@▷ 60 W Power Amplifier for 8 Ohms Load | Diagram for Schematic

60 W Power Amplifier for 8 Ohms Load


The design of the circuit originated from the 80’s version which aims to support 8 Ohms load with a power amplifier having a power output of 60 W.

TerminologyPower 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 distortionBC550 – an NPN general purpose transistor with low current and low voltage used for low noise stages in audio frequency equipmentBD591 – a power MOSFET with gate protection provided by integrated back to back Zener diodes with temperature given by the change in forward voltage drop of two antiparallel electrically isolated poly-silicon diodes, fully avalanche rated, ultra low ON resistance, dynamic dv/dt rating, Zener gate protected, and 175ºC operating temperature2N5630 – an NPN complementary silicon power transistor manufactured by the epitaxial base process, mounted in a hermitically sealed metal case designed for high voltage and high power amplifier applicationsCircuit Explanation

The circuit is being used even from the 80’s era without any problem being reported especially in the way it was constructed. Better attention was given to the operation and reliability of the power supply, the suitable heatsink that was used, and the good matching of the transistors that would serve as drivers for the circuit. This is how Class AB was imposed with the use of two driven resistors. The appropriate heatsink of the power transistor, which was used, is where the diodes D3 to D5 will be placed above. The purpose of this setup is to create a thermic coupling between the output transistors to avoid overheating during the process.

In the output of the amplifier, the 22K Ohms trimmer R15 is used to regulate the continuous voltage which will occur regularly with thermal stability. The continuous voltage is developed since the circuit uses Class AB operation where a small idle current is flowing from positive supply to negative supply even when there is no input signal. This idle current slightly increases the power consumption but does not increase it anywhere near as much as Class A. Almost all of the nonlinearity associated with crossover distortion is corrected by this idle current.

The bias current of the amplifier will be adjusted by the 4.7K Ohms trimmer TR1 for the value of 50 mA up to 100 mA. The bias current can be measured during the measurement of the voltage drop across the resistors R36 & R37. The circuit will have a current value of approximately 75 mA in accordance to the value of the voltage given in the circuit. The value of the voltage specified in the circuit was originally set but it can have a tolerance of up to +10% of the actual value.


The 60 W power amplifiers are designed to be applied in 8 Ohms load not only the low impedance line loudspeaker system but also with home theater systems. They can be very useful in on-stage monitoring, recording studios, and public address systems like those used in information counter in shopping malls, where they are being used to provide amplification as well as advanced mix functionality for distributed background music.

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