A Low Distortion Audio Amplifier

The paper presents a design and assembly of a high-quality audio amplifier. The design is simple, and it can achieve extremely low Total Harmonic Distortion (THD) less than -100 dB for a bookshelf speaker with output power levels up to 10 W. The solution for a high-quality output are transistor pairs used in the input stage along with a simple topology that does not need matched transistor pairs for a power stage. Such input and output stages were closely analyzed at different bias currents. It was found out that there is an optimal power stage bias current of 20 mA for lowest distortion. The THD of the proposed topology was simulated with the LTSpice simulator and measured with the audio spectrum analyzer U8903B from Keysight and by a simple solution using a handheld recorder and an integrated Digital to Analog Converter (DAC). The Keysight was able to measure -104.5 dB THDR, whereas simple solution did measure -92.8 dB.