The 550A EQ is a reissue of the original that has been a very popular model since the late 60s. It combines API’s 2520 op amp and “Proportional Q” circuitry. Reciprocal equalization is provided at 21 points in 7 steps of boost to a maximum of 12dB of gain 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 to the signal path. A small toggle switch is used to insert the band-pass filter into the 550A.

• 3 bands of classic API EQ, with 7 selected frequency centres

• Reciprocal and repeatable filtering

• Maximum 12 dB of boost/cut per band

• EQ Band 1 and 3 offer shelf/peak switching