When happens: we remove the jumper cap of jumper A to limit super low to bring distortion lower. When we remove the cap of jumper B, is increased while cap of Jumper B is on, 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. | Jumper A, B are used to control audio effect of range. A for lower limit while B for upper limit of the audio range. For some speakers, when is turned up too much, the speaker will show distortion. | 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 , you'd sooner or later find your best sound effect. Anywya, all effects still on your actual test.)