The principle of making a 40W transistor HiFi power amplifier

The amplifier can maintain high-fidelity quality at higher output, and can provide 2×73/44 watts of output power to a 4Ω/8Ω load. The offset voltage is less than ±40mV, the input impedance is 100KΩ, the harmonic distortion is less than 0.015%, the intermodulation distortion is less than 0.02%, the frequency range is 10Hz-30kHz (±2dB), and the signal-to-noise ratio is greater than 72dB at 100mW output.

Circuit Principle

Here we only give the principle and production process of one of the channels. Figure 1 is the circuit principle. The input signal is introduced from the base of transistor T1. T1 and T2 form a differential pair. T3 is used as a constant current source for T1 and T2. The output of T1 is directly used to drive T4. T4 and T5 form a series tube pair. The constant current source of T4 and T5 is composed of T6 and T7.

Transistors T8 and T9 are used as diodes to compensate for the temperature of transistors T10 and T11. T10, T12, T14 and T11, T13, T15 form two groups of triple composite transistors. In this way, once the VBE of T10 or T11 changes slightly, the output current can change greatly. Since the output uses a composite transistor, the current through T4 and T5 can be maintained at a small value (about 6mA).

The function of coil 11 is to prevent interference to the amplifier when the output is connected to a capacitive load. A fuse is connected in series between the output and the speaker. Once the speaker is subjected to a DC voltage drop, the fuse will blow to protect the speaker. The gain of the amplifier is determined by (R7+R8)/R8, which is about 22 here. The circuit uses dual power power supply.

First solder the resistors and capacitors , then solder the transistors, note that T12 and T13 should use a suitable heat sink. L1 is made of 20# standard wire wound 20 times around resistor R20.

After the installation is complete and everything is checked, start debugging. First, set VRl to the minimum position, turn on the power switch , and then connect a millivoltmeter across any 0.33Ω resistor. Adjust VRl until the millivoltmeter shows 16mV, which means a current of 50mA.

Connect the sound source and adjust the sound source level control to make its output 2W. Connect a dummy load (without speaker) to the output of the amplifier. Let the amplifier work for six hours. Check each transistor. It should not be overheated. The power tube can be slightly hot.