500 Series Compressor Module
The API 525, originally released in the 70's, is a feedback type compressor, such as the 1176 and 660, etc. An input (threshold) control and output (make-up gain) control are provided. An additional Ceiling fine tune function modifies threshold and make-up gain for precise changes in compression without changes in output.
A compression/limit switch sets ratio at either 2:1 or 20:1. Attack time is as fast as 15u/s. Four auto release modes are offered via two switches, 0.1s, 0.5s, 1.5s, & 2.0s. A De-Ess function inserts an inverse vocal energy curve filter in the detector for effective sibilance/p-pop reduction.
The API 525 is perhaps the most successful solid state feedback compressor in history. It delivers dynamics control behaviour unlike VCA feed forward compressors so common today.