The 40/70 watts amplifier circuit is built around transistor, configured in various modes. The single 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 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 T5 in this amplifier circuit.
Temperature coefficient of Transistor s T10 and t11 is compensated by Transistor T8 and T9 which act like diodes. Transistor T10 with T12 & T14 and Transistor T11 with T13 & T15 form a triple Darligton pairs.
The current through T4 and T5 is kept at a value of this output of this circuit consists of triple Darlington pair. The small change in the Vbe of T10 & T11 changes output current of hi-fi amplifier greatly.
In order to prevent distortion in the 40/70 watts hi-fi amplifier when capacitive loads are connected to the output, coil L1 is used to prevent DC voltage across the speaker and is connected in series to speaker. The total gain amplifier circuit is approximate 32 and is value is calculated by (R7 + R8)/R8.
PARTS LIST:
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
Capacitors
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
Semiconductors
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
Miscellaneous
L1 = Coil having 20 turns of 20 SWG over thin
pencil
F1 = 3.5 Ampere fuse
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