Welcome back, sound smiths! Last time we skimmed the surface of dynamic range compression, getting familiar with thresholds, knees, attack, and release. This time around, we’ll go a bit deeper into the world of compressors, unearthing the nuances that make each type distinct and pivotal in sculpting our sound.

Picking out the right compressor can feel as crucial and personal as finding that perfect guitar pedal. While they may share similar controls, each one brings its own flavor and character to the mix. So let’s take a closer look at these dynamic tools and uncover the unique essence of each one.

Analog vs. Digital Compressors

Like guitar pedals, compressors started off in the hardware realm, and the effect was achieved entirely from analog electrical components. While many engineers still prefer the hardware, buttons, and sound of analog components, digital technology has come a long in the last decade. The low cost, no maintenance, no footprint, and save-able settings make for a compelling case in favor of ditching the hardware right from the start.

Analog Compressors

Analog compression is where this whole party started, We typically divide compressors into 4 categories:

1. Tube
2. Optical
3. FET
4. VCA

Let’s take a brief look at each how each type works, and what makes them sonically unique.

Tube Compressors

Tube compressors are the old souls of the audio world, known for infusing warmth into anything they touch. This warmth is not just a matter of EQ; it’s about harmonic distortion that adds depth and life.

Tube compression works through thermionic valves (tubes) which subtly distort the signal as they compress. When the signal gets louder and crosses the threshold, the tubes start to saturate. This saturation doesn’t just lower the volume; it enriches the sound with even-order harmonics, giving it a musical and full-bodied character. Tube compressors are known for their slower response, which can smooth out the dynamics in a way that’s less about control and more about tonal shaping. They’re fantastic on sources where you want to preserve dynamic expression while still adding a touch of sonic glue.

For more info on tube compressors, take a look at some of these classics:

• Fairchild 660/670: The big daddy of tube compressors, the Fairchild is as legendary as they come. Often found on master busses and mix busses of countless classic records, its rich, euphonic character and unique compression curves have made it a prized possession in top-tier studios. They are so famous and rare, a single working model can go for over $50k!
• Manley Variable Mu: A modern take on the classic variable-mu design, the Manley is revered for its smooth compression and is often the go-to for vocals and bass that require a sophisticated touch.
• Pultec HLF-3C: While Pultec is more famous for its EQs, this lesser-known gem incorporates tube circuitry to achieve a gentle and musical compression, preferred by those in the know for its subtlety.

Optical

Optical compressors are known for their smooth, musical compression, handling signals in a way that is inherently gentle and unobtrusive.

The core of an optical compressor is the photocell and light source combo; as the input signal increases, so does the intensity of the light. The photocell reacts to the brightness and reduces the gain accordingly. This process is inherently smooth, and the reaction time tends to be a bit slower than other types of compressors, which contributes to the natural compression character they’re known for. Because of this, optical compressors excel at leveling out performances in a way that retains the performance’s natural feel — perfect for vocals, bass, and anything that benefits from a soft touch.

Icons of the Optical world:

• Teletronix LA-2A: The LA-2A is practically synonymous with optical compression. Its optical attenuator, called the T4 cell, responds to the incoming signal in a way that’s famously musical, making it a favorite for vocals and bass.
• Universal Audio LA-3A: A solid-state sibling to the LA-2A, the LA-3A offers a more focused control set and a faster response, perfect for guitars and keys where you want that smooth optical compression but with a bit more bite.
• Summit Audio TLA-100A: This unit blends tube technology with optical compression for a sound that’s both warm and transparent. It’s user-friendly and an excellent choice for those newer to compression or for tasks requiring a delicate touch.

FET

Field Effect Transistor (FET) compressors are celebrated for their ability to add an aggressive edge and punch to signals, making them a favorite for drums and other transient-heavy sources.

A FET compressor uses transistors to emulate a valve’s response. They’re known for their fast attack and release times, which allows them to respond quickly to transients. This quick response means they can catch fast peaks and add punch and clarity, making them ideal for bringing definition to a mix. By taming the peaks swiftly, they allow the rest of the audio to be amplified, effectively increasing perceived loudness without squashing the life out of the dynamics.

The aggressive FET workhorses:

• Universal Audio 1176: When you think FET, you think 1176. With its lightning-fast attack and the distinct “All Buttons In” mode, known as “British mode,” it can add a ton of character to drums, bass, and vocals.
• Empirical Labs Distressor: While not purely a FET compressor, the Distressor emulates the 1176’s FET design and brings additional features to the table, making it one of the most versatile compressors in any studio rack.

VCA

VCA (Voltage Controlled Amplifier) compressors offer precise control, making them versatile tools for a wide range of applications.

VCA compression operates on a simple principle: it uses a voltage-controlled amplifier for the gain reduction element. This technology allows for very precise control over every aspect of the compressor’s behavior. VCAs can react very quickly and with great accuracy, which is why they’re a staple for tasks that require tight dynamic management—everything from subtle bus compression to clamping down on fast, percussive sounds. Plus, their predictability and control make them a solid choice for both studio work and live sound environments.

• SSL G-Series Bus Compressor: Found on the master buss of SSL consoles, this VCA compressor is famous for “gluing” mixes together. Its ability to subtly tuck in the elements of a mix while imparting a punchy sound is why it’s a fixture in modern mixing.
• dbx 160: A staple for drums and percussive elements, the dbx 160 stands out for its simple control set and its very musical form of VCA compression, which adds a certain solidity to whatever it touches.
• API 2500: A favorite for bus compression, the API 2500 is versatile, allowing for both new and old VCA characters. Its ability to shape the tone and dynamics of a mix has cemented it as a go-to for engineers across genres.

Digital Compressors

With our tour of analog compressors concluded, we’re ready to journey into the realm of digital compression, where the emulation of these classic sounds meets the innovation of the digital age. At their core, digital compressors manipulate the digital signal using mathematical algorithms rather than physical components. This allows for perfect recall, total recall, and looking ahead in time, which bring a wealth of new sonic capabilities to compressors.

Inside the digital domain, everything is reduced to numbers. When your audio hits the digital compressor, it’s no longer about voltages and currents; it’s about bits and samples. The compressor analyzes the incoming digital audio and applies gain reduction based on the parameters you’ve set—just like analog compressors, but with the cleanliness and repeatability that come from its digital nature. There’s no noise, no aging components; it’s clinical precision that’s all about the zeros and ones.

Modeling and Emulation

One of the most fascinating aspects of digital compression is the ability to emulate the beloved characteristics of analog units. These digital models use complex algorithms to replicate the behavior of their hardware counterparts—the nonlinearities, the subtle distortions, the way they react to different frequencies and dynamics. It’s like having a virtual museum of classic compressors at your disposal, often at a fraction of the cost and space.

What’s more, digital compressors can go beyond emulation. They can offer a range of sounds and features that don’t exist in the analog world. Look out for digital-exclusive features like lookahead compression, where the compressor anticipates peaks before they happen, or dynamic EQ, which combines compression with frequency-specific attenuation.

Plugin Innovations

In the digital realm, compressors are no longer bound by physical limitations. We see innovations like multiband compression, where different frequency ranges can be compressed independently, or side-chain inputs that allow external signals to dictate the compressor’s action, opening up creative gates for ducking effects and rhythmic pumping that define modern music production.

Some of the most intriguing developments come from hybrid plugins that blend digital precision with analog warmth. These tools allow you to dial in the exact amount of analog character you want, offering a palette that can be as modern or as vintage as the project demands.

Conclusion

I would encourage you all to do a bit of research on the compressors mentioned above, and get up-to-speed on the options for digital emulations and new innovations. Keep experimenting, keep learning, and above all, keep those mixes breathing!