How to Make a Hi-Fi 100 Watt Amplifier Circuit Using 2N3055 Transistors

The amplifier circuit explained here was built and tested by me and am extremely pleased by its performance and also ruggedness as far as maintenance and handling is concerned.
Basically, the entire configuration is a symmetrical class A amplifier incorporating an input filter stage, an intermediate driver stage and a powerful symmetrical output stage consisting of the versatile 2N3055 power transistors. The circuit efficiently drives a 100 watt 4 Ohms speaker with inputs derived from any audio source like a cell phone or DVD player etc.

Before you learn how to build this interesting and useful 100 watt amplifier circuit using 2N3055 transistors, a prior understanding of the involved circuit configuration would be very handy, let’s begin the explanation with the following points:

Circuit Description

A quick glance at the given circuit diagram makes us conclude that the output configuration is not symmetrical, since the transistors T15 and T16 are both NPN types.

The input stage of the circuit begins or initiates with a symmetrical differential preamplifier stage consisting of the transistors T1, T2 and T3, T4.

T5 and T6 are positioned as the current sources which are further extended as the driver stage consisting of the transistors T7 and T8.

However a closer inspection tells us that of course the wiring is symmetrical, having the transistors T11, T13, T15 at the upper section acting like special booster transistor package.

Similarly the lower section also employs identical super booster stage consisting of the transistors T12, T14 and T16.

The above two sections are perfectly complementary to each other, with reference to the diagram which indicates their emitters being terminated to a common point through the resistors R25 to R27 and via R28 to R30, this effectively that the wiring is exclusively symmetrical by nature.

The output stage is able to produce a massive 200,000 times amplification factor with comparatively very low quiescent current drain.

The quiescent can be set by the adjusting the preset P1.

Due to a non critical nature of the circuit, the entire project can be easily built over a general purpose PCB, however the layout of the components or rather the placement and the ratio of the distance of the components must be kept as identical as possible to the layout of the circuit diagram.

Though a common heatsink may be used for the entire set of the output devices, I personally used separate individual heatsinks for each of the transistors. This saved me from the headache of using the cumbersome and low efficient mica isolation kit between the transistors.

The inductor is kept for improving the dynamic nature of the circuit. It is built by winding 20 turns of super enameled copper wire over the 1 Ohms resistor itself. The wire is selected to be close to 1mm in thickness.

Though not absolutely necessary, for better thermal stability the transistors T9 and T11 and also T10 and T12 should be glued together, preferably by attaching the respective pairs face to face.

The quiescent current should be ideally set to 50 mA through the following initial procedure:

Short the speaker terminals,

Connect a DMM set at current range in series with the positive of the power supply to the circuit,

Next adjust the preset such that the meter reads an input of 50mA, that’s all, the amplifier’s quiescent current is set and now the connections may be restored for the normal operations of the system.

The power supply circuit is also shown along side and as can be seen there’s nothing special about it and may be built using the shown ordinary sets of components.




Parts List of 100 watt amplifier circuit

R1 = 430 K,

R2 = 47K,

R3 = 330 Ohms,

R4, R5 = 12 K ,

R6, R7, R20, R21, R22, R23, R24 = 1 Ohm, 3 Watt, Wire Wound Type

R8, R17 = 68 Ohms,

R9 = 100 K,

R10, R11, R12, R13 = 5K6,

R14, R15 = 12 K,

R16, R19 = 100 Ohms,

R25 = 10 Ohms / 2 Watts,

P1 = 100 Ohms Preset, Linear

C1 = 1 µF / 25V,

C2 = 1 n, CERAMIC,

C3, C4 = 100Pf

C5 = 100 n,

C6, C7 = 1000 uF / 35 V,

L1 = 20 turns of enameled 1mm copper wire over R24,

D1, D2 = RED LED 5mm,

All other diodes are = 1N4148,

T1 = Matching BC546 pair,

T2 = Matching BC556 pair,

T3 = BC 557B,

T4, T7, T8 = BC 547B,

T5, T12 = BC 556B,

T6, T9 = BC 546B,

T10 = BD 140  

T13 = BD 139  

T11, T14 = 2N 3055  

General Purpose PCB,
All the transistors T10, T13, T11 and T14 ae mounted on suitable heatsinks



The original design, (courtesy - elektor electronics)

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