An audio-band full digital amplifier is presented using a low-pass sigma-delta modulator. The modulator is a nonlinear feedback system which outputs a sequence to control the power MOSFET. By rigorous analysis of the stability and performance, an effective design method can be derived. This paper presents the modulator design using both 1- and 1.5-bit quantizer. The designs are successfully implemented by using an FPGA and other discrete elements. Experimental results and comparison are shown by driving the loudspeaker load.