Power MOSFET BD522 and BD512 is now obsolete, so you can use other power MOSFET according to its characteristics. Q1 and Q2 are wired as a pair Darlington transistor that serves as a preamplifier. R3 Preset controls the quiescent current while R2 providing feedback. The output coupled to the speaker through capacitor C4. capacitor C5 Is a power filter andC2 is decoupling capacitor for DC input.

Mosfet Audio Amplifier Parts list :
R1 : 39K
R2 : 560K
R3 : 4.7K Variable
R4 : 10K
C1 : 33pF
C2 : 1uF
C3 : 4.7uF
C4 : 220uF
C5 : 2200uF
D1 : 1N4001
Q1, Q2 : BC107
Q3 : BD512
Q4 : BD522

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This circuit have Power output is 200 Wrms in 8 ohms or 350 Wrms into 4 ohms. Less than 0.02% distortion, damping factor greater than 300, the ratio signal / noise 112 dB (A balanced at full power), input sensitivity is 1.2 volts (200 W / 8 ohms).

Current source of + / – 1 mA carried around T5 and T6. Diodes D1 to D6 allow the use of low-noise type BC560C and BC550C transistors, transistor T7 to T10 is the driver stage. Potentiometer P1 allows setting the quiescent current of 100 mA per output transistor.

Symmetrical power entrusted to the large transformer 625 VA, 2 * 51 volts + bridge rectifier and 4700 uF capacitor 6, which gives the output voltage of + and – 70 volts per rail.

Note :

• The capacitor C3 is not reproduced on the diagram and on the PCB (it is correct).
• Transistors T9 and T10 must be assembled on a common heatsink with 5° c/w thermal resistance or on the output stage heatsink.
• The ground (earth) points of PCB’s and loudspeaker ground (earth) must compulsorily be connected in star with the 0 Volts of the PSU.
• The power transistors must be assembled on a heatsink with a thermal resistance less than 1 ° C/W. They must also be electrically insulated from the heatsink by using a mica insulator + heat-conducting compound or a silicone insulator.

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25 Watt power amplifier based MosFet,  can be connected directly to the signal source and does not require pre-amplifier to drive. The amplitude of the input signal must reach the level of 200 mV. This signal is normally present on the linear output recorder, CD player, computer sound card and other devices. To adjust the volume can be placed at the entrance to the potentiometer 10 kohm. Trimmer adjustable quiescent current. It must be within 100 mA.

Technical data:
Output power :  well in excess of 25 Watt RMS @ 8 Ohm (1KHz sine wave)
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

Power supply for this 25 watts power amplifier based MosFet can be found here.

Parts list :
Resistor
R1,R4 : 47K  1/4W
R2 : 1/4W
R3 : 1K5  1/4W
R5 : 390R   1/4W
R6 : 470R   1/4W
R7 : 33K   1/4W
R8 : 150K   1/4W
R9 : 15K   1/4W
R10 : 27R   1/4W
R11 : 500R   1/2W Trimmer Cermet
R12,R13,R16 : 10R   1/4W
R14,R15 : 220R   1/4W
R17 : 8R2    2W
R18 : R22   4W Resistor (wirewound)
 Capacitor
C1 : 470nF  63V Polyester Capacitor
C2 : 330pF  63V Polystyrene Capacitor
C3,C5 : 470µF  63V Electrolytic Capacitors
C4,C6,C8,C11 : 100nF  63V Polyester Capacitors
C7 : 100µF  25V Electrolytic Capacitor
C9 : 10pF  63V Polystyrene Capacitor
C10 : 1µF  63V Polyester Capacitor
 Semiconductor
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 : IRF530  100V 14A N-Channel Hexfet Transistor
Q9 : IRF9530 100V 12A P-Channel Hexfet Transistor

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