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.


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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