UAD API 2500 Plug-in Review

With the announcement of the new UAD 9 software comes the release of new plug-ins. This new version sees the addition of three new plug-ins: the UAD API 2500 compressor developed by Universal Audio, The Chandler Zener Limiter, and the Townshend Labs microphone software control. For this review, we will focus on Universal Audio's emulation of the studio staple API 2500 compressor. UAD 9 KMR Author Button Paul Paul Wolff initially designed the 1U API 2500 as a versatile mix buss compressor using an all-discrete circuit path and the legendary API custom transformers and as well as 2510 and 2520 Op-amps and added features that made it stand out from any other compressors at the time. Some of these functions included fixed and variable attack and release times, Feedforward and Feedback controls and a unique Thrust function, which imparted an incredible punch. For this emulation, Universal Audio took an uncompromising approach, by analysing schematics of two versions of this compressor: one rackmount and one in-console versions comparing each design and components to provide the closest emulation possible.

A Truly Unique Compressor

The layout of the plug-in will feel very familiar to users of the original hardware, but instead of laying all the features horizontally like in the 1U rack, the plug-in sees the VU meters moved on top of the compressor controls. This layout also sees the addition of a Mix control for parallel compression, a feature that is not present on the original hardware. Another small difference is an HR (headroom), which replaces the original L/R tilt but we’ll cover that later. Each section is very clearly laid out with the section named COMPRESSOR that includes all the standard threshold, ratio, attack and release controls. The Release control sees a VARIABLE RELEASE control next to it, which changes the behaviour to continuously adjust the compressor’s release time. The Variable release is activated when the Fixed Release control is set fully to the right. API 2500 Compressor types The second section is called TONE and offers controls of the knee, compressor type: feedback (Old) or feed-forward (New) and a unique Thrust feature. THRUST is a powerful feature and works by adding a high-pass filter in the control side-chain of the RMS detector, limiting its reaction to low-end frequencies. However, it does more than that and the three settings offer behaviours that a simple side chain HPF in other compressors cannot do. With three settings Normal, Medium and Loud, this function controls side chain frequencies in unique ways, which avoid unwanted pumping while retaining the punch of the program. In normal mode, the compression works across the whole spectrum and is equal to having no side-chain. In medium mode, the low-end is reduced and high end increased while the medium range is left untouched. The Loud setting generates a gradual linear filter ranging from -15dB at 20Hz to +15dB to 20kHz. This mode dramatically increases the low-frequency punch. API-2500-hardware Next to the Thrust is the Type, which lets you choose between Feed-forward (new) and Feedback (Old) mode. The New feed-forward mode works like modern VCA type compressors and provides harder compression. The Old setting offers a smoother compression but with more character. The Final low-tier is section is called LINK and lets you decide how each Left and Right channels work. Once again API has gone the extra mile by adding a raft of features that go beyond a simple channel link. First is the L/R Link which not only allows you to link or unlink both left and right channel in the detector circuit but also lets you choose the degree of linking ranging from fully independent to 100% linked. These controls are stepped and range between the already mentioned IND (independent), 50%, 60%, 70%, 80%, 90% and 100% linked. What’s more, it also includes a clever Link SHAPE that offers high-pass and low-pass filters to exclude these frequencies from the linking process.

In Use

The API 2500 is a very flexible and comprehensive compressor and truly unique. Since it was meant as a buss compressor for this test, I decided to try it as a buss compressor on drums, acoustic guitar and bass, Piano, vocals and finally on the mix buss. Despite its impressive feature set, the UAD API2500 is a joy to use. Starting with the Compressor section lets you dial in the classic compressor settings very quickly and delivered very musical results straight away. The drums of these songs consisted of very minimal mic'ing techniques with overheads on snare microphone but sounded fairly flat. To add punch and life, I went for extreme settings with setting the ratio in limiting mode, with a fast attack and release and brought the mix knob down to approximately 20%. This setting made the drums incredibly punchy, and I then decided to play with the TONE to improve its performance. Because of the extreme settings I wanted to prevent the Kick drum from affecting the rest of the compression and using the LOUD setting on the Thrust setting helped me achieve that while also keeping the cymbals under control. I also put the LINK control to wholly independent simply due to personal preference. The result is a drum that is full of life and punchy despite having minimal microphones. api-drums I then decided to use the compressor on Claps using much more subtle settings and dialed in a ratio of 3:1 with medium attack and variable release time. It immediately tightened the Claps helping them sit with the snare drum nicely. Because of the nature of the song I also went for a feedback compression (Old) for a more relax feeling which I completed with the HR set to 24dB which seems to add a bit of vintage tone. I then inserted it on Bass where I would usually go for an Optical-type of compressors like the LA-2A or TLA-100*. I set it at a ratio of 3:1 with a long attack of 30ms and medium release, again with a soft knee, no thrust and Feedback mode to reinforce the "vintage" style of the song. Dialing the Mix back to 70% wet re-injected a bit of movement. The bass felt more solid, rounded and engaging. At this point, I started to notice not only a change of movement but also a change of tonality that was building up. I felt the midrange was gaining a lot of definition without me resorting to EQ without the tracks or the song sounding harsh. I tried bypassing the compressors, and as well as the song feeling less glued together, it sounded darker. Moving on to the lead vocals I decided to play with a couple of presets provided (and developed by some of the greatest engineers) and found them to be extremely well thought out and after flicking through a few decided on Jeff Balding's Vocal Easy preset and tweaked a couple of settings. I was extremely surprised how even at the lowest 1.5:1 ratio, it was doing a great deal to the vocal. Although I usually go for harder compression, I felt this worked great. I then moved onto the acoustic guitar and tried Adam Hawkin's acoustic guitar preset, which was a fantastic starting point. I only changed the threshold and the Link to entirely independent. It just made the acoustic guitar sing that little bit more adding a little sparkle along the way. api-acoustic-guitars For the Piano, I used the only preset included for piano and just found it ideal, so I didn't want to do anything to it, except tweak the threshold for it to do a bit less compression. Working on backing vocals I like more assertive settings so went for a 4:1 ratio with the fastest attack time and medium release but using a soft knee to keep the compression always moving around the threshold. Once again I used an Old Feedback style compression setting because it seemed more appropriate to the style of the song. Finally, I added an instance on the mix buss with a subtle 1.5:1 ratio, 3ms attack and fixed 10ms release. Again I used a soft knee to minimise the effect of compression but to keep the low-end punch I decided to use a Loud Thrust setting to let the kick and the bass come through while maintaining the cymbals and high frequencies in check. I also reduced the Headroom a little as I feel it has a more vintage tone. It immediately brought the song together like we come to expect from great compressors is expected to do. While my preference on the mix buss compression to go towards Vari-mu types, I was pleasantly surprised by what the API2500 did to the song. It sounded upfront, controlled, and all instruments sitting together with a characteristic punch that I found very desirable.


Compressors are probably my favourite tools for mixing. I feel they can do so much more than taming dynamics or making things loud. When used properly they can breathe new life in recordings, add character and tone and in some circumstances be used instead of an EQ to achieve the desired tone. I feel this is the case here; not only does the API2500 does a great job at keeping dynamics in checks, but it also adds punch, life and clarity effortlessly. This compressor is a very special compressor that Universal Audio has emulated here, and the result is nothing short of stunning.