A design methodology for audio amplifiers with no global negative feedback: proposal and validation

This paper proposes a structured methodology for the design of high-performance audio amplifiers without the use of global negative feedback. This type of amplifier is being suggested in order to minimize supposed deleterious effects on audio quality. The proposed methodology uses a minimum number of stages with local negative feedback operating in a high voltage range in order to minimize harmonic distortion without the need to use global feedback. As an example of applying the methodology, a 50W power amplifier is designed and built. In this design example, three stages are used with field effect transistors operating with a symmetrical voltage of }90V in the voltage gain stages and }45V in the output stage. The practical results obtained satisfied the design requirements, such as: Slew Rate above 3.6V/μs (14V/μs), Upper Cutoff Frequency above 40kHz (125kHz), THD+N below 0.5% (0.2%). Also, to ensure that the designed amplifier has a good linearity, the IMD intermodulation distortion was measured. The value of −62.5dB was obtained. In this way, the effectiveness of the proposed method for the design of audio amplifiers without global negative feedback was proven.