In 1967, Saul Walker designed the first 12-track recording console for Apostolic Studios (New York City), a facility used by Frank Zappa on many of his legendary recordings. The console project led to Walker’s founding of API (Automated Processes, Inc). Ironically, API didn't build consoles until a few years later—instead focusing on creating OEM and retrofit modules for custom mixers, which were common in most studios in that era. Still highly prized today, perhaps the most beloved API module is the 550A, a three-band equalizer with five switchable frequency centers on each band.
Walker’s design combined his 2520 discrete Class-A/B op amps with an ingenious Proportional Q circuit that alters the filter bandwidth (Q) with changes in gain boost/cut—three octaves wide at ±2 dB, narrowing to one octave at ±12 dB. The result? A highly musical unit offering simple, fast operation. In 2004, API resumed production of the 550A (and the four-band 550B), offered in both modular and rackmount formats.