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  • API-550B-999-022

    API 550B Discrete 4 Band EQ for 500 Series

    Based on the API 550 EQ from the late '60s - the 550b is an update to the classic EQ that played a major role in the history of music recording but adds an additional filter band and several new frequencies. Incorporating API's exclusive circuitry and proprietary components (such as the legendary API 2520 Op Amp), the 550b artfully blends the past with the present. So many hit records still depend on the unique 550 sound that engineers around the world find it to be an invaluable tool.

    Requires a 500 series rack unit to operate


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

    In fact, the 550b design has been taken from the original blueprints and spec control drawings from the API archives. It is unlike any other EQ you will ever use.

    Rather than offer a huge assortment of complex features, the API 550b provides exactly the right number of controls. Its four EQ bands are overlapped significantly to aid in dual roles as problem solvers and sweetening devices with each band offering seven switchable filter frequencies that span four to five octaves. These frequencies, purposely selected to be musical rather than numeric, were selected by an experienced "who's who" list of the industry's most proficient engineers.

    Making use of API's "Proportional Q," an innovation designed by Saul Walker in the '60s, the 550b intuitively widens the filter bandwidth at minimal settings and narrows it at higher settings without the need for additional bandwidth controls. This unique feature minimizes the "phase-shift" sound found in many equalizers. In addition, the reciprocal nature of the 550b enables the user to "undo" what has been done previously with exact precision.

    The benefits of the API 550b are most obvious to those who work with EQ on a continuous basis. If major tonal restructuring is required, the extraordinary headroom made possible with API's 2520 Op Amp offers predictable and warm analog performance, even under duress. With a surprisingly wide range of tonal variations, the 500b is an invaluable and professional audio tool with great flexibility and excellent sonic ability.



    Number of Bands: Four
    Filter Type: "Low" and "High" bands are Peak/Shelf switchable.
    Low-mid and Mid-High are peaking type
    Filter Frequency Centers: Low: 30, 40, 50, 100, 200, 300, 400Hz
    Low-Mid: 75, 150, 180, 240, 500, 700, 1Khz
    Mid-High: 800, 1.5K, 3K, 5K, 8K, 10K, 12.5K
    High: 2.5K, 5K, 7K, 10K, 12.5K, 15K, 20K
    Filter Boost/Cut: ±12 dB
    Filter Slope: 12 dB per Octave
    Input Impedance: 18.5 K +4 in, 7.5 K -2 in
    Output Impedance: Less than 75 Ohms
    Nominal Levels: -15 to +15 dBu
    Clipping Level: +30 dBu @ ±16 V
    Frequency Response: +0, -.3, 30 to 20 kHz
    Noise: Actual, unw noise, <-98 dBu, 20-20 kHz
    Distortion: .07% max, @ +18 dBu Output
    Connectors: 15 position .156sp edge connector
    Power Consumption: ±12/18 Volts/DC,@ 40 ma.
    Size: 1.5" X 5.25" X 7.5" Deep
    Size (Boxed for Shipping): 4.5" X 6.25" X 10"
    Actual Weight: 2.02 lbs.
    Shipping Weight: 2.43 lbs.