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 15 points in 5 steps of boost to a maximum of 12dB of gain at each point. The 15 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 push-button "in" switch allows the EQ to be silently inserted into the signal path. A small toggle switch is used to engage the band-pass filter.
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 550A Three-Band EQ Features
- Three bands of classic API equalization
- Each band offers five API selected frequency centers
- Reciprocal and repeatable filtering
- Maximum 12 dB of boost/cut per band
- EQ bands 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
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