550b 4-band Equalizer

The API 550b is the logical evolution of the legendary 550A, expanding on its classic design to offer even greater flexibility and control. This 500 series module is a 4-band equalizer that retains the all-discrete, Proportional Q circuitry of its predecessor while adding an extra band of overlapping midrange control. This makes the 550b an incredibly powerful tool for both corrective and creative equalization, capable of everything from broad tonal shaping to precise frequency sculpting. It has become a modern classic in its own right, favored by engineers who love the sound of the 550A but require more surgical control in the critical midrange frequencies.

Like the 550A, the 550b features API's "Proportional Q" design, which automatically narrows the filter bandwidth as gain is increased, ensuring musical results at any setting. The module provides four bands of equalization, each with seven selectable frequency centers and up to 12dB of boost or cut available in stepped increments for easy recall. The two inner bands offer peaking (bell) curves, while the high and low bands can be individually switched between peaking and shelving curves. This combination provides immense versatility, allowing the 550b to handle any equalization task with ease. Driven by the legendary API 2520 discrete op-amp and custom output transformer, the 550b delivers the classic punch, warmth, and headroom that is synonymous with the API name.

Features

• Versatile 4-band equalizer for the 500 series format.
• Evolution of the classic 550A design.
• Features API's proprietary "Proportional Q" circuitry.
• Seven selectable, musically-chosen frequency centers per band.
• Up to 12dB of boost/cut per band via stepped controls.
• High and Low bands are individually switchable between Peaking and Shelving curves.
• Utilizes the API 2520 discrete op-amp for classic analog sound.
• True hardwire bypass via the "IN" switch.