100w Transistor Power Amplifier Schematic: Learn How to Build It

100w Transistor Power Amplifier Schematic: Learn How to Build It
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If we compare the simplicity of the proposed 100w transistor power amplifier schematic design to its power output, which is a good 100 watts, indeed it looks very impressive.

The entire circuit utilizes commonly available components and may be simply built over a general purpose board. If all the connections are done accurately as shown in the diagram, the circuit should immediately start “pumping” your loud speakers with a high quality music output. I have personally tested this circuit and believe me its response is outstanding, build a couple of them and it becomes compatible with stereo inputs- that also means now you are producing 200 watts of brain-pounding music power.

Let’s examine the circuit functioning.

Circuit Description

At the first glance the circuit rather appears to be unsymmetrical in design, due to an unbalanced looking output stage. However a closer look will prove this wrong. Transistors T9, T10, T11 and T12, T13, and T14 form two well-balanced halves of the circuit, perfectly complementing each other.

The input stage employs the standard R/C filter configuration. R1 and R2 fix the input impedance, and the inclusion of C1 creates a high-pass filter that blocks all frequencies around 1.5 Hz. C1 also functions as an input stage DC bias isolator.

The presence of R2 and C2 ensures no frequency above 250 KHz makes its way into the circuit, thus blocking most of the high frequency RF intrusions.

Transistor T1 and T2 are wired up in a standard differential amplifier mode.

The remaining portion of the circuit is mainly the output stage and is responsible for amplifying the differential stage into the loud speakers.


Power output is 60 watts into 8 Ω and 100 watts into 4 Ω loudspeaker.

Total harmonic distortion is less than 0.01 %.

Frequency range is within 20 Hz and 20 kHz.

The input sensitivity is in the vicinity of 750 mV.

The frequency characteristics lie in between 1 dB from 15 Hz to around 100 kHz.

Due to its very high amplification factor of around 20,000, the output stage may have an ideally low quiescent current drain of about 40 mA.

The quiescent current can be set through P1 with a digital multimeter connected across resistors R6 and R7.

Adjust P1 until the meter reads about 40 mV, corresponding to 50 mA current.

Important Technical Parameters to be Followed

Although the circuit parameters are not critical and may be built over a general purpose board, care should be taken that the component layout does not differ from the circuit diagram by much.

Preferably use separate heatsinks for the transistors T10, T11, T13, T14, to avoid the involvement of messy mica isolators, heatsink paste, etc.

The output stage of the circuit is virtually unaffected by temperature variations, however ideally T8, T9 and T7, T12 may be coupled with each other (by gluing them together) to enhance thermal stability of the circuit.

The output inductor L1 is made by winding 20 turns of 0.8 mm super enameled copper wire right over the resistor R24.

The current consumption may shuffle in between 1 and 3 Amp depending on the volume level of the unit.

Parts List

You will require the following parts to build this 100w transistor power amplifier schematic design.

All resistors are 1/4w, CFR unless otherwise specified.

R1 = 470K,

R2 = 47K,

R3 = 330E,

R4, R5 = 10K,

R6, R7, R20, R21, R22, R23, R24 = 1E/2W,

R8, R17 = 56E,

R9 = 100K,

R10, R11, R12, R13 = 4K7,

R14, R15 = 10K,

R16, R19 = 100E,

R25 = 10E/2W,

P1 = 100E Preset,

C1 = 1µ/25V,

C2 = 1n, CERAMIC,

C3, C4 = 100Pf

C5 = 100n,

C6, C7 = 1000uF/35V,

L1 = see text,

D1, D2 = RED LED 5mm,

Rest of the diodes are = 1N4148,

T1 = Pair of well-matched BC546,

T2 = Pair of well-matched BC556,

T3 = BC557,

T4, T7, T8 = BC547,

T5, T12 = BC556,

T6, T9 = BC546,

T10 = BD140 (Mount over “C” channel heatsink)

T13 = BD139 (Mount over “C” channel heatsink)

T11, T14 = 2N3055 (Mount over large finned-type heatsink)

General Purpose Board,

Power Supply = 25-0-25V, 5 Amp.

Fuse, Mains Cord, Metallic Enclosure, Switch, External Sockets etc.

If you would like more information about identifying components and building up circuits, please see my article “What You Need to make the Electronic Circuits at Bright Hub.”