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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