API Audio 5500 Dual Equalizer handleiding

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Handleiding

5500 Dual Equalizer API

1.0 Overview

1.1 History of the API Equalizer

The lineage of the 5500 circuit can be traced back to the original 550 equalizer designed by

Saul Walker, the founder of Automated Processes, Inc. That three band EQ was originally

used in the earliest API consoles built in the 1960s. Engineers were immediately captivated

by its fully reciprocal boost/cut characteristic and its proportional Q design, one of API’s

many industry firsts.

The sound of the 550, 550A, and 550B console equalizers was so highly prized by engineers

that many purchased API equalizers separately or took apart old consoles in order to install

the equalizers into a hand-made chassis. This allowed them to have that same API console

sound available no matter where they were working. Today, API manufactures the six and

eight-space Lunchbox and the 10-space VPR rack, making it easier for engineers to bring

the API sound to all of their recordings.

The 550 was designed as a console equalizer which, due to the architecture of the recording

console, uses an unbalanced input. Also, the ergonomics of a console dictates that controls

take up little space, sometimes limiting the number of functions that can be included in a

design. The 5500 is specifically designed to continue the tradition of API’s precision

equalization. It features a balanced input, a true straight-wire bypass, an integrated power

supply with noiseless muting, and a range control that expands its versatility to mastering

applications.

1.2 All-Discreet Design

Like all API products, the 5500 contains no integrated circuits in its signal path. The gain

comes from two hand-built 2520 operational amplifiers in each channel and the balanced

input is handled by a 2510 discrete operational amplifier. Besides being a key component of

the API sound, the 2520 coupled with the API 2503 output transformer is capable of

delivering +32dBu before clipping. With this much headroom, it is unlikely that the 5500

can be driven to distortion.

1.3 Proportional Q Circuitry

The Twin “T” topology is found in many equalizer circuits, but the legendary Saul Walker

included a novel Proportional Q section that makes it truly musical. Proportional Q works by

spreading the equalizer’s action over a broad bandwidth at low degrees of boost or cut. This

provides delicate shading and subtle coloring of the signal’s tone, without leaving the

impression that signal processing was applied. As the amplitude is advanced to more

extreme settings, the equalizer’s bandwidth narrows to provide the user with surgical

precision. This allows the equalizer to be useful in removing undesirable noises or radically

emphasizing an aspect of an instrument’s character, such as its attack.

The additional circuitry necessary to provide proportional Q is not included in most other

equalizers because it adds nothing to the spec sheet. Listening to musicality in the signal is

the only way to appreciate the benefits of the added cost and complexity required for

Proportional Q design.

1.4 Fully Reciprocal Operation

Before the 1960s, few equalizers had the same characteristics in boost mode as in cut

mode. (In fact, many earlier designs did not have both boost and cut to begin with.) Today,

it is common for equalizers to be reciprocal, but again, Saul Walkers design is special. When

switching an API EQ from boost to cut, the exact same components are used, reorganized

around the amplifier for the appropriate function. This guarantees 100% reciprocity. The

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