Home » Recording Resources » Featured Reviews » Universal Audio API 2500 Bus Compressor for UAD-2 and Apollo

A beloved flagship dynamics processor gets the UAD treatment


Review by Alex Hawley

The API 2500 stereo compressor has staked its claim as a modern-day classic. Famous for its unique tone-shaping capabilities, the 2500 has an all-discrete circuit path with two types of compression, a Thrust® circuit, and the versatility to sound great on anything from a drum crush to the 2-bus. I have had the pleasure to work with the 2500 on a couple of occasions, so needless to say, I was anxious to load up Universal Audio’s plug-in version and get to work.

The team at Universal Audio has meticulously modeled every nuance of the API 2500, capturing the details from the transformers to the op amps. API made its proprietary schematics available to the folks at UAD, in order to ensure that every digital nut and bolt faithfully represents its iconic hardware counterpart. The API 2500 Stereo Compressor plug-in runs exclusively on UAD-2 hardware DSP systems, including Apollo interfaces.


So much Tone, so little time

The API 2500’s main section has fixed attack times ranging from 0.03 ms to 30 ms. The release time also includes fixed (50 ms to 2 seconds) or variable options. The ratio ranges from 1.5:1 to infinity:1, and the threshold from +10 dB to –20 dB. There is an indicator light that flashes just above the threshold when gain reduction engages. The 2500 features an auto-makeup gain setting, in addition to the manual makeup gain option—a rarity on hardware compressors.

The Tone section has a few parameters that require a little more explanation. One of the key features that makes this compressor so unique is API’s patented Thrust circuitry. The Thrust control essentially uses a sidechain filter to balance the low and high frequencies of the signal, so that neither side of the spectrum will trigger the compressor more than the other. This allows for more control over the tone, which ultimately helps to maintain punchy low-end transients while minimizing audible pumping.

The Loud Thrust filter is a linear EQ that rises at 3 dB/octave across the full frequency range of the spectrum. The filter starts at 15 dB of attenuation at 20 Hz, and ends up at 15 dB boost at 20 kHz. The resulting signal hitting the RMS detector is completely balanced in terms of energy per frequency octave (this should sound familiar; it’s the inverse of pink noise). The resulting sound creates a significant low-frequency punch, while the high frequencies drive a bit further into gain reduction. However, the overall compression character is a bit more transparent in comparison to a flat-response sidechain filter with an equal amount of gain reduction. I find the Loud filter to be an invaluable tool on the drum bus; it can de-emphasize cymbal harshness in the top end, while adding punch to the snare and chest-thumping lows for the kick.

The Medium Thrust filter has the same slope as the Loud filter in the highs and lows, but is flat in the mids (200 Hz to 3 kHz), The resulting sound maintains a punchy bottom end, allowing lower frequencies to breathe and not slam the compressor, while the high-frequency content gets slightly boosted, which triggers more gain reduction. Since the midrange is left flat, it’s able to cut through a bit more than it would with a typical compressor.

The API 2500 offers two types of compression. The New setting implements a feed-forward design, much like the routing of modern VCA-based compressors. The sidechain input signal is taken from the uncompressed audio signal, which creates more aggressive-sounding compression. The ‘Old’ setting uses a feedback design, which is more typical of vintage compressors. The control sidechain input signal is taken from the compressed audio signal. The resulting compression sounds smoother, with more character.

The final piece to the tone section is the Knee setting, which changes how quickly the signal transitions into compression. Hard snaps into compression the fastest, and is the most common knee type; there’s also Medium and Soft settings.


To Link or not to Link?

The L/R Link section allows for some pretty cool stereo widening, which can be done by unlinking the channels so both the left and right side are compressed individually. For most compressors, and when the link is set to 100%, the sides are fully linked—if one channel has a loud transient, both sides will be compressed equally to maintain center definition and a cohesive stereo image. By gradually changing the stepped Link setting toward Independent, the channels approach being treated individually. L/R Link offers a range of options, trading a wider stereo image for the risk of losing some center definition in your mix or subgroup.

That’s where the Filter Shape comes into play; by applying a filter (high-, low-, or bandpass) to the L/R Link parameter, you are able to shape which frequencies remain linked. This is a great technique to get some stereo depth in the high frequencies while maintaining a consistent image in the vocal range, for example.


Perks of going digital

In addition to all of the same hardware functions (minus the L/R Tilt control, as it is not needed in the digital realm), UA’s 2500 model has added a few features only available with the plug-in version. First is the Mix knob, making it simple to create parallel compression (see Bill Stunt’s tutorial elsewhere in this issue) without any extra routing.

Another digital-only feature is labeled HR, for headroom. This setting lets you adjust the internal operating reference level. Turning the control clockwise makes it easier to push the emulation into higher gain reduction with tasty harmonic distortion, while counter-clockwise will create more headroom before gain reduction engages. By default, the headroom value is 16 dB, but using the HR control it can range from 4 dB to 28 dB.


In use

Once I got the UAD API 2500 up and running in my studio, it found its way into just about every mixing session I’ve had this month, most often on the drum bus. The depth and punch it gives to the body of the kit can add so much energy into the mix, I quickly found myself wondering how I got by for so long without it! This is definitely a piece that rewards practice and know-how to take advantage of its full potential. After spending time getting used to the unique Thrust and Link filters, I now know why it’s such a widely-sought-after compressor for many engineers’ 2-bus.

How does it stand up next to the real deal? Well, even though I don’t own the hardware myself, I was able to reach out to Nick Pelc, a good friend and local studio owner, to help me out. I took a couple of sessions over to his mixing room to see how his hardware 2500 responded next to the UAD emulation.

I started by dialing in the plug-in version on the 2-bus of a session mix that was pretty close to finished. I landed on a 1.5:1 ratio, Medium Thrust setting, and 50% of the L/R Link engaged with the highpass filter to widen the high frequencies. This setting added a nice glue to the mix, with a healthy amount of that API harmonic coloration. Next, after saving the settings, it was time to dial up the same parameters on the hardware counterpart.

On my first pass, the hardware compression felt much more grabby and the gain reduction was driving harder, despite the source material being identical. After further investigation, I learned that this particular API 2500 unit was calibrated to the reference level of –18 dB, and the plug-in defaults to –16 dB. Accounting for this change using the headroom parameter on the UAD emulation, I went back for another comparison.

This time, it was really unbelievable just how close they sounded. The signature 2500 low end sounded nearly perfect on the UAD platform; it had all of the punch and depth of the hardware. There was just a small amount of harmonic character that seemed to be elusive in the emulation—particularly in the high end. I heard the difference mostly in the cymbals; the UAD emulation felt slightly brighter and more airy, but this was only as I painstakingly A/Bed them, trying to nitpick at details in a critical listening environment.

Switching gears to the drum bus, I tried out a wide range of compression settings. There weren’t many settings where the UAD emulation didn’t stand toe-to-toe with the hardware. The Thrust filters translated very accurately. At times, I could hear a slight difference between the two when exploring the Link parameters; in a full mix, I’d be hard pressed to differentiate between them in a blind listening test.



Universal Audio continues to raise the bar with its meticulous emulations of iconic studio hardware. The API 2500 Stereo Compressor was a highly anticipated addition with the UAD Version 9 update, and UA nailed it.

This is one of those special pieces that inspires you to revisit and rework previous mixes. I know it will certainly have a place in all of my sessions moving forward (he says, laughing, as he reaches for his credit card). While I would love to own the actual hardware, it’s great to have the same sound along with the flexibility that comes with working in the plug-in realm. This is one piece that every mixing engineer should have.


Price: $299

More from: Universal Audio, www.uaudio.com


2017 Reviews