500 Series Classic 3 Band EQ

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 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.

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The sought-after API 550A earns a degree of reverence and esteem that few other equalisers enjoy. This subtle EQ was first utilised as a modular OEM equaliser and devised by the now legendary Saul Walker in the late 60s. The API 550A rapidly made its way into multiple custom console designs by Frank DeMedio and other leading engineers as the industry quickly embraced its sonic excellence. A lot of these consoles are still used to this day.

The API 550A has played a significant role in the recording trade for decades and, 40 years later, remains the benchmark against which other EQs are measured. Since the company began constructing consoles in 1971 the often copied but never duplicated 550A became API's default channel module EQ. Popular demand for this EQ caused API to finally resume production in 2004 - with nearly all of the existing units taken or in use.

Seven levels of boost are provided to a maximum of 12dB of gain at each 21 points of reciprocal equalisation. The 15 equalisation points are split into three overlapping ranges. A band-pass filter can be added independently of all other selected equalisation settings and the high and low frequency ranges are individually selectable as either peaking or shelving. A pushbutton "in" switch enables the EQ to be silently added to the signal path and frequency ranges and boost/cut are selected by three dual-concentric switches. The band-pass filter is inserted into the 550A using a small toggle switch.

Acoustically superior equalisation is created in a simplified manner for the 550A by Walker's incomparable 2520 op amp and "Proportional Q" circuitry.

This EQ, that has played an incredibly significant role in the history of recording, is continuing to make history in today's audio productions thanks to its long anticipated and desired reissue.

  • 3 bands of classic API equalization
  • Each band offers 7 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
More Information

Input Impedance:

  • 66 kΩ high level
  • 17 kΩ low level

Output Impedance: Less than 85Ω (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


  • .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:  All equalization is in steps of 2, 4, 6, 9 and 12 dB either plus or minus

  • 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

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


Dimensions and Weight

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.