Description

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:How to build 25 Watt Audio Amplifier Circuit - circuit diagramParts:
  • 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:How to build 25 Watt Audio Amplifier Circuit - circuit diagramParts:
  • 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.
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