Legacy AXS API
550A Discrete 3-Band Equalizer
Features
3 bands of classic API equalization
Each band offers 7 API selected frequency centers
Reciprocal and repeatable filtering
Maximum 12dB of boost/cut per band
High and Low EQ bands offer shelf/peak switching
"Proportional Q" narrows filter Q at extremes
Traditional API fully discrete circuit design
High headroom: +30dB clip level
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.
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 twenty-one 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 the 2520 op amp, and "Proportional Q" circuitry gives the 550A user an
uncomplicated way to generate acoustically superior equalization.
The controls for the 550A EQ function as follows:
IN (in/out switch): Activates the channel EQ
Hard bypass when not engaged
A LED indicator illuminates when engaged
HF (High-Frequency): Peaking/Shelving Switch: Selects between peaking and shelving EQ curves for
the high-frequency band.
Frequency Selection (Center Knob): Selects the center frequency for the selected band. Center
frequencies vary by band:
High: 2.5kHz, 5kHz, 7kHz, 10kHz, 12.5kHz, 15kHz, 20kHz
Mid: 200Hz, 400Hz, 600Hz, 800Hz, 1.5kHz, 3kHz, 5kHz
Low: 30Hz, 40Hz, 50Hz, 100Hz, 200Hz, 300Hz, 400Hz
Frequencies are indicated in blue numbers
Boost and Cut (Ring): Sets the amount of boost or cut for the selected band.
+12dB of boost or -12dB of cut
Decibels are indicated in white numbers
LF (Low-Frequency): Peaking/Shelving Switch: Selects between peaking and shelving EQ curves for
the low-frequency band
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