PFMOD-Sub (high pass) adjustable sub-sonic filter. By removing factory jumpers you can select 20hz, 25hz, 30hz, or 35 hz at 12 db per octave. Each channel crossover frequency can be selected independantly. This is a two channel device (stereo). There are two RCA input and two RCA output jacks and two bypass RCA jacks that pass on the input signal to other equipment. Passive device so there is no noise or distortion (transparent to your system).