DIY - MosFet 40W Audio Power Amplifier

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  • Very low parts count
  • Symmetrical clipping and high rail to rail output swing
  •  Uses unique "double bootstrap" technique
  • 40W continuous output power into 8 ohm @ <0.1%THD20
  • Low noise and hum
  • PCB available
Schematic, Parts list, PCB

Walk through the project

Below is a schematic of a fully functional and well behaving power mosfet amplifier. It is very compact, cheap and well performing. It is inteded for 8 ohm loads and should not be used with 4 ohm speakers.

It also utilizes an unique "dual-bootstrap" technique,  invented by the author. This simple idea guarantees full output stage symmertic saturation at both power supply rails, without additional supply voltages above and below the supply rails. Typically the VAS (voltage amplifying stage, in this case the BC489 transistor) collector would be connected to the gate of the uppes source-follower N-channel MOST transistor (MTP10N08). The other gate would be pulled down by a bootstrap circuitry (1.5K resistors and the 100µF cap). In this amplifier the collector is connected to the lower voltage follower P-channel MOS transistor's gate, and an additional upper bootstrap is added (3.3K resistors and the 47µF capacitor).

The upper bootstrap's current is half of the lower bootstrap's current. At equilibrium the VAS transistor needs to supply the other half of current. This arrangement makes it possible for the VAS collector to swing a more negative than the negative rail so that the P-channel mosfet gate can swing about 7 volg more negative than the negative rail. When the VAS transistor is fully saturated then again the N-channel mosfet gate is about 7 volts more positive than the positive power rail. This ensures that both MOSFETs clip symmetrically, and that they are driven very close to saturation. This increases the peak power output at a given rail voltage dramatically.  A quite elegant detail, if I'm allower to say that myself :)

Practical considerations

Adjust the idle current to about 80mA. Measure the current in series with either supply line. During this the output musts be disconnected from the loadm and the input terminal shorted to the input ground. Be sure not to open circuit the ammeter during the measurement. Repeat the measurement when hot, and readjust if necessary.


The amplifier has no output short circuit protection! Avoid shorting the output or overloading the amplifier. It is  rated to
Fuses of suitable ratings (4AT) should be placed in both power supply lines, to prevent fire or hazards.
An output series inductor (in the order of 4.7µH) must be added in series with the output if driving capacitive loads or extremely long speaker wires (>10m).
The shown PCB does not have a place for the gate resistors shown in the schematic. These resistors are mandatory and must be soldered between the PCB and the gate wire (using very short wires).
The output transistors need a reasonable sized heat-sink, immediate failure will follow if not properly heatsinked. Also isolation is mandatorybetween the transistor tabs and the heatsink, since the case metal stubs are at the same potential as their drains (at power supply voltage). See picture for details.
The third small transistor touching the heatsink is for temperature feedback and stabilisation if the idling current. It is mandatory that also this transitor is  in a good thermal contact with the heatsink.  Thermal compound is recommended.

The picture here is an example of a stereo amplifier. The bracket is mounted to a bigger heatsink in the final enclosure. All wires (power supply, speaker, input) are connected via the Molex "minifit" connector. Such amplifiers have been built in reasonable quantities, or at least I have sold more than 120 PCB's.

You need reasonable skills in electronics to build this amplifier. It is not recommended for beginners or as the first amplifier you build. 

Fig. 2 - PCB artwork and component placement

Fig. 3 - Bottom view of PCB

Fig. 4 - Stereo pair readily built. The bracket must be assemblet to a bigger heatsink. Note the thermal paste under the TO-92 thermal feedback trasistors to guarantee good thermal contact with the heatsink.