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API 2500


Hugh Robjohns looks at a pedigree mix compressor with a number of rather unusual extra functions up its sleeve...

They may not be one of the best‑known manufacturers of pro‑audio equipment — at least not in the UK — but API are a very highly respected company. The closest home‑grown equivalent would probably be Focusrite, if that helps to position API in your mind. With a long and interesting history (see 'The API Credentials' box later), the current product line up includes the legendary Legacy recording studio console, as well as several ranges of outboard equipment, including mic preamps, compressors, and equalisers.

The API 2500 bus Compressor under review here is primarily set up for stereo compression, though it may also be used to process unrelated signals, albeit with shared compression settings. It employs API's unique discrete op‑amp circuit building blocks within a fully balanced design in order to bestow a very high‑quality signal path, but it also provides a rather unusual collection of facilities.


The 1U rackmount unit occupies a rack depth of 250mm (not including connectors) and features a matt‑black front panel illuminated by a plethora of traffic‑light LEDs. The blue, extruded aluminium sides provide rugged support for the base, front and rear panels.

Audio connections on the rear consist of XLRs for the inputs and outputs of both channels, plus TRS quarter‑inch sockets for balanced side‑chain inputs (rather than the more usual unbalanced side‑chain inserts). The outputs are transformer balanced and isolated. The IEC mains inlet is accompanied by a fuse and mains voltage selector. The front panel looks, initially, to be quite 'busy', with its seven rotary controls, nine switches, 18 LEDs and two VU meters. Everything is clearly labelled, however, and the unit is very simple to use once you have grasped the operation of its somewhat unusual facilities.

In addition to balanced audio I/O, there are also balanced side‑chain input jack sockets, rather than the more usual unbalanced TRS side‑chain insert points.In addition to balanced audio I/O, there are also balanced side‑chain input jack sockets, rather than the more usual unbalanced TRS side‑chain insert points.

Internally, the unit is packed full of electronics, with a substantial linear power supply and two audio boards mounted one above the other. The discrete op‑amp blocks are evident: 2510 modules handle the inputs and 2520s drive the outputs. The dynamic processing is handled by THAT Corporation chips, and there are four 2180 VCAs and one 2252 level detector per channel.

Overall the construction is to high standards, although I was disappointed initially that the lights used to indicate the metering status did not work. Closer inspection revealed that the LEDs were missing and, on removing the lid, I discovered that the small circuit board supporting the LEDs had broken free — it had been glued to the side of a potentiometer — and was rattling around inside! Not impressive on a machine of this alleged pedigree.


The right‑hand side of the front panel contains the power switch (with blue LED), a screwdriver trimmer to optimise the level balance (±2dB) between channels during compression, and a grey button to select the function of the neighbouring VU meters. The options here are Input, Output and Gain Reduction, the current mode being indicated by the three associated LEDs.

Immediately on the left of the meters are a group of controls which determine the operating condition of the unit. The first button puts the compressor in circuit for both channels, while the second activates relays to engage a Bypass mode. Both buttons have LEDs to indicate their status. The third button (again with its own LED) disables the automatic gain make‑up circuitry provided by the unit and allows the adjacent rotary control to be used to manually correct the output level instead. Up to 24dB of make‑up gain is available.

Whereas most compressors make do with a simple stereo‑linking switch, API have provided a button and a switched rotary control. The button selects one of four filtering modes (indicated by a pair of LEDs) which modify the manner in which one channel affects the other. The modes are low‑pass, high‑pass, band‑pass and flat, and allow the user to minimise unwanted cross‑modulation between channels, for example because of a bass‑heavy instrument on one channel. The rotary switch provides six degrees of coupling between channels, marked from 50 to 100 percent, plus a fully independent (non‑linked) option. I'm not sure quite why this facility is provided, as anything less than full linking between stereo channels will cause the stereo image to wander. However, it may prove of use with independent signals, where one may be used to control the dynamics of another — rather like gentle gating with an external key.

The Tone section contains three more buttons, two with traffic‑light LED arrays and the third with just a pair of LEDs. The first button is labelled Knee and offers three options (Hard, Medium or Soft), determining the transfer characteristic at the onset of compression. The next button is labelled Thrust, and this controls a unique filter stage applied to the level sensing circuitry. This feature borrows patented technology from parent company ATI, where the Thrust circuit has been employed for many years in the Paragon series of live sound consoles. In the API 2500 compressor, the system inserts a filter in the sensing circuit which, it is claimed, results in "a chest‑hitting low‑end punch" and "puts the mix in your face". The three options are for no filtering at all (Norm), a 2dB/octave high‑pass filter (Med), or the full 4dB/oct slope (Loud).

The third button in this trio is marked Type, with the options of Old or New. The Old mode configures the system to use the traditional 'feed‑backwards' approach, where the amount of compression is determined by sensing the output level. This technology is used in many 'classic' compressors and has the benefit that the system operates in a closed loop, so its parameters can be tightly controlled. However, the danger with this system is that a high‑level transient could overload the gain‑controlling element before the sensing circuitry knows anything about it.

The New mode uses a feed‑forward technique based on sensing the input signal directly. This potentially allows the sensing circuit to respond more quickly, and is an approach used by most Dbx and SSL console compressors, for example. In practice, both approaches have various technical advantages and disadvantages, but the point is that they sound different. By offering both options, API have enabled the user to make a decision based on which provides the most musical result.

That just leaves us with the main Compressor control section to the extreme left of the panel, which contains all the usual controls: Threshold, Attack, Ratio, Release and Variable Release. The first and last are continuous rotary controls, while the other three are rotary switches. The Threshold can be set anywhere from +10dBu to ‑20dBu and has a red LED to show the onset of compression — handy if the meters are being used to show input or output levels rather than gain reduction. The Attack switch spans the range from an incredibly fast 0.03mS up to a very genteel 30mS in seven increments. At the fastest setting all transient life is squeezed from the programme, whereas the slowest setting retains transient dynamics almost untouched.

Ratios can be selected from 1.5:1 up to 10:1 and Infinity, again in seven steps, while the Release control offers six preset times from 0.05S to 2S, with a Variable position which passes control on to the fifth knob. This control provides continuous adjustment of the recovery time from 0.05S to 3S and allows the user to modify the release time manually, in order to track the changing nature of the music.

Hopping On The Bus

The API 2500 is fairly straightforward to use when you're familiar with its unusual facilities. I found it worked very well in a broad range of applications involving everything from brickwall protective limiting to gentle mastering compression. It certainly sounds very sweet and clean, with inaudible noise or distortion.

The ability to introduce filtering in the stereo‑linking side chain proved useful with some material, helping to minimise unnecessary gain modulation caused by strong off‑centre signals towards the top or bottom of the spectrum. Changing the knee characteristics resulted in the expected 'softening' of the compression effect, but the Thrust control came as something of a surprise. At its Loud setting, the extra punch created was impressive and seemed to work very well with a wide variety of material, lifting the attack and weight of rhythm instruments in a totally different way to conventional EQ.

The automatic gain make‑up facility made it easy to compare different ratio and threshold settings without having to continually compensate for level changes. However, one trap to be aware of when using manual make‑up gain is that, in the feed‑backwards mode, the level sensing is derived from after the make‑up gain control. With high ratio and threshold settings the output level, naturally, is reduced, but increasing the make‑up gain results in even further compression.

Overall, I found the API 2500 to be a flexible, creative and capable tool with a very distinctive musical quality. Apart from the one aspect of dubious build quality mentioned earlier, my only criticism would be that I would have preferred an automatic release‑time mode, instead of the continuously variable option which was presumably provided at the behest of API's loyal users.

What we have here is a highly individual compressor which compares closely — from the financial point of view — with devices such as the Millennia Media TCL2 valve/solid‑state stereo compressor and the Avalon AD2044 stereo opto‑compressor. While the API's UK price tag is hefty, its unique blend of characteristics certainly makes it worthy of short‑listing if you are looking for a master compressor at this price point.

The API Credentials

Automated Processes Incorporated (API) may not be particularly well known in the UK, but the company possesses an impressive reputation for its very high‑end studio equipment. In the States, API started off in 1968 manufacturing audio broadcast and recording studio consoles, and many are still in daily use over thirty years later!

The company has many significant technical milestones to its credit, including the first computerised console fader automation system (1973) leading on to the first programmable console with automation of EQ, aux sends, pans and faders (1974). This system, far ahead of its time, was eventually known as Total Recall. In 1991 API also produced the first console using touchscreen computer assignment for resetting switches across the console.

API joined the ATI Group two years ago, a group which also owns Uptown Automation Systems, manufacturers of moving‑fader automation systems. ATI has been at the forefront of high‑performance audio manufacturing since 1988 and produces the Paragon live mixing console, and a range of audio processors based around proprietary high‑voltage preamp designs and a patented compressor circuit.


  • Extraordinary flexibility.
  • Unique Thrust control.
  • Configurable side‑chain topologies.
  • Highly musical sound.


  • Slight concerns over build quality.
  • No programme‑dependent release setting.


A well‑specified stereo compressor with some unique attributes which make it unusually flexible. Superbly musical, however hard it is driven.