PURPOSE: To eliminate the need for the adjustment of an input or output level of both channels after balance adjustment by connecting a center tap of a slide resistor to ground and connecting a variable resistor means between a slider of a slide resistor and ground.

CONSTITUTION: A slide resistor 6 has one terminal 14, other terminal 16, a slider terminal 18 and a center tap 20. The shaft of the resistor 6 is coupled with a balance knob 26. When the knob 26 is turned, the resistance between the one terminal 14 and the slider terminal 18 and between the other terminal 16 and the slider terminal 18 change as the shaft is turned. The shaft of a variable resistor 8 is coupled with a knob 28 of a control range. When the knob 28 is turned, its resistance R3 is continuously changed. Thus, the resistors 6, 8 are adjusted to adjust the balance and the range of balance, then one output level is kept constant and it is not required to adjust the input or output level of both the channels after the balance adjustment.