DIY Motorola Hi-Fi Amplifier Guide
Uploaded by
Ureche IlieDIY Motorola Hi-Fi Amplifier Guide
Uploaded by
Ureche IlieMotorola 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 BD898, BD896, BDX34 MJ900G 2N6052,
Darlington) BD678 BD676 MJ2501
T4 (NPN BD897, BD895, BDX33 MJ1000G 2N6059,
Darlington) BD677 BD675 MJ3001
C1 10 ÷ 12 pF
C2 4,7 ÷ 5,6 nF
C3 (not electrolitic) 1 ÷ 10 uF
C4, C5 2200 uF / 4700 uF / 4700 uF / 4700 uF / 4700 uF /
35 V 35 V 35 V 50 V 50 V
R1, R2, R9, R12 10 kΩ
R3, R4, R5, R6, 3,3 kΩ
R14, R15
R7, R8 680 Ω
R10 100 kΩ
R11 47 kΩ
R13 10 kΩ
(potenciometer)
F1 (fast) 1,2 A 1,6 A 2A 2,2 A 4A
BR1 B40C1500 B40C2000 B40C2800 B40C3200 B40C5000
Cooler (Rth) 4,5 2,7 2,0 1,7 1,5
Transformer 20 + 20 V 20 + 20 V 20 + 20 V 22 + 22 V 22 + 22 V
seconder voltage (
± 20 % )
Transformer 0,8 A 1,2 A 1,3 A 1,5 A 2,5 A
seconder current
Power (RMS) 20 W 25 W 35 W 50 W 80 W
Music power 35 W 40 W 60 W 80 W 125 W
Minimal speaker 8Ω 8Ω 8Ω 8Ω 4Ω
impedance
Supply voltage ( ± ± 25 V ± 25 V ± 25 V ± 30 V ± 30 V
20 % )
Maximal current 0,54 A 0,85 A 1,0 A 1,15 A 2,0 A
Input voltage for 1,8 V 1,5 V 1,7 V 1,95 V 1,7 V
Pmax output
Input resistance 10 kΩ
Frequency band (- 0 – 100.000 Hz
3 dB)
THD at ½ P 0,04 %
THD at Psin max 0,18 %
Loudness 93 dB
I am very contented with this amplifier. It gives a very good sound quality.
Have fun with it!
Jacint Chapo
6. 5. 2005.