25 Watt Audio Amplifier Circuit

High Quality simple design, No need for a preamplifier

This is a 25 Watt basic power amp that was designed to be (relatively) easy to build at a reasonable cost. It has better performance than the standard STK module amps that are used in practically every mass market stereo receiver manufactured today. When I originally built this thing, it was because I needed a 25 Watt PC amp and did not want to spend any money. So I designed around parts I had in the shop.

Circuit diagram:



Parts:

R1 = 47K
R2 = 4K7
R3 = 1K5
R4 = 47K
R5 = 390R
R6 = 470R
R7 = 33K
R8 = 150K
R9 = 15K
R10 = 27R
R11 = 500R-1/2W
R12 = 10R
R13 = 10R
R14 = 220R
R15 = 220R
R16 = 10R
R17 = 8.2R-2W
R18 = 22R-4W(wirewound)
C1 = 470nF-63V
C2 = 330pF-63V
C3 = 470µF-63V
C4 = 100nF-63V
C5 = 470µF-63V
C6 = 100nF-63V
C7 = 100µF-25V
C8 = 100nF-63V
C9 = 10pF-63V
C10 = 1µF-63V
C11 = 100nF-63V
Q1 = BC560C
Q2 = BC560C
Q3 = BC560C
Q4 = BC560C
Q5 = BC560C
Q6 = BD140
Q7 = BD139
Q8 = IRF530
Q9 = IRF9530

Power supply section:


Parts:

R1 = 3K3-1/2W
C1 = 10nF-1000V
C2 = 4700µF-50V
C3 = 4700µF-50V
C4 = 100nF-63V
C5 = 100nF-63V
D1 = 200V 8A Diode bridge
D2 = 5mm. Red LED
F2 = 3.15A Fuses with sockets
F2 = 3.15A Fuses with sockets
T1 = 220V Primary, 25 + 25V Secondary 120VA Mains transformer
PL1 = Male Mains plug
SW1 = SPST Mains switch

Notes:

• Can be directly connected to CD players, tuners and tape recorders. Simply add a 10K Log potentiometer (dual gang for stereo) and a switch to cope with the various sources you need.
• Q6 & Q7 must have a small U-shaped heatsink.
• Q8 & Q9 must be mounted on heatsink.
• Adjust R11 to set quiescent current at 100mA (best measured with an Avo-meter connected in series to Q8 Drain) with no input signal.
• A correct grounding is very important to eliminate hum and ground loops. Connect to the same point the ground sides of R1, R4, R9, C3 to C8. Connect C11 to output ground. Then connect separately the input and output grounds to power supply ground.
• An earlier prototype of this amplifier was recently inspected and tested again after 15 years of use. Results, comments and pictures are shown here.