The 500 series form factor, first innovated by API in the 1960s, has established itself as one of the most popular and convenient form factors for outboard gear—get yourself an API Lunchbox or an equivalent and you can mix and match modules, swap gear in and out, and customize the chain of your outboard gear in whichever way you please, all in a portable form factor that you can carry from studio to studio with ease.
500-series modules range from unique and original hardware designs to emulations, clones, and even near-exact recreations of classic outboard gear from throughout recording history. In this entry, I'll round up and discuss some of the most impressive 500-series compressors we have to offer at B&H, covering some different compression types. If you want an overview of different kinds of compression and a list of some legendary rack-mount studio gear, check out our compressor guide before moving on!
FET Compression—API Audio 525A 500 Series
The original API 525 was one of the first 500-series compression units first released in the 1970's and reissued in the early 2000's and quickly became one of the most celebrated pieces of recording gear in history. The more recently developed API 525A maintains the discrete circuitry of the original compressor, along with some of its unique features, particularly its analog VU meter and especially its "ceiling" knob. The ceiling knob can be used in conjunction with the threshold and make-up controls to increase or reduce gain reduction and output level simultaneously. This means you can control the amount of compression in your signal without having to worry about changing the level.
The API 525A's expanded features include variable attack times for 15 microseconds to 15 milliseconds (FETs are lightning fast!) and seven selectable release modes from .5 to 2.0 seconds. With this simple but significant change, the API 525A brings the classic API sound into the modern era.
Diode-Bridge—Rupert Neve Designs 535 Diode-Bridge Compressor and Portico 543 Compressor/Limiter
Neve is lauded for their legendary preamps, EQs, and consoles, but their compressors are comparatively slept on. With a reputation like Neve's, you shouldn't overlook anything they create. The Rupert Neve Designs 535 500 Series Compressor is designed as a recreation and update to the classic console-integrated (and later rack-mounted) Neve 2254 compressor released way back in 1969.
The luxurious and silky (and expensive) Neve sound is maintained in the Neve 535 500-series unit. It has all the typical controls for threshold, attack, release, and ratio, but Neve has added updates to the 535 like expanded timing controls, stepped controls for easy recall, internal parallel processing (via a wet/dry "blend" knob), a side-chain high pass filter (12dB/octave at 150Hz), and a lower overall noise floor. All this along with stereo-link capability makes the Neve 535 an impressive module to add to your arsenal.
If you need the same versatility but with a brick-wall limiter, you might want to go with the Portico 543. This single-spaced 500-series compressor is based on the rackmount 5043 model and is one of the most versatile compressors in this format. In addition to the typical controls over threshold, attack, release, and ratio, you can change the detection mode between the vintage-styled feedback mode to the faster attack time of feed-forward. Add to the ability to change detection from RMA (or average) to Peak, increasing what kind of content the 543 can shine on. Add stereo-link ability, and the ability to disengage the compressor completely and use the module as a line amplifier, and the 543 can be at home in any studio.
VCA—Solid State Logic E-Series Dynamics Module and G-Comp
You can't talk about compression and not mention SSL. The company's Stereo Bus Compressor (naturally, also available in a 500 Series version) essentially popularized the idea of master bus compression in the late '70s and throughout the '80s and is still used as the glue on countless records made today. If you are after classic SSL dynamics, the single-channel E-Series Module offers the compressor/limiter/expander/gate found on the SSL 4000 E console.
VCA—EMPIRICAL LABS PUMP
Empirical Labs is known for their prolific, digitally controlled analog compressors, most notably the EL8X Distressor. The Emperical Labs PUMP is its 500-series style younger sibling, providing the same VCA style compression that delivers smooth transparent 2:1 glue to oppressively crushing 20:1 soft-limiting.
The PUMP keeps some of the Distressor's best features while throwing in some excellent new ones, making it a powerful tool in a tiny package. Similar to the Distressor, the attack and release controls are digitally stepped knobs that allow for prefect recall of settings. The added "AtMod" feature reveals a bit more transparency and impact for any transients by slowing down the initial slope of the attack envelope, preserving the sharp thwacks and bangs in your audio—excellent for aggressive guitars and basses and perfect for drums.
Vari-Mu with Digital Integration—Wes Audio Rhea
The Wes Audio Rhea is an incredibly functional vari-mu stereo compressor taking up two spaces in the rack. It is emblematic of the vari-mu sound, giving smooth and powerful leveling to a signal—perfect for vocals. Besides the digital integration (see below), it comes with some convenient features like a mix control for parallel compression, three sidechain filter toggles, and true bypass.
With its DAW and plug-in integration, the Rhea offers true tube-based compression in a modern hybrid form. Download the included ng-series control plug-ins and you can run a program that can communicate between hardware and software: control the hardware compressor via the software and vice versa, program automation, and save perfect static or dynamic digital recall settings. The Wes Audio Rhea is perfect for anyone looking to build out a hybrid studio workflow.
Optical—Chandler Limited TG Opto Compressor
Another Dual-space 500-series module, the Chandler Limited TG Opto Compressor harkens back to the compressor and limiter found in the EMI TG12345—the legendary console used in Abbey Road Studios in the late 1960s— especially with its simple layout and classic four-color chicken head knobs. Speaking of the knobs, most opto compressors are program dependent, meaning you can only control attack and release via the amount of level you send to the input. The Chandler Limited TG Opto has attack and release knobs and a toggle switch for adjusting the knee, making for an opto compressor with an impressive amount of control.
The Chandler Limited Opto's internal design and external interface make dialing in soft and wide compression easy to accomplish with your ears. Those looking for a classic, refined, and subtle compressor should check out this one.
Conclusion
There are a ton of manufacturers producing 500-series compressors. Unfortunately, we can't list them all, but if you are in the market, do check out the current offerings from our extensive 500 series listings on our store page. Hopefully, this article has opened your eyes to the vast array of dynamic circuits, and you're on your way to building the perfect 500-series setup. For a complete overview of what the 500-series format offers, check out our other articles on rack enclosures, microphone preamps, compressors, and effects.
