Great River PWM-501, 1-Channel Pulse-Width Modulated Compressor/Limiter, Enables a fast response as with an FET or VCA compressor, Sidechain filter: High pass, 20 to 600 Hz, switchable, Controls for output level, threshold, ratio, attack, release, Controls for continuous feedback/feed forward mixing, LED Meter, Stereo coupling with two devices, Design: 500 Series module