Legacy AXS API
HF (High-Frequency): Peaking/Shelving Switch: Selects between peaking and shelving EQ curves for
the high-frequency band
550b Discrete 4-Band EQ
Features
4 bands of the famous API equalization
Each band offers 7 API selected frequency centers
Reciprocal and repeatable filtering
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
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.
The controls for the 550b 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
High-Mid: 800Hz, 1.5kHz, 3kHz, 5kHz, 8kHz, 10Hz, 12.5kHz
Low-Mid: 75Hz, 150Hz, 180Hz, 240Hz, 500Hz, 700Hz, 1kHz
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.
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