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Motorola Hi-Fi Power Amplifier

Source: InternetPublisher:公子李 Keywords: Power amplifier power amplifier circuit HIFI Updated: 2024/06/28

This is a very simple, low cost, hi-fi quality power amplifier. You can implement it in 5 ways, as indicated in the diagram (from 20W to 80W RMS).

Some explanations:

- The first thing you should do is to test the gain factor hfe or β of the final power tubes. If their difference is greater than 30%, the amplifier will not give you a clear sound. I used MJ3001 and MJ2501 transistors and their difference was within 5%.

- Before turning on the power, you must short-circuit the input terminals, connect an ammeter in series with the output of the amplifier, then turn on the power supply and adjust R13 so that the ammeter current reaches the microampere level. If you are lucky enough, you may be able to reach 0. It is easy to achieve an ammeter current of 10 microamperes.

Power amplifier circuit diagram:

Power circuit:

Component and property 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 to 12 pF
C2	4.7 to 5.6 nF
C3 (non-polarized capacitor)	1～10uF
C4, C5	2200 uF / 35 V	4700 uF / 35 V	4700 uF / 35 V	4700 uF / 50 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 (Trimming resistor)	10 kΩ
F1 (Fast)	1,2 A	1,6 A	2 A	2,2 A	4 A
BR1	B40C1500	B40C2000	B40C2800	B40C3200	B40C5000
Cooler	4,5	2,7	2,0	1,7	1,5
Transformer secondary voltage (± 20 %)	20 + 20 V	20 + 20 V	20 + 20 V	22 + 22 V	22 + 22 V
Transformer secondary current	0,8 A	1,2 A	1,3 A	1,5 A	2,5 A
Power (RMS)	20 W	25 W	35 W	50 W	80 W
Music Power	35 W	40 W	60 W	80 W	125 W
Minimum speaker impedance	8Ω	8Ω	8Ω	8Ω	4Ω
Supply voltage (± 20 %)	± 25 V	± 25 V	± 25 V	± 30 V	± 30 V
Maximum current	0,54 A	0,85 A	1,0 A	1,15 A	2,0 A
Required input voltage at maximum power	1,8 V	1,5 V	1,7 V	1,95 V	1,7 V
input resistance	10 kΩ
Bandwidth (-3 dB)	0 C 100.000 Hz
THD at ½ P	0,04 %
THD at Psin	max 0,18 %
Frequency Response	93 dB

I am quite happy with this amp, it has a nice sound quality. Have fun with it!

公子李

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