Circuit diagram:

25 Watt Amplifier


 R1, R4_________47K 1/4W Resistors R2____________4K7 1/4W Resistor R3____________1K5 1/4W Resistor R5__________390R 1/4W Resistor R6__________470R 1/4W Resistor R7___________33K 1/4W Resistor R8__________150K 1/4W Resistor R9___________15K 1/4W Resistor R10__________27R 1/4W Resistor R11_________500R 1/2W Trimmer Cermet R12, R13, R16__10R 1/4W Resistors R14, R15_____220R 1/4W Resistors R17___________8R2 2W Resistor R18____________R22 4W Resistor (wirewound) C1___________470nF 63V Polyester Capacitor C2___________330pF 63V Polystyrene Capacitor C3, C5________470?F 63V Electrolytic Capacitors C4, C6, C8, C11_100nF 63V Polyester Capacitors C7___________100?F 25V Electrolytic Capacitor C9____________10pF 63V Polystyrene Capacitor C10____________1?F 63V Polyester Capacitor Q1-Q5______BC560C 45V 100mA Low noise High gain PNP Transistors Q6_________BD140 80V 1.5A PNP Transistor Q7_________BD139 80V 1.5A NPN Transistor Q8_________IRF532 100V 12A N-Channel Hexfet Transistor Q9_________IRF9532 100V 10A P-Channel Hexfet Transistor  

Power supply circuit diagram:

Power supply


 R1____________3K3 1/2W Resistor C1___________10nF 1000V Polyester Capacitor C2, C3______4700?F 50V Electrolytic Capacitors C4, C5_______100nF 63V Polyester Capacitors D1__________200V 8A Diode bridge D2__________5mm. Red LED F1, F2_______3.15A Fuses with sockets T1__________220V Primary, 25 + 25V Secondary 120VA Mains transformer PL1_________Male Mains plug SW1_________SPST  Mains switch  


  • 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 in series with Q8 Drain) with no input signal.
  • A correct grounding is very important to eliminate hum and ground loops. Connect in the same point the ground sides of R1, R4, R9, C3 to C8. Connect C11 at output ground. Then connect separately the input and output grounds at power supply ground.

Technical data:

Output power: well in excess of 25Watt RMS @ 8 Ohm (1KHz sinewave)

Sensitivity: 200mV input for 25W output

Frequency response: 30Hz to 20KHz -1dB

Total harmonic distortion @ 1KHz: 0.1W 0.014%
1W 0.006% 10W 0.006% 20W 0.007% 25W 0.01%

Total harmonic distortion @10KHz: 0.1W
0.024% 1W 0.016% 10W 0.02% 20W 0.045% 25W 0.07%

Unconditionally stable on capacitive loads