Few equalizers enjoy the respect and admiration of the coveted API 550A. Designed by the now legendary Saul Walker in the late 60s, the discrete 550A was first used as a modular OEM equalizer. As the industry quickly embraced the 550A's sonic quality, it quickly found its way into many custom console designs by Frank DeMedio and other leading engineers. Many of these consoles are still in use today.

Forty years later, the 550A remains the standard against which other equalizers are measured and has played an important role in the recording industry for decades. Still copied but never duplicated, the 550A became API's standard channel EQ module when the company began manufacturing consoles in 1971. With virtually all units in existence, popular demand for this EQ resulted in API finally resuming production in 2004.

The 550A provides 21-point reciprocal equalization in 5 boost steps for a maximum of 12dB of gain at each point. The fifteen equalization points are divided into three overlapping bands. The high and low frequency ranges are individually selectable as peaking or shelving, and a bandpass filter can be inserted independently of all other selected EQ settings. Frequency ranges and boost/cut are selected by three dual concentric switches, and an “in” switch allows EQ to be introduced silently into the signal path. A small toggle switch is used to insert the bandpass filter into the 550A.

The combination of Walker's incomparable 2520 operational amplifier and its “Proportional Q” circuit offers the 550A user a hassle-free way to generate acoustically superior equalization.

With the long-awaited reissue of this unit, an EQ that played so much of a role in recording history continues to make history in today's music.

Characteristics

• 3-band classic API equalization;
• Each band offers 7 API-selected frequency centers;
• Reciprocal and repeatable filtering;
• Maximum 12 dB boost/cut per band;
• EQ Band 1 and 3 offer shelf/peak switching;
• "Proportional Q" narrows the Q filter at the extremes;
• Traditional API fully discrete circuit design;
• High headroom +30 dB clip level.

Specifications

• Number of channels: 1;
• High-pass filter: yes;
• Low-pass filter: yes;
• Number of bands: 3 bands;
• Boost/cut range: ±12dB;
• Q type: proportional Q;
• High frequency range: 2.5kHz–20kHz;
• Mid frequency range: 200Hz–5kHz;
• Low frequency range: 30Hz–400Hz;
• Frequency response: 30Hz–20kHz.