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API 5500

Dual-channel Equaliser
Published May 2007
By Hugh Robjohns

API 5500

API remind us that the best analogue gear is still that bit more classy than some digital recreations...

API — Automated Processes Inc — have been working at the highest end of the analogue audio business for the best part of 40 years, with a long pedigree of consoles and outboard equipment to their credit. Indeed, 'discrete analogue' and 'API' are almost synonymous!

One of the company's longest-standing designs is the 550 Equaliser module, which dates back to 1967, when it was designed by API founder Saul Walker. It was originally conceived as a modular equaliser for use in custom studio console designs, which were the norm at that time. The two key elements in its success were the use of bespoke and all-discrete '2520 op-amp' gain stages, and the application of a 'proportional Q' response. Proportional Q means that the filter bandwidth narrows as the amount of boost or cut is increased, allowing both surgical precision and gentle musical sweetening, just by varying the gain control. Another claimed advantage of this approach is the minimisation of the 'phase-shift sound,' which afflicts many equalisers. Personal experience confirms that proportional Q certainly does lend it a very attractive and musical sound quality, combined with almost subconscious operation.

Another important aspect of the design (an industry first at the time) is that the cut and boost curves are reciprocal in their responses, meaning that you can precisely 'undo' any tonal modifications applied previously: a handy side-benefit to have! The circuitry also has a massive headroom of over +30dBu — twice as much as most professional audio equipment — which allows the use of extreme EQ settings, when necessary, without fear of internal overload.

When API started producing their own console range in 1971, the 550 module became the standard channel equaliser, and its reputation extended far and wide. Popular demand eventually forced API to put the 550 back into production in 2004, as the 550A, along with a four-band 550B version. The original designs had unbalanced interfaces, since they were intended for inclusion within a console, whereas the current stand-alone versions have balanced interfaces, but other than that the designs remain the same as the originals.

The latest addition to the family, launched at last year's AES Convention in San Francisco, is the API 5500: a dual-channel version of the 550B four-band equaliser in a compact, 1U, rackmount case.


API state that the 5500 is designed for tracking, mixing and, perhaps surprisingly, even mastering applications. This two-channel equaliser features the same kind of all-discrete signal path as other API products, and expands the functionality and sound quality of the existing 550B four-band EQ with the gain-setting precision of the specialist mastering versions of the equaliser.

The four frequency bands have stepped, but broadly overlapping, frequency ranges, spanning 30Hz to 20kHz. The gain-control steps can be switched from API's standard ±2dB per step, to either ±1dB or ±0.5dB options, for very fine resolution indeed. This is really a formal implementation of the factory modifications for mastering versions (the 550D and 550M, respectively), and certainly increases the versatility of the equaliser enormously.

Each channel has an actively balanced input via both XLR and TRS quarter-inch socket (plugging into the latter disconnects the XLR feed). Unbalanced inputs are accepted without problems and the input impedance is rated at over 100kΩ. The input buffering is handled by a discrete API 2510 operational amplifier stage, which is similar to the better known 2520 design, but lacks its high-current output stage. A transformer input option is also available.

Each channel has an actively balanced input via both XLR and TRS quarter-inch socket (plugging into the latter disconnects the XLR feed). Unbalanced inputs are accepted without problems and the input impedance is rated at over 100kΩ.Each channel has an actively balanced input via both XLR and TRS quarter-inch socket (plugging into the latter disconnects the XLR feed). Unbalanced inputs are accepted without problems and the input impedance is rated at over 100kΩ.On the output side, the 2520 discrete op-amp drives a model 2503 transformer, which balances and floats the output signal before it is presented on an XLR. The specifications state that the output can drive more than +28dBu into a 60Ω load, which is very impressive. Distortion is given as better than 0.1 percent at +20dBu and noise is below -70dBu (unweighted). Perhaps not quite state-of-the-art figures, but certainly more than adequate, and not actually reflective of the perceived sonic quality of this equaliser.

The internal mains power supply is designed with noiseless muting — so there are no nasty thumps when you turn the box on or off — and the power is connected via an IEC socket. An input voltage selector and fuse are accessible on the rear panel. The on-off button is cunningly coloured black and resides on a black-painted part of the front panel, on the far right hand side, making it a little tricky to find the first time around!

Turning the unit off automatically engages a hardwire bypass mode, and this remains in force for a few seconds after turning the unit back on, to ensure the power supply and electronics stabilise before processing audio signals. The same global bypass can be activated with an orange, illuminated button, in the centre of the front panel. This bypasses all the electronics and transformers, via relays and hardwire connections between inputs and corresponding outputs.

The front-panel controls are straightforward in their design and operation, with dual-concentric knobs for frequency and gain on each of the four bands. The knobs are solid and distinctive, and adjusting them by feel alone is quite easy. The larger, silver base knob controls the boost or cut up to ±12dB in five steps either way, while the smaller blue top knob sets one of seven centre frequencies. The low band spans 40 to 400Hz, with the others covering 75Hz to 1kHz, 800Hz to 12.5kHz, and 2.5 to 20kHz.

All four bands normally have 'peaking' or 'bell' responses, but bright-blue, illuminated push-buttons change the top and bottom bands independently to shelf responses. An 'In' button for each channel switches the equalisation into circuit, and this differs from the global bypass in that the input and output gain stages remain in circuit at all times.

Two further three-way rotary switches towards the centre of the front panel configure the overall gain resolution for each channel. This is presented in the form of a gain multiplier, with normal, half- and quarter-resolution steps. At the half-resolution setting, the maximum ±12dB cut/boost is reduced to ±6dB, and to just ±3dB at the quarter-resolution setting — allowing very fine tonal tailoring indeed. Combined with the resettable frequency and gain parameters, this facility makes the 5500 suitable for critical mastering applications.


At more than £2000 in the UK this is a serious purchase for anyone, but it sits very comfortably amongst its peers such as the Focusrite Red 2, Manley Enhanced Pultec EQP1A, the new Neve 8803, TF Pro P9, and the Oram Octa EQ, to name but a few. They are all worth auditioning but I have to say that I really loved using this unit and was sad at having to pack it away and return it!

In Use

The first word that comes to mind when you pass an audio signal through the API 5500, even before dialling in any EQ, is 'class'. It sounds smooth, detailed, transparent, fast and very analogue — in the best possible way.

The controls are well-spaced, unlike so many other outboard equalisers, thanks mainly to the use of dual-concentric knobs and fewer controls than most equivalent designs. This makes it easy to understand and to use, and reminds me of the classic Neve and Calrec modular console equalisers I cut my professional audio teeth on, so many moons ago.

The absence of variable Q (or bandwidth) controls isn't an issue at all, since the way in which the bandwidth reduces as the gain is increased is completely natural and appropriate. I never once felt the need to change the bandwidth of any band during use, regardless of whether I was applying savage corrective EQ or gentle tonal polish to a final master.

Each band's frequency control spans at least four octaves, and the way in which the bands overlap each other ensures that you can always reach a problem frequency range. The frequency jumps between the seven steps per band are also very well judged for musical applications (apparently the result of feedback from a 'who's who' list of top engineers).

I've not used analogue equalisation for some time now (all hail the mighty digit) but I have to say that this unit has reminded me of the quality and musicality that is possible from high-end analogue. It also highlighted just how poor some digital implementations still are!

If you are considering a top-notch outboard equaliser, and want the versatility to use it for tracking, mixing and mastering, with a simple user interface and bullet-proof signal path, the API 5500 should be high on your auditioning list. API also provide a five-year warranty, which is reassuring. 

Published May 2007