API 527 Compressor-Limiter

Possibly the most flexible compressor our reviewer has ever used, the 527 is a “one-stop compression shop.”
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Possibly the most flexible compressor our reviewer has ever used, the 527 is a “one-stop compression shop.”

The API 527 is a discrete, single-channel compressor based on the API 225L. It features variable Attack, Release, Ratio, and Output Gain controls as well as API’s patented Thrust circuit, first offered on the API 2500 stereo bus compressor. Those who know the 2500 or 225L will recognize the similarities since the 527 incorporates many of the same features including the gain reduction process selection between feed-back (called Old) and feed-forward (called New) gain reduction.

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The well-constructed 527 incorporates all surface-mount components and is built into a rugged steel case. The 527’s circuit utilizes an API 2520 discrete op amp as the differential input stage, a 2510 VCA output driver and a second 2520 to drive the API output transformer. Made to install into the 500 Series frame VPR rack, Lunchbox, or an API console with 500 series slots, the VCA-based 527 has continuously variable Threshold (+10 to -20 dBu), Ratio (1:1 to 1:Infinity), Attack (1 to 25 ms), Release (0.3 to 3 sec), and Output Level (-Infinity to +10 dB) controls as well as a 10-segment red LED gain-reduction/VU meter. When set to gain reduction, the LED indication is inverted, meaning that all LEDs are illuminated when no compression is present and as the circuit compresses, LEDs go dark signifying the gain reduction. When switched to read output level, the LEDs illuminate from the bottom to the top signifying output gain of -20 to +3 dB. A +27 dBu overload indicator illuminates when the circuit is on the verge of clipping.

The Soft/Hard switch provides either soft/over-easy type compression, which sounds smooth, natural and transparent or a hard/sharp knee type lending itself to more obvious compression. The 527 is capable of output levels up to +28 dBu through API’s 2520 discrete amplifier and output transformer. The 527’s output level remains reasonably constant regardless of the threshold or ratio control, allowing for adjustments on the fly without noticeable gain changes in the program level. The Thrust function applies a high-pass filter (10 dB/decade slope with a corner frequency of 1 kHz) before the RMS detector circuit resulting in less low-frequency detection, thus preserving the bottom-end punch. The LINK function allows two 527s to be combined for stereo applications via a motherboard bus connection but is not as easy as flipping a switch. It requires wiring a jumper cable within the lunchbox between the slots that will hold the linked units. The “interdependent linking” shares the control voltage so that attack, release, threshold and ratio parameters are shared between units.

In Use

I was never a huge fan of the API 525 compressor, as it always seemed to take too long for me to fine-tune its performance. On the contrast, the 527 is easy to use and extremely quick to set up. Even though the controllability is very flexible, the box just won’t sound bad no matter how you turn the knobs. With the continuously variable controls, this single 500 module is capable of extreme sonic variation.
I initially put the box to work on a lead vocal during a mix session, and I was initially amazed at how the vocal clarity and presence improved with the vocal sitting at the same level in the mix. I went on to use the 527 several times while tracking vocals and was never disappointed. While it doesn’t lend itself to extreme coloration, it is capable of running the gamut from a smooth, transparent R&B vocal to a punchy, aggressive rock sound while providing meticulous gain control.

While recording bass guitar, I found the Thrust feature to be indispensible. When engaged, the 527 provided extreme dynamic control without losing any punch. Decreasing the attack time allowed me to attenuate an overly bright attack resulting from the bass’ active pickups.

Using a Royer R-122 and a Neve 33115 along with the 527 to record electric guitar yielded amazing results. With the Thrust feature engaged I was able to significantly compress a heavily distorted beefy guitar tone without any pumping whatsoever. It worked equally well on a clean guitar through a Vox AC-30.

During my review period, I was able to use the 527 on acoustic guitar, violin, cello and banjo, and I had great results in every instance. This unit is one of the most musical compressors I’ve used, and it works wonders on acoustic instruments.

During a mix, I used the 527 set to 10:1 with a significant amount of compression on the snare track to pull some nearly silent grace notes out of the performance, and the results were amazing. I set up a mono drum bus using the 527 for parallel compression, which resulted in a thunderous drum kit sound. I’ve successfully used the API 2500 for this application many times in the past, and the 527 worked every bit as well. I regret having only one 527 as I’m sure the results would have been even better if my parallel bus had been in stereo. Adding the Thrust function kept the kick and low toms at the front of the mix and eliminated the pumping.

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As with the API 2500 stereo bus compressor, I love being able to instantly alternate between feed-forward and feed-back compression with the New/Old switch. Feed-forward, which works by feeding the input audio into the sidechain detector circuit of the VCA, is the compression method used by many new designs, and the feed-back method operates like most classic circuits by using the VCA’s output to feed the sidechain. I’ve found that there are no hard rules regarding which sounds better in a specific instance, but I tend to prefer the OLD setting on acoustic guitars, strings and less aggressive vocals, and I like the NEW setting on virtually everything else.

I have only two gripes with the 527. The first is with the inverse metering which seems backward. Indicating compression by having the LEDS light up from the top going down would be more familiar. The second is with the layout. The 527 is laid out so the Ratio knob, the largest knob located in the lower right corner away from the other controls, is the easiest control to grab in the heat of the moment. Unfortunately, the control I most often need to adjust once a session is in progress is the Threshold. Typically, I spend a bunch of time getting a sound and then, when the musician or vocalist gets into the performance and starts playing or singing louder, I back off the Threshold so that is the “go to” control that should be quick and easy to grab. I rarely adjust the Ratio once I have a sound I’m happy with and begin recording. These gripes are minor though and thankfully neither affects the stellar sound quality of the 527.


The 527 provides extremely controllable compression with a wide variety of sonic approaches ranging from smooth and transparent to aggressive, bold and colored. In several months of use, I never encountered a situation where the 527 wasn’t able to provide my desired result, making it the most flexible compressor I’ve ever encountered. Anyone needing to expand their range of compressor options should give top consideration to the 527, as it truly is a one-stop compression shop.

Russ Long is a producer, engineer and mixer. He owns the Carport studio in Nashville and is a senior contributor to PAR.

by Rob Tavaglione

Although I didn’t have a lot of time with the API 527, the deeper I got “in use” with it, the clearer it became that this is no one-trick pony. The most important variables here are the old versus new modes and the profound difference between these feed-back and feed-forward methods. Throw in a switchable knee and the essential Thrust sidechain circuit, and you’ve got a complex and versatile dynamics controller that can delight — or mangle if you don’t watch your Ps and Qs.

Acoustic guitars highlighted the 527’s ability to cleanly contain levels without frequency coloration or obvious audibility, all nice and hi-fi with plenty of sheen. I went with the new option (the subtler of the two modes), hard knee, Thrust and 2.5 or 3:1. My only wish was for a lower threshold than -20 to dig in a little deeper and create artifacts before backing off.

On electric bass guitar, the 527 was more forgiving and eminently capable of clean squeezing, but high ratios and “dirty” attempts weren’t all that successful. I really did like the old mode (which generally sounds much more compressed than new), with soft knee and Thrust for downright invisible operation, healthy punch and superb level consistency.

I found the 527 to be very pristine and clear on both drum sub-mixes and whole mixes (although I had to create mono mixes with only one 527 in hand). It is sensitive enough to attack/release times and mode/knee choices to be a bit fickle, but carefully I could get the bottom really thumping, the top end pleasantly open and levels nice and steady without any audible artifacts (actually, a whole lot like the R22 in this app). I used the new mode (old was too processed) with soft knee, Thrust, low ratio and high threshold for ideal density without any frequency voodoo or squishiness. For a slamming 1176-type sound, try high ratios and the old mode for some dirty leveling that works best in parallel.

It was only on vocals I was a little disappointed with the 527, as I was hoping for some heavyhanded “squeezy” fun, but I never did quite dial in a great-sounding, no-dynamic, “over-compressed” thing. The 527 has an auto gain control that keeps output level close to unity as you dial in higher ratios or lower thresholds — not so good here (I tried from 4:1 up to brickwall), as I was bothered by pumping in my noise floor (even with careful attack/release adjustments), inconsistent output and some unnatural artifacts. In fact, you might have to attenuate output to lower than unity to prevent overload. Don’t get me wrong, I received some wonderfully present and “normal” sounds with the old mode, soft knee, no Thrust (to help contain occasional “chestiness”) and 3.5:1 ratio: smoothly flattering, almost ELOP-y and very sexy.

In my very personal opinion, the 527’s higher ratios aren’t as easy to dial in with satisfaction (so don’t bump those knobs on the crowded front panel), and it was sometimes encumbered by that unity gain output control. Conversely, the R22 may not have many non-clean options, but the few it does have were very cool for drums and parallel compression tasks, where a little blending got some very good results.

All things considered, the 527 and the R22 are, in fact, sonic “cousins.” Both work great as Windex-clean massagers of drum submixes, acoustic tracks and full mixes. Both are even trustable on your L/R bus, keep it real with minimal affectation on your sound, and leave a very subtle signature that is quite similar in both units (especially with Thrust). From there, though, these compressors go wildly different ways: the 527 offers limiting and lots of tweakable options, and the R22 goes to 10:1 with far fewer parameters.

Rob Tavaglione owns and operates Catalyst Recording in Charlotte NC. Contact him atrobtav@bellsouth.net.