A Direct PWM Loudspeaker Feedback System

A loudspeaker feedback system is built in this project. A class D switching amplifier and the loudspeaker are placed in a closed loop. The switching power amplifier is a simple pulse width modulator, but without the conventional output filter that is made up of expensive inductors and capacitors. The band-pass loudspeaker replaces the output filter. Feedback is achieved by an electret condenser microphone and a lag-pole compensator in the low frequency range of the audio spectrum. For a typical closed box type loudspeaker, the closed loop system frequency response is flat from 35Hz to 300Hz. The total harmonic distortion is also minimized to acceptable levels in the same frequency range. Various loudspeakers, drivers, and compensators are also studied for the completion of the final system. Thesis Supervisor: James K. Roberge Title: Professor of Electrical Engineering This thesis would not have been possible without the help of many people. The names that follow are not necessarily listed in the order of importance. I would like to thank Professor J. K. Roberge, my thesis supervisor, for his technical comments and criticisms. His graduate class "Advanced Circuit Techniques" (6.331) though took a portion of the time I should have spent on my thesis, it taught me the necessary tools for the design of the final system. The suggestions from the other 6.331 staff, Kent Lundberg and Tracy Clark, and my classmates in 6.331 were very helpful. I would also like to thank Dr. Bruce D. Wedlock for taking me as a Teaching Assistant in his 6.071 class, "Introduction to Electronics." I relearned many basic electronic concepts while trying to help 6.071 students. In addition, the TA appointment granted me the tuition support, office space and full access to the EECS lab, all of which became helpful later in my research. The staff at the lab's instrument desk, John Sweeney, Lorenzo Pires and Pat Orr consistently tolerated me. They loaned me electronic equipment and parts, and let me stay in the lab for however long hours. I sincerely thank the Lee's family, in particular Tommy and Josephina, and their daughter Christina. Their technical and mental support helped me through the final stage of my research. I also thank John Tucker for his encouragement and Anne Hunter for her constant pressure and help with my thesis. In addition I appreciate the help and support from my friends: James Chen, Robin Huang, Mei Hu, Beethoven Cheng and Matilde Kamiya. My thanks also go to National Semiconductor Corporation. Without its sample parts this thesis would have been delayed. Additionally I thank the Bose Corporation, whose job offer in the fall made my M.Eng. degree a prerequisite which kept me going for the whole summer. Finally I would like to dedicate this thesis to Dr. B. D. Wedlock and my parents, from whom I received financial support, to the Chiu family and to all my dear friends in America and abroad.