8-Ohm Power Amplifier with 60W Output

Overview

The simple circuit was built to create a power amplifier that would produce 60W under 8 ohms or 80W under 4 ohms load.

Terminology

• Power Amplifier – a type of amplifier built and designed for the purpose of delivering and supplying sufficient and maximum high output power to directly drive loudspeakers or loads, within a given percent of distortion
• Class AB Amplifier – functions as a class B amplifier that eliminates virtually all of the crossover distortion by having a small amount of bias current that is constantly flowing through the output transistors
• BC557 – PNP general purpose transistors used for amplification and switching due to its low current and low voltage
• BD139 – NPN power transistor used for driver stages in hi-fi amplifiers and television circuits because of its low voltage at 80 V maximum and high current at 1.5 A maximum
• TIP2955 – a general purpose complementary power transistor manufactured in epitaxial-base planar technology which is suitable for audio amplifier, linear power, and switching applications because of its low collector-emitter saturation voltage

Circuit Explanation

The circuit consists of large quantities of components to produce the desired output. In the output of the amplifier, the DC voltage is being regulated by the presence of 100 ohm trimmer TR1 in preferred low levels from 20 mV up to 50 mV. The characteristics of the transistors Q1 & Q2 are completely matched at the differential amplifier. The bias current of the transistors at the power output stage is being regulated by the 470 ohm trimmer TR2 where an approximately 50 mA current is imposed for each division.

The adjustment by TR2 is done even without the signal in the input stage. A digital multimeter can be place in the terminals of resistors R22-23-24-25 while adjusting TR2 to produce an output voltage of approximately 25 mV DC with reference to 50 mA current. During this testing stage, the amplifier is left to operate for a few hours and if needed, regulation can be done again. A suitable heatsink should be used for the transistors Q6 up to Q12 while providing enough insulation between the support screws and the heatsink.

Application

The 60W power amplifier circuit can be used with home theater systems and a perfect choice for distributed background music systems to provide simple amplification and advanced mix functionality. Some designs are made to operate for the completion of power amplification requirements of commercial and professional sound systems. They made their name perfectly for use in powering public address systems, on stage monitoring, recording studio, and home theater applications.

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