Many electronics circuits and projects for learning in simple ways.

50W OCL main amplifier using LF351-2N3055-MJ2955 with PCB

This 50W OCL main amplifier circuit has high quality and economical. Because it is designed simple, It can be easily created in your friends, An electronics rookie. And cheap when compared to other speakers, the as same volume level.

The main power amp.
This circuit is designed in such a way “Direct Coupling All Complementary IC Drive” will be seen that, the heart of the circuit is IC1(LF351), are the differential amplifier and pre driver circuit.
Note: I chose LF351, because the noise reduction circuit. And high frequency response than IC741, or you may use a number TL071 or NE5534N is good quality as well.
The output signal of IC1 pass through two diodes D1,D2(both is 1N4148) to the transistor drivers Q1(BD139),Q2(BD140). Then, their current is entered to two main transistor output Q3(MJ2955),Q4(2N3055).
The small Potentiometer VR2-1K is fined the idle current of this amplifier circuit that easy to fine who not has an amp meter to do not need measure the IDLE current, you just fine the VR2 at middle only (very simple!)


The dc dual power supply +35V GND -35V, 3A for Mono and 5A for stereo system.

Instructions on how to create.
1. Check the completed of PCB that defective, short circuit or not.
2. First, do not connect to the output transistors(MJ2955,2N3055).
3. Use a voltmeter measure voltage at output speaker and ground point.
4. Supply to this circuit And must meter reading is zero volts. Or error must not exceed 0.5V.
5. put the output transistor into this circuit.
6. The voltage must be OV the same, If non-zero, indicating that the bad output transistors.
7. Connect speakers and input signal to this amplifier circuit. Let’s to listen the melodic music.


The Electronic parts.
ZD1,ZD2___15V 0.5W Zener diode
VR1______50K Potentiometer
VR2______1K pot
C1,C9____10uF 16V Electrolytic capacitors
C3,C4___47uF 25V Electrolytic capacitors
C2______100uF 16V Electrolytic capacitors
D1,D2____1N4148 Diode

The caution.
Holding the output transistors onto the heat sink you must sure that:
1. The screws tight, not fell, the loose easily.
2. Do not the short circuit between the collector Pin(transistor body) and heat sink, Due to the installation of mica insulation is not well.

And….. This circuit may be good for you!


Besides this, we introduce these projects sometimes, you may like them.

1. 55 watts rms classic OTL integrated amplifier easy to builds

2. The 30 watts rms integrated amplifier projects using cheap transistors.

3. Power Amplifier OCL 50W by Mosfet (K1058 + J162)

This Post Has 32 Comments

  1. I require Simple universal audio music system of 6volt

  2. To whom it may concern,

    I am working in a powder coating plant as a manager. We have electrostatic machines that uses +/- 70 – 80 kV and only micro amps to charge the powder.

    I have several diagrams of how to build a voltage multiplier but I need a little more help.

    If the input voltage is max 16 volt AC but can be varied to less with a pot to approximately 50% to produce 35-40kV output. The transformer circuit also goes through a TIP3055 transistor and have 2 x 5W 220ohm resistors. I do not have a complete diagram as yet due to time limitations.

    Can you think about this and assist in designing a proper circuit with components so that I can build these units for my self.

    The machines we have were Staticote machines and I don’t get repair assistance without being severely overcharged.

    Thank you for your time. I hope you can assist me in this regard.

    Christo Smit

  3. Hello
    can you tell me the The Electronic parts. for all at The main amplifier 50 watt.
    and what is BD1 and the Transformator 40 or 35 V 2 or 3A

    Thanks Andreas

  4. Hello,

    Look forward to building this amp!


  5. Hi,Andreas Sohn.

    This project is work so well.
    You can use 5A 24V CT 24V transformer for stereo.
    It power full 50 watts rms.

    good luck.

  6. I like how concise your notes are ! It’s one of my greatest dreams to mould an audio amplifier but unfortunately, I lack the basic knowledge about the components ;this problem ranging from how each component looks like (thus their pictures) ,their functions to their connections. I will be very grateful to have access to any material that helps me manoeuvre this problem . Thanks !

  7. What to do if it sound noisy . It only has great sound when half of its volume but noisy onwards.

    Please reply thanks


  9. hi
    where should i connect the grand wires in amp board?
    +35 in power board connect to +35 in amp board
    _35 in power board connect to _35 in amp board
    ground in amp board ???
    T1 = 24-0-24 5 A
    output voltag in power supply = 66 volt why?
    amp not working please help me

  10. Hi amir,
    Thanks for your question.
    First I am sorry if I understand you not cleared.
    As Figure power supply circuit “GND = Ground”
    Yes, the DC power supply voltage will be “33.6V + 33.6V about 66V.

  11. Is it okay to use NT955G Power Mosfet instead of using MJ 2955 ?

  12. this circuit is quite alright but pls i need a description of how the circuits operates pls reply asap. Thanks

  13. i ll appreciate if i get the description about how both the amp and power circuits on this page operate asap

  14. hey. i just want to ask . the power supply that we have produces only +35V and 0V how can i make it +/- 35?

  15. Great, just finished it and it works a charm.

    Thank you.

  16. I am working in a power amplifier circuit on power system not on audio system. the power must be amplifier. for examples when we give 2 watts it gives 40 or 50 …. watts. and also it must be proportional.

  17. so how to design the circuit? pleas help me every body. I want the circuit diagram

  18. DC output speakers What should I do. quick Reply!!

  19. Values 470 ohm the resistor burns

  20. Hello, Adjustable resistance 1k VR2 to be set by reference to what?. Thank you

  21. IC1 LF351 pin 3 DC biasing is missing , the circuit wil not work that way!

  22. lf351 mistake I see in + entry should be connected to ground via a resistor

  23. Very simple circuit to build. But documentation is inadequate. Wattage of resistors should be mentioned. Regards.

  24. super good amp

  25. Did all 17 resistor are 1/2w. Please help

  26. Thanks request my transformer 125+125-
    Old crown amplifier diss barn myching design PDF wanted Bass request

  27. An fascinating discussion is worth comment. I feel that it’s best to write more on this subject, it won’t be a taboo topic however usually people are not sufficient to talk on such topics. To the next. Cheers

  28. can it stand for 6amperes?

  29. cause the 100 ohm resistor always burn right next to that bd139 transistor. but only on that positive side.

  30. Montage très simple et performant ,les resistances R 10 et R 11 doivent être supprimées pour avoir un courant de repos correct

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