I propose an explanation to why tube amplifiers have ``fat sound" that solid state amplifiers lacks. The explanation is, the interaction of (1) the nonlinearity of the output tube, (2) output impedance of the amplifier, and (3) the nonlinearity of the output transformer inductance caused by core material B-H curve, results in a frequency selective nonlinear feedback system that softly limits the speaker cone excursion for low frequency music signal with excessive amplitude, while has little effect on high frequency music signal or low frequency music signal with low to moderate amplitude. Better yet, when low frequency music signal with excessive amplitude is superposed with high frequency music signal, this system selectively limit low frequency music signal and has little effect on the superposed high frequency music signal . Comparing to that of a typical solid state amplifier, this mechanism trades some amplifier nonlinearity for less speaker nonlinearity, resulting in less overall nonlinearity of the music sound waves people's ears perceive.
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