50 Watts Audio Power Amplifier

50 Watts Audio Power Amplifier


The circuit was originally based on the design of OSU IEEE student group which aims to produce an audio power amplifier with 50 Watts output.

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 equipment2N3906 – a common PNP BJT transistor intended for medium voltage, lower current and power, which can operate at moderately high speeds, used for general purpose switching and low-power amplifying applicationsMPSA56 – a PNP general purpose transistor enclosed in a TO-92 package and used for amplification and switching with features of low current with 500 mA maximum and low voltage with 80 V maximum Circuit Explanation

The construction of this power amplifier circuit involves the typical procedures such as etching, drilling the single sided printed circuit board, soldering all components, and attaching the pre-developed heatsink assembly without the output transistors. This was done in a workshop by the students during the year 1995 to 1996. The frequency response of the circuit is approximately 40 KHz which is believed to have an impressive performance. These characteristics include a reasonably fast slew rate, very low noise, and an unregulated power supply with the accurate voltage of 40 V while providing a true RMS power of approximately 50 Watts. Since TIP33C and TIP34C have been discontinued and generally not available elsewhere, they were replaced with BD249C and BD250C respectively.

There should be some testing needed to be done on the power amplifier by way of visual inspection. The following are the steps to consider:

1.    The variable resistor should be turned fully counter clockwise with 24V supply connected and no input/output connected. A current meter is used to monitor the current from power supply.
2.    If the current being measured reaches above 25mA after applying power, shut it off immediately.
3.    Measure the voltage across the 1K Ohm resistor connected to the input stage and Vcc which should be around 2V or 2mA.
4.    Measure the DC voltage in the output line which should be around 0.05V.
5.    Turn the variable resistor slowly until the consumption of current is approximately 50mA.
6.    Connect an oscilloscope to the output and apply a low frequency of 20KHz square wave to the input.
7.    Solder the capacitor C1 then shut the power while connecting an audio input and a speaker with the volume at the minimum range.
8.    Apply power while watching the current meter and slowly turn up the volume to check if the amplifier is working. Make sure that the supply used to operate the amplifier is not less than 30V before turning the volume to maximum since the circuit is not used for high volume without a power supply rated for at least 2A of current or 8 Ohms load.


The 50W power amplifier is useful especially if a distributed audio system is existing and there is a need to add a local line level source to only one local zone. This is applicable in a residence where some pair of speakers are installed in the ceiling and the television in that room can produce the sound via those speakers. Other 50W power amplifier may be used in FM radio broadcasting where it functions by shutting down automatically to conserve power in the absence of RF input in reference to DC to power amplifier.

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