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API 550A

Designed by the now-legendary Saul Walker in the late '60s, the API 550A is a discrete, 3 band EQ that was first used as a modular OEM equalizer.

As the industry rapidly embraced the sonic quality of the 550A, 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 EQs are measured, and it has played a major role in the recording industry for decades. Still copied but never duplicated, the 550A became API's standard channel module EQ when the company began manufacturing consoles in 1971. With virtually all existing units spoken for, popular demand for this EQ resulted in API finally resuming production in 2004.

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

The combination of Walker's incomparable 2520 op amp and his "Proportional Q" circuitry gives the 550A user an uncomplicated way to generate acoustically superior equalization.

With the long-awaited reissue of this unit, an EQ that has had such a part in the history of recording is continuing to make history in today's music.
  • 3 bands of classic API equalization
  • Each band offers 5 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
  • Traditional API fully discrete circuit design
  • High headroom +30 dB clip level
  • Specifications
  • Input Impedance: 66 k high level, 17 k low level
  • Output Impedance: Less than 85 (transformer isolated)
  • Nominal Operating Level: -15 dBm to +12 dBm
  • Insertion Loss: None (bridging)
  • Frequency Response, Unequalized: ±0.5 dB 30 Hz to 20 kHz
  • Noise: Unequalized, output noise of -90 dBm, high level strapping and -98 dBm, low level strapping, 20 Hz to 20 kHz unweighted
  • Distortion: .05% THD max, 30 Hz to 20 kHz @ +4 dBm, .07% THD max, 30 Hz to 20 kHz @ +18 dBm
  • Clipping Level (3% THD): +30 dBm
  • Power Requirement: ±15 volts to ±18 volts @ 70 ma
  • Controls: Dual Concentric Rotary: LF select: 30, 40, 50, 100, 200, 300, 400 Hz and stepped degree of equalization -12dB to +12 dB
  • Dual Concentric Rotary: MF select: 200, 400, 600, 800, 1500, 3000, 5000 Hz and stepped degree of equalization -12dB to +12 dB
    Dual Concentric Rotary: HF select: 2500, 5000, 7000, 10,000,
    12,500, 15,000, 20,000 Hz and stepped degree of equalization
    -12 dB to + 12 dB
    All equalization is in steps of 2, 4, 6, 9 and 12 dB either plus or minus
  • Toggle Switch: 50 Hz to 15 kHz band-pass filter In or out, Push Button: LF range Peak or Shelf, Push Button: HF range Peak or Shelf, Push Button: Equalization In or Out, Tally light when In
  • Connector: 30 pin PC connector supplied
  • Size: 1.5" X 5.25" X 7.75" Deep
  • Size (Boxed for Shipping): 4.5" X 6.25" X 10.25"
  • Actual Weight: 2 lbs.
  • Shipping Weight: 2.53 lbs.
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    Additional information
    For more information about the 550A, please visit the website of API
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