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API Audio 550A Saul Walker Edition 500 Series EQ  From API

Single space EQ going back to the original Saul Walker Discrete 3 band EQ design.

API Audio

Manufacturer's Description from API

This limited edition EQ is a continuation of the equalizers produced by API exactly as they were in the 1960's, featuring hand-wired switches, the original circuit board design and API founder Saul Walker's signature on the chassis.


  • 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


The 550A Saul Walker Edition uses the same circuitry as the current reissue of the API 550A. Few equalizers enjoy the respect and admiration of the coveted API 550A. Designed by the now legendary Saul Walker in the late 60's, 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 it's 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 EQ's 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 15 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.

API Audio

About Manufacturer

API (Automated Processes, Inc.) is world famous for its all-discrete audio products: studio and broadcast mixing consoles, and a full line of rack mount mic preamps, equalizers and processors. It all began with a dream. It was 1968. The company: Automated Processes Inc., formed by men with a vision. Best known for their now legendary "2520 amplifier", this unique amp has been and will continue to be the heart of all API discrete products. It continues to provide reliability and sonic purity unmatched by the competition. The resulting console met the needs of the music, commercial and broadcast industry. API became the leading audio broadcast console manufacturer for radio and television networks and high profile stations. In addition, recording studios, large and small, began using API.....and receiving rave reviews from engineers and producers. There are over 700 API consoles across the nation and around the world, including the three major networks. Over thirty years later, many of these consoles are still in daily use in some of the most prestigious recording and broadcast facilities in the world because, to date, there are few, if any consoles of equal sound quality at a cost-effective price.



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


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


Dual Concentric Rotary: LF select: 50, 100, 200, 300, 400 Hz

and stepped degree of equalization -12 dB to +12 dB


Dual Concentric Rotary: MF select: 400, 800, 1500, 3000,

5000 Hz and stepped degree of equalization -12 dB to

+12 dB


Dual Concentric Rotary: HF select: 5000, 7000, 10,000, 12,500,

15,000 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


30 pin PC connector supplied


1.5" X 5.25" X 7.5" Deep

Size (Boxed for Shipping):

4.5" X 6.25" X 10.25"

Actual Weight:

1.93 lbs.

Shipping Weight:

2.37 lbs.