API Audio The Channel Strip - Manual (Page 21)

5.0 Spectral Processing: 550A Equalizer
5.1 550A Equalizer Overview and Features
Few equalizers enjoy the
respect and admiration of the
coveted API 550A Equalizer.
Designed by the now-
legendary Saul Walker in the
late '60s, the discrete 550A
Equalizer was first used as a
modular OEM equalizer.
As the industry rapidly
embraced the sonic quality of
the 550A Equalizer, 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 Equalizer 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 Equalizer 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 Equalizer provides reciprocal equalization at 21 points in 7 steps of boost to a maximum of
12dB of gain or attenuation 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 into the signal path. A small toggle switch
is used to insert the band-pass filter into the 550A Equalizer.
The combination of Walker's incomparable 2520 op-amp and his "Proportional Q" circuitry gives the
550A Equalizer 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.
Features
• 3 bands of classic API equalization
• Each band offers seven (7) API selected frequency centers
• Reciprocal and repeatable filtering
• Maximum 12dB of boost/cut per band
• Low and High EQ bands offer shelf/peak switching
• "Proportional Q" narrows filter Q at extremes
• Band–pass filter (-3dB @ 50Hz and 15kHz)
• High headroom +30dB clip level
• Traditional API fully discrete circuit design
• Audio circuit uses the 2520 discrete op-amps
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