70/40 Watts Hi-Fi amplifier

Here is a circuit of the world best Hi-Fi amplifier you may never have to replace by a better one. The high output, good specification, and small size make this 70/40 watts hi-fi amplifier more versatile.

Circuit description of 70/40 watts hi-fi amplifier

The 70/40 watts hi-fi amplifier circuit is build around transistors, configured in various modes. The signal to be amplified is given to base of transistor T1 configured as differential amplifier with T2. Transistor T3 is used to maintain the current of differential amplifier.

Transistor T4 with T5 forms a cascaded pair derived directly by transistor T1. For better result transistor T6 and T7 again configured as cascaded pair and provide constant current source to cascade pair T4 and Tin this amplifier circuit.

Temperature coefficient of transistors T10 and T11 is compensated by transistors T8 and T9 which act like diodes. Transistors T10 with T12 & T14 and transistors T11 with T13 & T15 form a triple Darlington pairs.

The current through T4 and T5 is kept at a low value of about 6mA because the output of this circuit consists of triple Darlington pairs. The small change in the VBE of T10 and T11 changes output current of hi-fi amplifier greatly.

In order to prevent distortion in the 70/40 watts hi-fi amplifier when capacitive loads are connected to the output, coil L1 is used. Fuse is used to prevent DC voltage across the speaker and is connected in series to speaker. The total gain of this amplifier circuit is approximate 32 and is value is calculated by (R7 + R8)/R8.

advanced hi-fi amplifier


Resistors (all ¼-watt, ± 5% Carbon)

R1, R7 = 100 KΩ

R2, R3, R9 = 1.2 KΩ

R4, R5, R6 = 2.7 KΩ/2W

R8 = 4.7 KΩ

R10, R13, R16, R18, R19 = 100 Ω

R11 = 3.3 KΩ

R12, R17 = 680 Ω

R14, R15, R22, R23, R24, R25 = 1 Ω/2W

R20 = 10 Ω/2W

R21 = 1o Ω/1W

VR1 = 100 Ω


C1 = 1 µF/polyester

C2, C3 = 25 µF/25V electrolytic

C4, C8, C9 = o.1 µF polyester

C5 = 10 µF/60V electrolytic

C6 = 4.7 µF/10V electrolytic

C7 = 56 pF ceramic disc


T1, T2, T3 = BC546B

T4 = BC558B

T5, T11 = 2N4033

T6, T10 = 2N3019

T7 = BC548B

T8, T9 = BC147B

T12 = BD140

T13 = BD139

T14 = 2N3055

T15 = MJ2955

ZD1 = 3.3V 400mW zener diode

ZD2 = 3.9V, 400mW zener diode


L1 = Coil having 20 turns of 20 SWG over thin pencil

F1 = 3.5 Ampere fuse


Output power (1 KHz, o.7% THD): 73W into 4-Ω and 44W into 8-Ω

Offset Voltage: Less than ±40mV

Input impedance: 100 KΩ

Harmonic distortion : 0.015%

Intermodulation distortion (70W): 0.02%

Frequency range: 10 Hz – 30 KHz, ±2dB

Signal-to-noise ratio (out = 100mW): Over 72dB



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7 thoughts on “70/40 Watts Hi-Fi amplifier”

  1. This 70/40 Watts Hi-Fi amplifier was published in electronics for you in oct. 1992 by the name of Mr. C. Sanjay and i had constructed the same for my stereo music system in the year 1998 till now this amplifier is performing very well. During construction i was faced the problem if i leave audio input point open (without volume pot) then both output transistors (2N3055 and MJ2955) got overheated so i connected a 10K ohm resistance before C1 and ground point. Problem was solved.
    But before few days this amplifier (Left channel of my stereo music system) become faulty (output transistor burnt four this) so i replaced almost all the transistors and related components but problem was as it is. I was very confused, so finally i have connected a diode IN4007 in forward bias across R18 and R19 resistor to limit the voltage drop across these resistor after that output transistors are very cool and the performance of amplifier is same. Can you please suggest why transistors were heating. Awaiting for your expert reply

  2. Hi,I was wondering if I could use this to make an amp for a guitar,and if so,where would be the input and the volume adjust? the output in the project i suppose it’s to an speaker right?Need an answer as soon as possible,please.

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