Jumper A, B are used to control audio effect of bass range. A for lower limit while B for upper limit of the bass audio range. For some speakers, when Bass is turned up too much, the speaker will show distortion. When that happens: we remove the jumper cap of jumper A to limit super low Bass to bring distortion lower. When we remove the cap of jumper B, Bass is increased while cap of Jumper B is on, Bass is decreased. Jumper C and D are used to control Treble range. When cap of both jumper C and D are on, the sound of treble is loudest. When only cap C is on, the treble sound is lower. When only cap D is on, further lower. When both caps of C and D are removed, there is no treble sound. (Generally speaking, with the four jumpers, you'd sooner or later find your best sound effect. Anywya, all effects still depend on your actual test.) How to use the splitter board: First connect as per our diagram above (Treble, Low, Input), connect to speaker, seal air gap, we can begin to hear music while optimize. Normally we use jumper A and let B idle. For jumper C and D: we need to hear the actual effect of treble. A general formula is: A+D = better Bass; A+C = clear Treble and Bass; A+CD = clear Treble and Bass (the overall effect inclined towards Treble); AB+CD = more mediant frequency. When Bass effect is unnecessarily high, we can remove cap A. Please optimize until you think the sound is best, and then fix the board with your speaker unit. Power: 400W or lower Resistance: 4-16ohm Frequency: Adjustable Colour: motherboard red Material: ABS + metal Size: 7 x 5 x 2.5cm Package Contents: 8 x Frequency Divider