When a knob is at a center position, transmission levels of first volumes on a path from a second effector to a first effector, and second volumes on a path from the first effector to the second effector are both zero, and the first effector and the second effector operate independently. When the knob is displaced to right side from the center position, the transmission level of the second volumes is controlled according to the displacement amount, but the transmission level of the first volumes stays at zero. Thus, the second effector is connected in series to the first effector. When the knob is displaced to left side from the center position, the transmission level of the first volumes is controlled according to the displacement amount, but the transmission level of the second volumes stays at zero. Thus, the first effector is connected in series to the second effector.