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

If you want a 50W OCL main amplifier circuit, high quality and save money. I recommend this circuit.

You can use it to amplify the normal sound in the home. At the level of pleasant sound power. It was the first amplifier circuit that I created. Also, we will often see this circuit in a generally affordable amplifier.

50W OCL main amplifier using LF351 2N3055 MJ2955 PCB


Because it has fewer components others circuits. It consists of transistors-2N3055-MJ2955, a low noise IC-LF351, a few parts. And, you can buy them easily.

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

Of course, everything has its disadvantages.

This circuit also has a disadvantage, it is an OCL amplifier. Someone who is interested in an audio system would know that if something goes wrong. It may easily cause the speaker to be broken.

However, we can reduce the problem as well. Add the speaker protection circuit.

And it has likely an interference easily as well. Which we also reduce this problem, choose IC with low noise.

This circuit is suitable for Experienced people, people who have already built other small amplifiers. However, if you are a beginner. We will build it meticulously and have a process. Yes, you can do it.

Main power amplifier working

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.

50W Main Amplifier Circuit 741 2N3055 MJ2955

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!)

35V Dual DC power supply

This main amplifier requires an unregulated power supply circuit that can supply sufficient electrical power.

50W main amplifier 35V dual power supply circuit

C1, C2, C3, C4 eliminates a high-frequency noise signal that may occur while BD1(Bridge Diode) is operating.

LED1 shows power on. The R1 limits a safe current for LED1.

This is DC Dual power supply, +35V GND -35V 3A for the MONO system.

But if stereo system, We need a power supply circuit that can supply twice the current.

  • Change transformer to 5A
  • Add more filter capacitors(C5, C6, C8, C9) 4,700uF to reduce a ripple voltage (noises or Hum…).
50W main amplifier OCL

How to Build

First of all, get the components as electronic parts below.

The Electronic parts

I think this kind of circuit High flexibility. Some people may use different components.

The Main Amplifier

IC1: LF351, TL071, LM741 op-amp; See text below
Q1: BD139 or 2N3053(TO-39)
Q2: BD140 or 2N4037(TO-39)
Q3: 2N3055 or TIP3055
Q4: MJ2955 or TIP2955
ZD1,ZD2: 15V 0.5W (1N5245) Zener diode
VR1: 50K (A) 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
PCB, 8 pin DIP IC Sockets, and more.

The Power supply

BD1: 6A 100V Bridge Diode
C5, C6, C8, C9: 2,200uF 50V Electrolytic Capacitors
C1, to C4, C7, C10: 0.1uF 100V Ceramic or Mylar Capacitors
T1: 24V CT 24V, 3A secondary transformer
R1: 6.8K 0.5W resistor
LED1: LED any color
F1: 0.5A Fuse

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
  • Check the completed PCB, Is it defective, short circuit?
  • Before assembling an electronic circuit, be careful with polarized devices. We only need to connect them correctly.

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polarized-components 50W amplifier

While working, the power output of the transistor is very hot. So we have to mount it on the heatsink. And we should have the correct mounting method. Because of its structure, pin C is connected to the body.

Proper way mounting 2N3055 in heatsink

See the pattern below for the correct mounting. After that check that does not shorted-circuit between each leads C, B, E, and the Heatsink, with using Digital Multimeter(DMM) to CONTINUITY.

You must sure that:
1. The screws tight, not fell, they lose 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.

Step by Step making

  • Assemble the power supply circuit first. Measure the output voltage level with a Digital multimeter. Set to dial of the Digital Multimeter(DMM) to DCV. Then, touch the end of both probes between +V and GND and -V. It is +33.9V (approx 35V).
  • Assemble the main amplifier circuit. Except, do not put IC1(LF351) and power output transistors Q3(2N3055) and Q4(MJ2955).

    Why does not put the transistors? Because while the transistors are working. They will have a high current. If there is an error. It has very damaged the circuit. So we have to check it thoroughly first.
  • Check the circuit again.
  • Connect the power supply to the circuit. And turn on it.
  • Use a voltmeter to measure the voltage at ZD1 and ZD2. Each Zener diode should read at 15V. Because the op-amp IC1 requires this regulated voltage supply. (+15V GND -15V)
  • Then, turn off the circuit. Put IC1 on the socket 8 DIP. Rotate, the VR1 (Volume) to a minimum.
  • Next, turn on the circuit. And, Measure voltage between the output speaker and ground point. We should read the meter is zero volts. Or error must not exceed 0.5V.
  • We need to figure out why the voltage is not zero. And solve it. Please check again.
  • Then, Put these output transistors into this circuit. The voltage must be OV the same. If non-zero, indicating that the bad output transistors.
  • Then, adjust Idle current with rotating VR2. Look at the block diagram above. Start with set VR2 in middle. Measure the current of the circuit with the Ammeter. Rotate VR2 until reading the meter at 20mA (approx).
  • Now you are done with the customization process. Connect speakers and enter signals to the input of this amplifier circuit.

Mistake and possible causes

Sometimes the circuit may not work as expected. You can consider these reasons.

If measuring the voltage is not zero.

1. It is a positive. The mistake point may be caused by:

  • Check IC1, it is an error.
  • Transistors – Q1 and Q3 may be broken in an internal short circuit.
  • Transistors – Q2 and Q4 may be broken in open-circuit, not runs.

2. It is a negative. Check these:

  • Check IC1
  • Transistors – Q2 and Q4 may be broken in an internal short circuit.
  • Transistors – Q1 and Q3 may be broken in an open circuit.

3. It is zero. But no sound. Check R15, R16 They may lose.

4. The sound is not clear. The sound has noise.

  • Check the B-E extension wires of the power output Transistors. There may be a problem.
  • Bad quality IC
  • Oscillation or noise within the circuit.

See the prototype of 50W amplifier circuit.


Back of the project

Read also:

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 the secondary transformer to 27V CT 27V 3A. It makes the voltage of the power supply up to 38.17V. So, the power of the amplifier also up. But it shouldn’t add too much voltage. Because power transistors can tolerate a voltage of no more than 60V. It may cause oscillated noise more easily.

This circuit is an OCL amplifier. So, you should a Speaker protector circuit, too.

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

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I always try to make Electronics Learning Easy.

Apichet Garaipoom

I love the electronic circuits. I will collect a lot of circuit electronics to teach my son and are useful for everyone.
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