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.
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.
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
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.
As with the API 2500 stereo bus compressor, I love being able to instantly alternate between
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
API 527by 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 at