100W DC servo amplifier circuit using Power MOSFET

If you are looking for DC amplifier circuit. This may be one of choice for you. This is the DC servo amplifier circuit, OCL100W.

It uses MOSFET 2SJ162 + 2SK1058 Or MOSFET 2SK134 + 2SJ49 (To-3).

The output powers of 112 wats at Speaker 8Ω. And, Requires Power Supply +56V/-56V 4A /Ch.

100 watts DC Servo MOSFET amplifier circuit

For those who want to be satisfied, so fun. This series of 100W power MOSFET amplifier be able to you for sure.

It have responding of sound frequencies very well.

Invite you to consider this circuit project below.

How it works

See in the circuit. You will see that characteristics of the circuit is similar to the OP-AMP power circuit.

Because of positive input for the signal input. And the negative input for connecting a feedback signal.

circuit diagram 100W DC servo mosfet amplifier
The circuit diagram of  DC Servo Power Amplifier  using MOSFET

This circuit can work well, even at low frequencies alike DC signal. So, we may call it the “DC Amplifier circuit”.

Also, you may apply it to the servo amplifier.

Gain and Output power

The gain of this circuit depends on the R8/R7. From values in the circuit to has amplified rates equal to 22.

If there is an input signal size of 1V. So, the output voltage is 22V.

From Ohm’s law.

We can calculate the power output equal :

Power = (ExE)/R
= 22 x 22 / 8
= 66.5W at load 8Ω.

or = 120W  at load 4Ω

So, if you want to output 100W at load 8Ω. We will need to enter the input signal is not lower than 1.28V.

Adjusting idle current

The transistor Q5 is constant current Circuits as the bias of output MOSFET. So, it can be adjusted by adjusting VR1.

It makes the circuits more good stability. Some called it the idle current.

The VR1 should be chosen carefully. For example CERMET.

Protection MOSFETs

At the gate of power MOSFET have the Zener diode to prevent the input signal is higher than 14V. Because MOSFETs will be damaged.

Recommended: High MOSFET Power Amplifier

The MOSFET runaway thermal

The advantages of the MOSFET power amplifier circuit. We know very well is preventing overload by itself.


When the over load. The MOSFET is higher the temperature. But resistance of the MOSFET is also higher. (In normal transistors, the resistance will decrease.) As a result, the current through the MOSFET lower.

If we install the heat sink the appropriate size. Then, can be easily shorted output. But the MOSFET will not be damaged.

MOSFET Amplifier Power Supply

The most important factor, which will make circuits work to its full potential is the power supply circuit.

We suggest using the power supply separately, for each amplifier.

56V 5A DC dual Power supply circuit
56V 5A DC dual Power supply circuit

The advantage of this power is that it helps reduce cross talk and low frequencies as well.

Then, the transformer should be able to provide the current at secondary coil than 3 Amps (recommended 5A).

And, has voltage output at 40 Volts center trap as well.

How to build

First, prepare all equipment.

The shopping list

0.5W Resistors, tolerance: 1%
R1: 56K
R2, R5: 3.9K
R3: 47K
R4: 1K
R7: 22K
R9: 100Ω
R10: 12K
R11, R12: 100Ω
R13, R14: 220Ω
R8, R21: 10Ω 1W

Ceramic Capacitors
C2: 47pF 50V
C10, C11: 27pF 50V

MKT, or MKP Capacitors
C5: 0.0056uF 100V
C7: 0.01uF 50V
C8, C9: 0.1uF 100V

Electrolytic Capacitors
C1: 10µF 25V
C4: 47uF 50V
C3, C6: 100uF 100V

Q1, Q2: 2SA1016 or A872, -100V 50mA, PNP TO-92 Transistor
Q3, Q4: 2SD666, 80V 50mA, PNP TO-92MOD Transistor
Q5: 2SB646, -80V 50mA, PNP TO-92 Transistor
Q6: 2SK1058( To-3P) or 2SK134 (To-3) N-channel Low-Frequency MOSFET
Q7: 2SJ162( To-3P) or 2SK49 (To-3) P-channel Low-Frequency MOSFET
D1, D2, D3: 1N4002, 100V 1A Diodes
D4, D5 12V 1W Zener Diodes

PCB, Transformer, and power supply circuit, etc.

To build, just install the components as the proper circuit layout.

See pinout of these components in this circuit

One thing to be extra careful about is installing the MOSFET on the heatsink. Do not short-circuit absolutely with the heat sink.

The PCB of 100 watt DC servo MOSFET amplifier

The Copper PCB layout of 100watts DC Servo MOSFET Amplifier

And See components Layout

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

Setting & Adjusting

When sure that everything is ok. Then, try to apply the power supply to this circuit. Without connecting the speakers.

Next, use a voltmeter to measure the voltage at the speaker. It needs to be no voltage or zero volts.

If it is not shown that the circuit failure at any point exactly.

Soldering is key
And most important, it should not be overlooked is the soldering, especially. Beginners usually soldering is not good enough.

When the output voltage have been zero. Then, the next step is to adjust the Idle current for this circuit. Connect the Ammeter at the positive terminal. And, then Adjust VR1 at only 35mA is enough.

Set idle current

The ground is best

For installation into a metal box, just like a typical high watt amplifier circuits. What needs to be careful, is to install ground.

If we installed it incorrectly. It could easily cause the oscillator.

The ground of input and output circuit. We should separate lines and the ground at the same point.

For the PCB of this circuit, the input and output ground have separate neatly.

And, the ground wire, especially output and the power supply, use a large, durable power more than 15A.

See the input that enters to this circuit. We can use both AC input and DC input, depending on the requirements. But usually, it is recommended AC Input is safer.

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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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