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

If you want a 50W OCL main amplifier circuit, high quality and save money. You can use it to amplify the normal sound in the home. At the level of pleasant sound power. It was the first amplifier circuits that I created.  Also, we will see this circuit in a generally affordable amplifier.

Because it has fewer components, transistors-2N3055-MJ2955, a low noise IC-LF351, a few parts. You can buy it easily.

If compare it at the same volume level is cheaper than other circuits.

Main power amplifier working

50W main amplifier OCL

Designers set this circuit in  “Direct Coupling All Complementary IC Drive”. The heart of the circuit is  IC1(LF351). It is the differential amplifier pre-driver circuit.


I chose LF351.  Because of the lower noise reduction circuit. Also, high-frequency response than IC741.   While you may use a TL071 or NE5534N is good quality as well.

The output signal of IC1 flows through two 1N4148 diodes D1 and D2 to the transistor drivers Q1(BD139), Q2(BD140).

Then, these current come to two main transistor output Q3 (MJ2955), Q4(2N3055).

The small Potentiometer VR2-1K is adjusted to the idle current of this amplifier circuit.  This circuit is so easy to adjust.

main amplifier circuit 50W using LF351 2N3055, MJ2955 with PCB

If you do not have an ammeter, you do not need to measure the IDLE current. You just adjust  the VR2 in the middle only (very simple!)


The DC dual power supply +35V GND -35V, 3A for MONO, and 5A for the stereo system.

Instructions on how to building

  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, we should read the meter is zero volts. Or error must not exceed 0.5V.
  5. Put these output transistors 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 signals to this amplifier circuit.

    Let’s to listen the melodic music.

Read also:

The Electronic parts

IC1: LF351, TL071, LM741 normal op-amp
Q1: BD139
Q2: BD140
Q3: 2N3055
Q4: MJ2955
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

Then, get the Actual-size of Single-sided Copper PCB layout and PCB layout. See below.

PCB copper layout of amplifier 50W
The copper PCB layout of 50w OCL main amplifier
The component layout of 50w main amplifier
The component layout of 50w main amplifier

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.

There is a circuit like the above circuit. I collect old circuits. Sometimes you may see them on a power amplifier in your house.

Class AB, 60W to 70W Power Amplifier

This circuit is similar to the first circuit. When increasing the voltage of power supply to + 38V and -38V. It will increase the output power to 60 watts to 70watts.


Since this circuit is an old circuit and I have not tried to build it. So, unable to confirm that it works.

Power Amplifier OCL 70 watts using IC-741, 2N3055

However, I believe that this circuit has good sound quality. And should learn it

Change for the better

  • IC-LM741 is a normal Op-amp IC. It is cheap and popular. But if you use LF351, low-cost high-speed JFET input operational amplifier. It works well.
  • You may use TIP3055 and TIP2955 instead of 2N3055 and MJ2955. Because they are small size but have high power.
  • Change the voltage of secondary transformer to 27V CT 27V 3A

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

Related Posts


I always try to make Electronics Learning Easy.

This Post Has 35 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. lf351 mistake I see in + entry should be connected to ground via a resistor

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

  23. super good amp

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

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

  26. 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

  27. can it stand for 6amperes?

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

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

  30. This is a opposite pcb

  31. What language is this written in
    “A little this equipment circuit”
    What the hell does that mean ?

    1. Hi Wendell Hammond,

      I am sorry for my bad English. I hope you get happy with this project.

      Thanks for your text.

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