Motorola Hi-Fi power amplifier
This is a very simple, low cost, Hi-Fi quality power amplifier. You can build it 5 ways, like it’s shown in the table (from 20 W to 80 W RMS).
Some comments:
- The first thing that you must do, is to measure the end transistors (T3 and T4) amplifying coefficient, the hfe or β. If their disagreement is bigger than 30 %, the amplifier would not give a clear sound. I used MJ3001 and MJ2501 transistors, and this disagreement was around 5%.
- Before the first “turning on” you must short circuit the inputs of the amp, and put a mA-meter on the output, than turn the amplifier on, and tune the R13 pot, to decrease the DC current on the output, to some uA-s, or in a lucky situation to zero. I was able to decrease it to 10 uA.
Amplifier schematic diagram:
Power supply schematic diagram:
Components and properties table:
20 / 35 W
25 / 40 W
35 / 60 W
50 / 80 W
80 / 125 W
T1
BC546
T2
BC556
T3 (PNP Darlington)
BD898, BD678
BD896, BD676
BDX34
MJ900G
2N6052, MJ2501
T4 (NPN Darlington)
BD897, BD677
BD895, BD675
BDX33
MJ1000G
2N6059, MJ3001
C1
10 ÷ 12 pF
C2
4,7 ÷ 5,6 nF
C3 (not electrolitic)
1 ÷ 10 uF
C4, C5
2200 uF / 35 V
4700 uF / 35 V
4700 uF / 50 V
R1, R2, R9, R12
10 kΩ
R3, R4, R5, R6, R14, R15
3,3 kΩ
R7, R8
680 Ω
R10
100 kΩ
R11
47 kΩ
R13 (potenciometer)
F1 (fast)
1,2 A
1,6 A
2 A
2,2 A
4 A
BR1
B40C1500
B40C2000
B40C2800
B40C3200
B40C5000
Cooler (Rth)
4,5
2,7
2,0
1,7
1,5
Transformer seconder voltage ( ± 20 % )
20 + 20 V
22 + 22 V
Transformer seconder current
0,8 A
1,3 A
1,5 A
2,5 A
Power (RMS)
20 W
25 W
35 W
50 W
80 W
Music power
40 W
60 W
125 W
Minimal speaker impedance
fred1144