Transistor Hi-Fi Class A Amplifier

Transistor Hi-Fi Class A Amplifier

The author made a high-fidelity Class A amplifier with reference to various amplifier circuits. The sound of this amplifier is pure and clear, and the principle is shown in the figure. V1 is a fixed bias Class A amplifier, providing a gain of >20 times. V2 is a voltage buffer, which amplifies the high impedance voltage signal output by V1 and drives the current amplifier. This current amplifier is a typical circuit of NE5534 driving high-power transistors that people were very familiar with more than a decade ago. The difference is that V3, V4, V5, and V6 are added in series to form a Darlington tube output, which still feels relaxed and comfortable in the high current state. At the same time, the current flowing through V2 is reduced, the distortion is low, and the working point is stable. At the same time, the resistance of the bias resistors R7 and R8 is increased to further increase the input impedance of the current amplifier. Q1, Q2, Q3 and peripheral components form a unique DC detection circuit with a starting control point of 0.7V. Since it is a shared power supply, the relay should use a high-sensitivity and high-quality DC 48v relay.

Debugging is very simple. First, adjust R2 to make the V2 E-pole voltage around 0V, and then adjust the resistance of R9 to make the quiescent current of the power amplifier tube 100mA. If the output midpoint voltage deviates at this time, the resistance of R7 and R8 should be adjusted.

In the figure, R5 is a 2W resistor, R11 and R12 are 5W resistors, and the other resistors are 1/2W metal film resistors. The withstand voltage of the capacitors is greater than 63V. Since this circuit is made to be simple, no thermal compensation and overcurrent protection circuits are set, and a larger area heat sink is required. The power of the power transformer should be ≥250W.

After debugging and auditioning, the imitation Swan No. 2 speaker was connected, and the treble was crisp and the bass was powerful.