API 550b Equalizer (4-Band 500 Series Module)
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  • Regular price $1,230.25

API 550b Equalizer (4-Band 500 Series Module)

Based on API's original 550 from the late '60s, the API 550b is a continuation of the EQ that played a major role in the history of music recording, but with 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. In fact, the 550b design has been taken from the original blue prints 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 solver and sweetening device 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 the predictable and warm analog performance, even under duress. With a surprisingly wide range of tonal variations, the 550b is an invaluable and professional audio tool with great flexibility and excellent sonic ability

  • Four bands of our famous sounding equalization
  • Each band offers seven API selected frequency centers
  • Reciprocal and repeatable filtering
  • 12 dB of boost/cut per band
  • EQ band 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


Number of Bands Four
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 Type "Low" and "High" bands are Peak/Shelf switchable.
Low-Mid and Mid-High are peaking type
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
Insertion Loss None (bridging)
Frequency Response +0, -.3, 30 to 20 kHz
Noise Actual, unw noise, <-98 dBu, 20-20 kHz
Distortion .07% max, @ +18 dBu Output
Clipping Level +30 dBu @ ±16 V
Power Consumption ±12/18 Volts/DC,@ 40 ma.
Connectors 15 position .156sp edge connector
Size 1.5" X 5.25" X 7.75" Deep
Size (Boxed for Shipping) 4.5" X 6.25" X 10"
Actual Weight 2 lbs.
Shipping Weight 2.43 lbs.