API 550A Special Edition Equaliser

The API 550A is one of the most respected equalisers in professional audio, originally designed by Saul Walker in the late 1960s. First introduced as a modular OEM unit, it quickly gained recognition for its sonic character and became a key component in custom consoles by engineers like Frank DeMedio. Many of these consoles remain in use today, a testament to the enduring quality of the 550A.

Adopted as API’s standard channel EQ when the company began producing full consoles in 1971, the API 550A set the benchmark for equaliser design. After decades of use and with original units in high demand, API resumed production in 2004, faithfully recreating the original unit.

The API 550A Special Edition features three overlapping bands of EQ, offering fifteen frequency points in total. Each band provides up to ±12 dB of boost or cut in five defined steps. The high and low bands can be switched between peak and shelf modes, and an optional band-pass filter can be inserted independently. All settings are selected via three dual-concentric switches, with a pushbutton for engaging the EQ and a toggle switch for activating the filter.

Powered by API’s 2520 discrete op-amp and incorporating Walker’s “Proportional Q” design, the API 550A Special Edition delivers musical, phase-consistent equalisation without complexity. Still widely used and revered, the reissued API 550A Special Edition remains a defining tool in modern audio production.

Features

• 3 bands of classic API equalization
• Each band offers 7 API selected frequency centers
• Reciprocal and repeatable filtering
• Maximum 12 dB of boost/cut per band
• EQ Band 1 and 3 offer shelf/peak switching
• “Proportional Q” narrows filter Q at extremes
• High headroom +30 dB clip level