (or extreme) changes in tonality based on a source's level, and is an incredibly powerful and versatile tool.

Pictured: SSL The Bus+ 2-Channel Bus Compressor and Dynamic Equalizer

SSL also provides a 500 Series version of the classic G Series stereo bus compressor, as well as a native plug-in version.

SSL The Bus+ Alternatives

Inspired by the SSL, the Warm Audio Bus Compressor delivers a more affordable stereo bus compressor that works well for mix or mastering. On the software side, Universal Audio, once again, has an excellent emulation of SSL's legendary bus compression on its UAD plug-in platform.

Final Words

So, there you go—a quick guide to the compressors that you really should be considering adding to your recording setup. In addition, if you're concentrating on getting a great vocal sound in your studio, you should check out our article, Building Out a Hardware Vocal Chain, for more on compression and some extra clues for excellent vocal capture.

A final thought about analog outboard gear in general, especially if you often find yourself fiddling with things in a final mix, then going back and trying something else. Paying more attention to how things sound at the front end of a recording project and making sure that it's what you want before you hit that record button may seem restrictive because you can't "uncompress" a track that was printed with outboard compression. But it can actually be freeing. By getting the sound you want up front, the final mix process can be more about nuance and vibe than about tweaking sonics that weren't quite where you wanted when you tracked. And that can be a very good thing. So instead of "fix it in the mix," try and make your personal studio mantra, "fix it in the room."