The Purple Audio Action is a FET compressor module designed for standard 500 series racks, inspired by the classic MC77 compressor. It features a constant impedance 600 ohm input attenuator feeding an input transformer, which protects the unit from overload by hot signals. The compression is performed by the same gain reduction FET used in the MC77, while the signal is buffered with discrete class A and class AB opamps driving a custom hexafilar nickel and steel output transformer. The total gain is 45dB, like the MC77.

It offers variable attack and release times that cover a broader range than the original MC77, which can result in distortion if pushed to extremes. The compressor has five ratio settings (4:1, 8:1, 12:1, 20:1, and an "all buttons in" mode), and the threshold varies with the ratio similarly to the MC77. It includes input and output tricolor LED metering and an eight-LED gain reduction meter.

Stereo linking is possible by cross-connecting pins on the edge connector or through a TRS cable in Purple Audio racks. The unit has true relay bypass, a compression on/off switch, and is built with a discrete signal path. It operates with an input and output impedance of 600 ohms and consumes 90mA of current.

The Action is particularly suited for processing drums and sources with fast transients, bringing the classic FET compression sound with modern usability and flexibility in a convenient 500 series format.