Based on the original API 550 from the late 1960s, the API 550b is a continuation of the EQ that played an important role in the history of music recording, but with an additional filter band and several new frequencies. Incorporating unique circuitry and proprietary API components (like the legendary API 2520 Op Amp), the 550b artfully blends the past with the present. So many hit records still rely on the unique 550 sound that engineers around the world consider an invaluable tool. In fact, the 550b design was taken from the original blueprints and specification control drawings in the API files. It's unlike any other equalizer you'll ever use.

Rather than offering a multitude of complex features, the API 550b provides just the right number of controls. Its four EQ bands overlap significantly to help with dual functions as a troubleshooter and smoothing device, with each band offering seven selectable filter frequencies that span four to five octaves. These frequencies, purposely selected to be musical rather than numerical, were selected by an experienced list of “who's who” of the industry's most capable engineers.

Making use of the “Proportional Q” API, an innovation designed by Saul Walker in the 1960s, the 550b intuitively expands filter bandwidth at minimum settings and reduces it at higher settings without the need for additional bandwidth controls . This unique feature minimizes the “phase shift” sound found in many equalizers. Additionally, the reciprocal nature of the 550b allows the user to “undo” what was previously done with pinpoint precision.

The benefits of the API 550b are most obvious to those who work with EQ on an ongoing basis. If major tonal restructuring is required, the extraordinary headroom enabled by the API 2520 Op Amp delivers warm, predictable analogue performance, even under pressure. With a surprisingly wide range of tonal variations, the 550b is an invaluable professional audio tool with great flexibility and excellent sonic capability.

Characteristics

• Four bands of our famous sound equalization;
• Each band offers seven API-selected frequency centers;
• Reciprocal and repeatable filtering;
• 12dB of boost/cut per band;
• EQ bands 1 and 4 offer shelf/peak switching;
• "Proportional Q" narrows the Q filter at the extremes;
• Traditional API fully discrete circuit design;
• High headroom +30dB clip level;
• Reissue of 1967 API 550 EQ with an extra band.

Specifications

• Number of channels: 1;
• High-pass filter: yes;
• Low-pass filter: yes;
• Number of bands: 4 bands;
• Boost/cut range: ±12dB;
• Q type: proportional Q;
• High frequency range: 2.5kHz–20kHz;
• Hi-Mid Freq Range: 830Hz–12.5kHz.
• Mid frequency range: 75Hz–1kHz;
• Low frequency range: 30Hz–400Hz;
• Frequency response: 30Hz–20kHz.