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
