The online audio community always cheers when Universal Audio announces a software update with new plug-in emulations. This happened again when the company announced UAD version 9 with the introduction of UAD API 2500. Developed using "for your eyes only" schematics, UA's team of DSP experts modeled everything from API's custom transformers to the 2510 and 2520 op-amps, resulting in a faithful recreation of one of the most popular and versatile compressors around.
Based on the in-console version of the hardware, UAD API 2500's front panel should look familiar to anyone who has used a hardware unit or other plug-in variant. The usual compression controls are there — threshold, ratio, attack, and release (which can either be set to variable or one of six fixed time constants). Where UAD API 2500 and the hardware that it emulates start differentiating themselves from other compressors is in the Tone section. A Knee control changes the character of the compression onset between soft, medium, and hard. Next is the Thrust feature, API's patented circuit, and in my mind, one of the 2500's shining features. Thrust inserts a proprietary filter at the input of the RMS detector, reducing the compressor's response to lower frequencies and boosting the incoming high frequencies entering the sidechain. In Normal mode, there is zero filtering. Medium mode attenuates lows and boosts highs, but leaves the midrange flat and intact. Loud mode employs a filter that is based on the principal of equal energy per octave (in the same manner our ears hear music), which is realized by a gradual filter that is down 15 dB at 20 Hz, and up 15 dB at 20 kHz. The result is a punchy, midrange-forward sound with reduced pumping and a noticeably tighter, more controlled top end and upper midrange. Lots of plug-in compressors have sidechain filters, but none that I've heard do quite what Loud does. Expansive left/right link capabilities and auto makeup gain are both equally useful as well. Lastly, Type switches the routing of the sidechain signal between New (feed-forward) mode for harder, VCA-style compression, and Old (feedback) for a smoother, more traditional style of compression. Blindly selecting between the two, I almost always preferred the Old setting for mix-bus purposes, but liked New when inserted on individual instruments, bass, and drum groups in particular.
Also available are two UAD-only additions not found on the original hardware: a Mix knob, and Headroom control. Headroom manipulates the internal operating level of the plug-in, providing greater control over the amount of compression and coloration. And though the Mix knob does not sound particularly sexy, in my mind, it's a necessity for the 2500. The unit often sounds the best to me when pushed into saturation and heavy compression, so being able to dial that in really expands its applicability.
In the past, I'd (ignorantly) only used the API 2500 and its other plug-in variants as crush boxes — perfect for room mics or drum buses, but often too colored for everyday mix-bus use. I remember trying the hardware version across a mix a few years ago and not loving it; it added a certain impact to the lower midrange that would've been great for a more aggressive track, but wasn't right for that particular mix. UAD API 2500 remedies that limitation for me, again thanks to the headroom and mix controls. I recently mixed a folky, Mazzy Star–esque EP, and I used UAD API 2500 on the mix bus for every song, changing only the release times and the dry/wet blend. It sounded fantastic. On the other end of the spectrum, the Loud feature mentioned above worked wonders on rap mixes, gluing the vocal and snare together in a way that sounded closer to "finished," without obscuring the TR-808 sounds. This is now the first thing on my mix bus. As for subgroups, I still think you'd be hard-pressed to find a better drum bus compressor.
It's safe to say that Universal Audio's in-house plug-in emulations are in a league all their own, and UAD API 2500 certainly continues that tradition. It's versatile, unique, and adds the punch and character you associate with API products. And at 1/10th of the price of the original hardware unit, I'd suggest giving it a go.