100 watts OTL amplifier circuit using transistor MJ15003-MJ15004

If you are looking for a 100-watt OTL amplifier circuit for the job a small PA system. For example, the news spread in the village or the sound system for the conference.

I recommend this circuit. Because the sound is very loud. When compared to other circuits at the same power.

100 watts OTL Power Amplifier circuit using transistors

And more importantly, It is very durable. Because It is OTL amplifier type. So, It does not hurt your very expensive speakers certainly.

But the speaker may sound a little annoying in the only turn it on.

In addition, we use a few components, easy to build, without any adjustment.

And It’s easy to find and buy these components replaced easily. Because we use all transistors.

Note : You can see Others amplifiers projects with PCB

How it works

Read the advantages of it is very much. Let’s see the details on this circuit. You will see that We increased the efficiency from the 50 watts OTL Amplifier circuit.

OTL power amplifier circuit using transistor

Figure 1: 100 watts OTL transistors amplifier circuit

In first pair differential transistors. We use MPS9682. Then, the preamplifiers drive, Q3. We use transistor 2SD357.

And the dual transistor drive are 2SD357 & 2SB527.

For the dual power transistors, we use MJ15003 matched pair with MJ15004. So, they have a full real wattage to 100 watts absolutely.

The 90V Power Supply

In figure next, this circuit we use the DC power supply circuit, 90 volts at 5A.

The AC voltage 62V from the transformer is converted by the Bridge diode rectifier to be DC90V voltage. Then, pass to the 10,000uF 100V Capacitor to filter to smooth DC voltage.

90V DC Power Supply circuit
90V 5A DC Power Supply Circuit

Build OTL amplifier circuit

First of all, we should get components as list below.

The Shopping Lists

0.5W Resistors, tolerance: 5%
R1, R8, R9: 10K
R2, R7: 50K
R3: 27K
R4: 1K
R5: 4.7K
R6: 120 ohms
R10: 22K
R11: 2.7K
R12: 5K
R13, R14: 300 ohms, 1W Resistor
R15, R16: 0.5 ohms 5W Resistor
R17: 10 ohms, 1W Resistor

Electrolytic Capacitors
C1,C4: 10uF 50V
C2,C5: 100uF 50V
C3: 47uF 50V
C6: 150pF 63V, Polyester Capacitor
C7: 0.022uF 63V, Polyester Capacitor

Semiconductors and others
Q1, Q2: MPS9682 PNP transistor
Some can find it. You may use BC557 (See this)
Q3, Q4: 2SD357
Q5: 2SB527

Q6: MJ15003, 20A 140V NPN, Power transistor
2SC3856 = NPN Epitaxial Planar Transistor 15A 300V 130W Silicon Transistors

Q7: MJ15004, 20A, 140V PNP Power transistor
2SA1492 = PNP Epitaxial Planar Transistor 15A 300V 130W Silicon Transistors

D1, D2, D3: 1N4002, 1A 100V Diode


This project is quite ancient. Now it also works well.
But when you will try to build a new generation still problems buying a transistor, sure.

Special instructions from a friend of mine.
On the transistor MJ15003 & MJ15004. Shape TO-03. Which is now hard to find.

I hope that friends many happy with this project. But do not forget about security. Check it carefully.

In Figure 3 is a PCB layout and positioning devices. In creating this project, be sure that the soldering is completed. The polarity of the diodes, capacitors electrolyzed is correct or not?

Copper PCB layout

The Figure 3 is PCB layout.

Click to view large size

Components layout of OTL amplifier 100 watts
Component Layout

Setting and Testing

For safety, always complete your circuit before attaching your power to the circuit.

You should remove the power transistors before. Then, attach the probe of the voltmeter at a point between R15, R16.

If you are using a multimeter that is not auto-ranging, set it to the 100V range.

Then, add a power source to this project. The voltage must be between 40V to 50V.

But if wrong of these shows, the circuit will not work correctly.

To check the value of the equipment used to assemble the proper place for it or not.

Once you make sure it, so can be connected to the MJ15003+MJ15004 in the circuit.

However, need to make sure that the installation of transistors on a piece of the heat sink all that is good. Do not lose, or short-circuit with a cooling strictly prohibited.

Then, the input power into the circuit again.

Measure voltage is equal when does not connect the power transistors. Then Input the signal from preamplifier tone control or audio CD to hear a sample sound.

So the complete process of creating this project it.

Caution! This project needs to use the speaker protection circuit. Otherwise, your speaker may be damaged.

You may like these circuit, too.


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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