Power amplifier circuit made with LM317

The power amplifier made of voltage-stabilized integrated amplifier is a positive attempt for electronics enthusiasts as a way to explore new ideas. This circuit is pure Class A and uses low-noise tubes for voltage amplification, so the THD, NF and other indicators are good, and the output power can reach 30W

The circuit is shown in the figure. Transistor VT1 is used as voltage amplifier. Since the input impedance of voltage regulator integrated circuit IC1 is high, its working current is only 0.6mA. R1 and C3 are power supply decoupling. C4 is used to prevent parasitic oscillation. R3 and R4 divide the voltage to provide bias and AC/DC feedback to VT1 to improve linearity and DC stability. VT2, R5, R6 and diode constitute a constant current source to improve the efficiency and output power of the circuit and increase the output dynamic range.

Since the circuit is very simple, the components can be overlapped, the constant current of the constant current source can be set at 10~15mA, and the output voltage of LM317 can be adjusted to half of the power supply voltage during debugging. The static current is about ten milliamperes. The power of the power transformer T1 shall not be less than 50W.

The three-terminal voltage regulator IC is LM317, the transistor VT1 is 9014, VT2 is 8050, the diodes VD5 and VD6 are 2CP20, and the power of the transformer should be greater than 50W