Fixed crossover point (-3db) low pass (passes low frequencies such as woofer amplifier use) passive crossover modules. The RCA plug is the output and "points" toward the amplifier. These may be combined to produce 24db per octave slopes  if a 3db attenuator is used between them. Keep in mind that two equal value FMOD low pass modules will increase the slope to 24db per octave and the crossover point will be 1/2 the FMOD value. Two 100 low pass FMODs with the 3db attenuator between them will become a 50 low pass 24db per octave crossover. This also holds true for combining FMODS with other equipment to alter the crossover point(s) and to increase the slope AND in that case you do not need the 3db attenuator. When ordering be sure to select the crossover point from the drop-down list. Passive device so there is no noise or distortion (transparent to your system). Color coded for crossover point.