API 550A and 550B Equalizers Modeled on the late 1960's legend, the API 550A EQ provides reciprocal equalization at 15 points in 5 steps of boost divided into three overlapping ranges. The high and low frequency ranges are individually selectable as either peaking or shelving, and a band-pass filter can be inserted independently of all other settings. The 550A is a uniquely powerful equalizer with that one-of-a-kind API sound. Featuring four overlapped EQ bands, the API 550B is invaluable as both a problem solver and a sweetening device. With 7 switchable filter frequencies spanning up to 5 octaves per band, the 550B's "Proportional Q" automatically widens the filter bandwidth at minimal settings and narrows it at higher settings. It even lets you undo previous processing, affect or even reverse tonal modifications. With its vast range of tonal possibilities, the API 550B is a versatile EQ for all your processing needs.