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140W Class AB Amplifier using MJL4281A and MJL4302A transistors with PCB

Fig. 1 - 140W Class AB Amplifier using MJL4281A and MJL4302A transistors with PCB

This is a simple 140W Class AB amplifier using MJL4281A and MJL4302A transistors, which stands out for its simplicity, quality, and is a moderate amplifier to build.

You can be making two of these boards, to be able to work with two outputs, in stereo, and make an amplifier with total power of 280W RMS.

This amplifier works with a simple power supply, that is, a unipolar, non-symmetric power supply.
 
This amplifier circuit can be used in almost any type of application that requires a simple amplifier, with great performance, low noise and low distortion, and good sound quality.

It uses 2 output power transistors, NPN transistor MJL4281, and PNP transistor MJL4302, forming a pair of complementary transistors.

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The Power Transistors

The MJL4281A and MJL4302A transistors are power transistors designed for high power audio, they have a collector-emitter sustaining voltage of 350 V.

It's high gain – 80 to 240, with the hFE = 50 (min) 8A collector current, and a low harmonic distortion, which makes a transistor excellent for high power operation and audio quality.

The Circuit

This circuit has a moderate complexity, it is not recommended for those who have no experience in electronics and in the assembly of amplifier circuits.

You should have minimum knowledge in an intermediate to advanced level to assemble this type of power amplifier.

The schematic diagram of the complete circuit, shown in Figure 2 below, is a very robust amplifier, with great sound quality and very stable, responding very well at all audible frequencies, with little attenuation in the 20Hz to 20Khz hearing frequency range.

Fig. 2 - Schematic Diagram 140W Class AB Amplifier using MJL4281A and MJL4302A transistors

Power supply

The power supply is symmetrical, with a voltage of +45V | 0V | -45V, and direct current, with at least 6 Amps of current. For continuous use, we recommend 4A, especially if used in a subwoofer.

In Figure 3 below, we have a suggestion for a power supply that we use in our projects. In this article, besides having the schematic diagram with the Printed Circuit Board, you will understand how to easily calculate your own Power Supply, with the desired voltage.

You can in the link below:

Fig. 3 - Symmetrical Power Supply for Power Amplifiers

Components List

  • Semiconductor
    • Q1, Q2, Q3 .... 2N5551 NPN Transistor
    • Q4, Q6 ........... BD139 NPN Transistor
    • Q5, Q7 ........... BD140 PNP Transistor
    • Q8 .................. MJL4302A NPN Transistor
    • Q9 .................. MJL4281A NPN Transistor
    • LED1 ............. Light Emitter Diode (general use

  • Resistor
    • R1, R3 ........... 2K2Ω resistor (red, red, red, gold
    • R2, R4, R8 ..... 22KΩ resistor (red, red, orange, gold)
    • R5, R6 ........... 560Ω resistor (green, blue, brown, gold)
    • R7 .................. 1K2Ω resistor (brown, red, red, gold)
    • R9 .................. 1KΩ resistor (brown, black, red, gold)
    • R10, R11 ....... 3K3Ω resistor (orange, orange, red, gold)
    • R12, R13 ....... 220Ω resistor (red, red, brown, gold)
    • R14, R15 ....... 0.22Ω / 5W resistor (red, red, silver, gold)
    • R16 ................ 10Ω / 1W resistor (brown, black, black, gold
    • RP1 ................ 2K Potentiometer
  • Capacitor
    • C1 .................. 2.2uF / 25V electrolytic capacitor 
    • C2 .................. 330pF ceramic, polyester capacitor
    • C3, C5 ........... 100uF / 65V electrolytic capacitor 
    • C4, C6, C7 .... 100nF ceramic, polyester capacitor

  • Miscellaneous 
    • P1, P2 ............ Screw Terminal Type 5mm 2-Pin Connector
    • P3 .................. Screw Terminal Type 5mm 3-Pin Connector
    • Others ............ PCB, tin, wires, heat sink, soldering Iron, etc.

Printed Circuit Board - Download

We are offering the PCB – Printed Circuit Board, in GERBER, PDF and PNG files, for you who want to do the most optimized assembly, either at home.

If you prefer in a company that develops the board, you can is downloading and make the files in the Download option below.

Fig. 3 - PCB - 140W Class AB Amplifier using MJL4281A and MJL4302A transistors

Files to Download, Direct Link:

Click on the link beside: GERBER, PDF and PNG files

If you have any questions, suggestions or corrections, please leave them in the comments and we will answer them soon.

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My Best Regards!!!

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