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This paper presents an analytical modeling of the mechanisms of total harmonic distortion (THD) of second-order based single-feedback and double-feedback class-D amplifiers (CDAs). We show that the overall THD in these closed-loop CDAs comprises the THD of their open-loop counterparts reduced by the Loop Gain+1 and the THD due to the combined phase and duty cycle error that is due to feedback, hence unique to closed-loop CDAs. We show that the latter THD can be large and is the dominant THD at high input frequencies ( > 3 kHz), and that the mechanisms therein are the phase and duty cycle errors. By means of double Fourier series analysis, analytical expressions for the harmonic components and thereafter a THD expression for closed-loop CDAs are derived. The derived expressions depict the parameters that affect THD, and are insightful to designers to optimize/vary pertinent parameters to reduce THD. The derived THD expression is verified against HSPICE and on the basis of measurements on a prototype CDA IC and other CDAs realized discretely.