API 550a Equaliser

The API 550A is one of the most respected and enduring equalisers in professional audio. Originally designed in the late 1960s by Saul Walker, the discrete API 550a began life as a modular OEM EQ but quickly gained widespread recognition for its sonic quality. It was soon incorporated into custom consoles by notable engineers such as Frank DeMedio, and many of those original consoles are still operational today.

When API began manufacturing complete consoles in 1971, the API 550a became their standard channel equaliser. Over time, it became a benchmark in EQ design, widely imitated but never truly replicated. Due to ongoing demand and limited availability of original units, API resumed production of the 550A in 2004, preserving its historic design and sound.

The API 550a features three overlapping EQ bands, each offering five fixed frequency points with up to ±12 dB of boost or cut. The high and low bands can be switched between peak and shelf modes, and a dedicated band-pass filter can be inserted independently of other settings. All parameters are selected via three dual-concentric rotary switches, with a pushbutton for silent EQ bypass and a toggle switch to engage the band-pass filter.

At the heart of the API 550a is API’s proprietary 2520 op-amp and “Proportional Q” design, which adjusts the bandwidth according to the amount of boost or cut applied, delivering musical results with minimal phase shift. This combination ensures the API 550a delivers the classic API sound with clarity and ease of use, making it a continuing presence in both vintage and modern recording environments.

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