API 5500 Stereo/Mono Dual 550 EqualizerThe beginnings of the 5500 circuit can be traced back to the original 550 equalizer designed by Saul Walker, the founder of Automated Processes Inc. The 550 was designed as a console equalizer, which uses an unbalanced input due to the architecture of the recording console. Because the ergonomics of a console dictate that the controls take up little space, sometimes the number of included functions can be limited. The 5500 is specifically designed to