
Features
Four bands of our famous sounding equalization
Each band offers seven API selected frequency centers
Reciprocal and repeatable filtering
12 dB of boost/cut per band
EQ band 1 and 4 offer shelf/peak switching
“Proportional Q” narrows filter Q at extremes
Traditional API fully discrete circuit design
High headroom +30 dB clip level
Re-issue of 1967 API 550 EQ with an extra band
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 500b is an invaluable and professional audio tool with great flexibility

and excellent sonic ability.
Automated Processes Inc.
8301 Patuxent Range Road
Jessup Maryland USA 20794
301-776-7879 sales@apiaudio.com