PURPOSE: To enable to continuously change the amplitude without individual preparation of the filters having many amplitude characteristics, by changing the amplitude only without shifting the frequency axis in the attenuation, through the change of one parameter.

CONSTITUTION: The input signal from the input terminal 7 is fed to the adder 8 and added with the output of the feedback circuit 9. The feedback circuit 9 consists of the cascade circuit of the transfer function T(P) of an odd function and the variable circuit 12 changing in the output level to change the variable parameter K. The output of the adder 8 is fed to the output terminal 13 as the filter output and fed to the input of the feedback circuit 11.