This master's thesis is explaines working principle of vacuum tubes: diode, triode tetrode and pentode, and reason of adding more grids to vacuum tube. Working principle of semiconductor is also explained, obtaining the p and n type of silicon and working principle of a diode and bipolar transistor. Basic circuits of tube and transistor amplifier were analysed, static operation point, and reading parameters from datasheet curves. Three common connections were described: common cathode, common grid and common anode on vacuum tube amplifiers, and common emitter, common base and common collector on transistor amplifiers. The thesis explained classes of audio amplifiers and their operational principles: class A, class B, class AB and class D. Negative feedback is shortly explained and analysed, and how negative feedback influences at amplitude-frequency characteristics, distortion and amplification of amplifier. Harmonic distortion is explained, classification on even and odd harmonics and influence of even and odd harmonics. Two amplifiers are made, vacuum tube and transistor, and based on measured data it's concluded that vacuum tube and transistor amplifier act differently regarding distortion, and vacuum tube amplifier has more even harmonics, and transistor amplifier has more odd harmonics.