The API 550B is a classic 500-series 4-band equalizer known for its musical sound and precision. Originally designed by Saul Walker in the 1970s, it features continuously variable boost/cut controls and fixed frequency points tailored for effective tonal shaping. The 550B offers overlapping frequency bands and proportional Q, meaning he bandwidth narrows as gain is increased, allowing for both broad tone shaping and surgical adjustments. Widely used in professional recording and mixing, the 550B is prized for its punchy, analog character and intuitive interface.