How do you choose the right compressor for the instrument you’re working with?
Are there better and worse compressors to use on specific sound sources like vocals, bass, drums, or mix bus—or is it all just a matter of taste? And let’s talk about the elephant in the room: WHY are there so many compressors out there to choose from in the first place? This is a reasonable set of questions to ask.
So many models of compressors have been released over the years, both in the analog and digital worlds, that it feels like it could take ages to get to know all of them. Plugin Alliance alone offers well over two dozen compressors in its Mega Subscription Bundle—many of them modeled after some of the most iconic and coveted analog compressors ever designed. (And some of them, totally new inventions).
How are you going to pick your new favorites from among them?
Easy: Simply get familiar with each of the main types of compressors. It’s the fastest path to being able to choose the perfect one, every time.
The truth is that despite all the variety in compressors out there, there are really only a small handful of compressor types in existence.
Once you get to know each of these compressor types intimately, and start to understand their distinct strengths and weaknesses, every compressor you see just becomes a subtle variation on these main themes.
This makes it so much easier to reach for exactly the right compressor the first time, and makes understanding compression in general much less daunting. Let’s get into it.
An Overview of Compressor Types
If you first got into audio or music production during the digital age, some of the first compressors you ever saw were probably the ones built into your DAW. Almost every DAW has an example of what I like to call a Fully Variable Digital Compressor built right in as a stock plugin.
This type of compressor usually offers dedicated controls for attack, release, threshold, ratio and makeup gain. Many of them will include additional bells and whistles like knee control, sidechain filtering, and maybe even a wet/dry “mix” knob if you’re lucky. This type of compressor has the benefit of being both very flexible, and very inexpensive. (The added cost to you is usually zero.)
But for all they offer in tweakability and cost-effectiveness, they have their weaknesses. One of the most common complaints about these fully variable digital compressors is what they lack in tone. In general, they simply don’t offer the kind of character and “vibe” that old school analog-style compressors do. But it’s not just mystical mojo that’s missing from most stock compressor plugins. It’s also ease of use.
To get the most out of stock digital plugins with their fully variable control over every parameter imaginable, you often have to be willing to tweak and tweak—making exactly the right choices for attack, release, ratio, knee and the like. It can feel just as easy to make them sound bad as it is to make them sound good, and they run the danger of leading to a workflow that inspires more second-guessing than creativity.
But working with some of the most popular analog and boutique compressors is a totally different experience. Throw up a classic analog modeled compressor, and they often just sound right, without touching a single knob. The intrinsic “box tone” of a good analog compressor or emulation can add a touch of intangible sheen, polish, attitude, growl or grime, seeming to do the work of several plugins in one, before you touch a thing. And, when you do go to tweak them, there is often a fairly small set of carefully curated controls, designed to help steer you in the right direction.
This is another big part of what makes classic compressors classic. They make the process less visual, less variable, and bring a set of welcome limitations to the table that tend to inspire a more intuitive and creative way of working. If a fully variable digital compressor is like mixing your own paints from scratch, choosing a classic analog-style compressor is like picking out the perfect shade of colored pencil. Pick the right shade, and it just works. If it doesn’t, pick another one. But just like there are only a handful of primary and secondary colors that make up all the shades in the rainbow, there are just a few primary and secondary compressor types.
The Primary analog compressor types that make up the overwhelming majority of the most popular compressors ever made are Opto, Vari-Mu, FET and VCA.
The Secondary compressor types include more uncommon analog designs, like Diode Bridge and PWM, and a handful of Speciality compressor styles as well: Multiband, Limiting and Upward compressor types. Let’s look at them all.
It’s easy to start with VCA compressors, because they are the classic compressor type that are closest in operation to the kinds of stock variable compressors you’ll find in every DAW. Compressors based around a VCA (short for “voltage control amplifier”) were arguably the pinnacle reached in analog compression technology before digital took over. They were one of the last types invented, and finally made it practical to offer new forms of control rarely seen on earlier compressor types. The kinds of independent control we take for granted today over attack, release, ratio and threshold were first made popular by analog VCA compressors. But classic VCA compressors and their emulations offer two distinct advantages over their more modern sanitized counterparts:
- Although they offer more variable control than earlier analog compressors, the limitations and tradeoffs of the analog domain often requires that designers think very carefully about just how much control to offer their users. In many cases, this leads to high-end designs with selectable attack, release and ratio settings, rather than fully variable ones.
With just a handful of settings available on some of these knobs, gear designers had an incentive to select a small number of the most useful and pleasing-sounding options. And, with these handful of settings used on countless hit records, helping each of them sound instantly “familiar” to our ear. A good example is the selectable attack and release settings found on the legendary SSL-style bus compressors, modeled by Brainworx in the popular bx_townhouse compressor. The attack times of 1, 5, 10, and 30ms each have an iconic and distinctive sound that encourages listening for more meaningful, bigger-picture changes in sound. The same goes for the release settings, which also features an exceptionally smooth-sounding, program dependent Auto mode that has been heard on countless major releases.
- Relative to the average stock compressors found in DAWs, these kinds of compressors have some built-in character. While not as colored as some types of classic compressors, the most popular VCAs add a little something special to the sound. The SSL style like the bx_townhouse are known for their tight, slightly forward and subtly aggressive sound, while the API style, exemplified by the Lindell Audio SBC, is often thought of as being assertive, clear, open, and punchy all at once. Each good VCA compressor is a variation on this theme with a unique personality of its own, from the snappy Vertigo VSC-2 to the full Maag Magnum K to the sleek and smooth elysia alpha compressor.
Some VCAs offer a completely unique take on what a compressor can offer, like the elysia mpressor, with its influential “Comp Limit” control, or the stunningly transparent Amek Mastering Compressor, with its unique control over dedicated slow, fast, and peak detector circuits, making it like 3 compressors in one.
The flexibility of VCAs makes them like the chameleons of classic compressors, suitable on practically any source. They are often cherished on mix bus, percussion, bass, and acoustic guitars. With the right settings, they can work well on vocals, and despite its name, the Amek Mastering Compressor is an absolute favorite there.
At the opposite end of the spectrum are Opto compressors, which are among the earliest types invented, and often some of the least flexible of the classic analog types. What they lack in tweakability however, they more than make up for with character, flavor and style.
These compressors use an internal light and a photoreactive cell. The hotter the signal, the more brightly the internal light glows, and the more vigorously the photocell tells the compressor to clamp down. Due to the unique physical properties of these components, practically everything in Opto compressors is originally “program dependent”, meaning the exact ratio, attack and release would depend on the signal running through it. For instance, the ratio on an Opto compressor might be higher for signals higher above its fixed internal threshold, and lower for more modest signals. Its attack and release might be faster for signals above a certain level, and slower below it.
On average, their attack and release times would tend to be slower than those of most other classic compressor types, at roughly 10ms for the attack, 60ms for the first 50% of release, and potentially up to a second or more for the rest of the release. Ratio tends to average around 3:1. This made these compressors focus more on average level than responding quickly to transient peaks. While this potentially limits their usefulness on percussion, it makes them a perennial favorite on vocals and bass in particular, and on those occasional acoustic guitars that want a little more retro jangle and chime.
In addition to these compression characteristics, the most iconic Opto compressors include tube components that give them a bit of extra earthy growl and saturation when pushed hard, for an organic, rootsy feeling.
The Acme Audio Opticom fills this role perfectly, offering smooth Opto compression with just a little bit of welcome tube grit, adding a variable speed control to tilt the attack and release times more in the desired direction, to make it useful on even more sources.
If you love the fat, smooth sound of Opto compression, but don’t want as much of the tube grit, the bx_opto is possibly among the smoothest, cleanest, simplest and most useful classic-style Opto compressors available.
Cleaner-sounding Opto compressor circuits also factor into the Millennia TCL-2 compressor, Shadow Hills Mastering Compressor and Tomo Audio Labs LISA Dynamic EQ.
FET compressors are the ying to the Opto compressors yang. Designed later in time, in part to make up for the Opto compressor’s perceived shortcomings, these solid state devices have variable attack times that can go from very fast to incredibly fast, selectable ratios and a wide range of release times.
These kinds of compressors, exemplified by the UREI 1176 offer assertiveness, speed, control, and the option to “pump and breathe” in a unique way that earlier compressors simply couldn’t.
At Plugin Alliance, this role is filled by the MC77 FET (“field effect transistor”) compressor from Purple Audio. It’s an exacting recreation of the beloved “E revision” of the original 1176. The Lindell 7X-500 offers an even more modern sounding take on this approach.
These types of compressors are prized on male vocals, bass, drums (especially in parallel or on room mics), acoustic guitars that want tight, fast transient control and more. They can give incredible character even at extreme compression settings, and despite being designed to potentially replace Opto compressors, they turn out to couple with them beautifully well when stacked in series.
Along with Opto compressors, Vari-Mu compressors are among the earliest designs, and unlike Optos, use tubes to control the compression itself. Similarly to Opto compressors they tend to have relatively low ratios and soft knees, but their nominal attack times tend to skew a bit faster, and they will often have some control over the timing of attack and release.
Another big part of the appeal of these units is in the tone of both their tube circuitry and transformers. These compressors can sound wonderfully warm, full, round and weighty, often without sacrificing detail. The perform splendidly well on vocals, mix bus, retro style acoustic guitars, and warming up sterile sounding synths and samples.
In the Plugin Alliance family, this role is primarily served by the SPL Iron, based on a modern high end design that marries the personality of classic tube compressors with far greater control and refinement over settings, and the option of switchable transformer options.
Neold’s V76U73 channel strip also includes a Vari-Mu compressor based on a much loved Telefunken design that offers a bit more color, saturation and gorgeous growl, with slightly simpler operation.
In addition to these primary types are few others:
- Diode Bridge compressors were most successfully marketed by one brand—Neve. These compressors offer many of the same benefits as VCA compressors, but with a sound that is a bit fuller, smoother, creamier and chunkier than most VCA compressors. They are absolutely wonderful on drum bus, mix bus—all manner of buses really.
The Lindell Audio 254E fills this time beautifully in the mega subscription bundle.
- PWM (pulse width modulation) compressors are perhaps among the rarest, cleanest, and most expensive to build of the significant types of analog compressors. As so few of them were ever found in the field, and some users believe they offer fewer advantages over modern digital designs due to their clean sound, they have been modeled less extensively than other types in the digital domain.
- Multiband compressors split the signal up into multiple frequency bands, each of which is compressed differently. They work wonders for solving problems in poorly recorded sources and already processed stems, and can also elegantly enhance even the best of sounds. They allow you to focus your compression on where you need it most, and back off where you don’t, potentially allowing for more compression in target areas with greater transparency overall.
The Lindell Audio 354E and MBC offer two different flavors of this kind of compression for the Mega Subscription bundle.
- Dynamic EQs are basically multiband compressors that allow for even more targeted compression by using the interface of an EQ to select and separate frequency bands. The Tomo Audio Labs LISA is the only analog example known to exist, and its divine-sounding Opto compressor-expanders have been expertly modeled for Plugin Alliance.
- Limiters are essentially compressors set to stun. With super fast attack times and ratios of 10:1 to infinity:1 they will lock your level in place and help you get much greater loudness. The bx_limiter and bx_limiter True Peak achieve this for Plugin Alliance. The latter is capable of capturing intersample peaks, essential for modern masters to translate properly to contemporary playback environments.
- Upward compressors are basically unheard of outside the digital domain, and barely ever heard of within it! Unlike conventional compressors, which reduce the the loudest parts of the sound source and then turn the whole thing back up with makeup gain, Upward compressors do practically the opposite. They take the quietest sounds, below the threshold, and turn those up directly, never touching the loudest peaks. This potentially offers more dynamic control with greater transparency. It can be found inside of ADPTR Audio’s Sculpt in the MEGA Bundle. It has potential applications in mastering, bus processing and new, as of yet undiscovered compression techniques.
And with that, we’ve covered… all the types of compression there are. Hopefully, this mental framework will help you think and feel your way through making even more seamless compressor choices in the future.
The best way to get an even better handle on these compressor tykes however, is to start trying them out for yourself on your own mixes. And when you find your own favorites… let us know about them.