Mini-MosFet Audio Amplifier

Including Preamp, Tone Controls, Regulated dc Power Supply

18 Watt into 8 Ohm - 30W into 4 Ohm loads

Power Amplifier Circuit diagram:

Mini MosFet Power Amplifier

Power Amplifier Parts:

R1_______________2K2 1/4W Resistor
R2______________27K  1/4W Resistor
R3,R4____________2K2 1/2W Trimmers Cermet or Carbon (or 2K)
R5_____________100R  1/4W Resistor
R6_______________1K  1/4W Resistor
R7,R8__________330R  1/4W Resistors

C1______________22µF  25V Electrolytic Capacitor
C2______________47pF  63V Polystyrene or Ceramic Capacitor
C3,C4__________100µF  50V Electrolytic Capacitors
C5____________2200µF  50V Electrolytic Capacitor

Q1____________BC550C  45V 100mA Low noise High gain NPN Transistor
Q2___________IRF530  100V 14A N-Channel Hexfet Transistor (or MTP12N10)
Q3__________IRF9530  100V 12A P-Channel Hexfet Transistor (or MTP12P10)


This project was a sort of challenge: designing an audio amplifier capable of delivering a decent output power with a minimum parts count, without sacrificing quality.
The Power Amplifier section employs only three transistors and a handful of resistors and capacitors in a shunt feedback configuration but can deliver more than 18W into 8 Ohm with <0.08% THD @ 1KHz at the onset of clipping (0.04% @ 1W - 1KHz and 0.02% @ 1W - 10KHz) and up to 30W into a 4 Ohm load.
To obtain such a performance and to ensure overall stability of this very simple circuitry, a suitable regulated dc power supply is mandatory. This is not a snag because it also helps in keeping noise and hum of the preamp to very low levels and guarantees a predictable output power into different load impedances. Finally, as the amplifier requires only a single rail supply, a very good dc voltage regulator capable of supplying more than 2 Amps @ 40V can be implemented with a few parts also.

Setting up the Power Amplifier:

The setup of this amplifier must be done carefully and with no haste:

  1. Connect the Power Supply Unit (previously tested separately) to the Power Amplifier but not the Preamp: the input of the Power Amplifier must be left open.
  2. Rotate the cursor of R4 fully towards Q1 Collector.
  3. Set the cursor of R3 to about the middle of its travel.
  4. Connect a suitable loudspeaker or a 8 Ohm 20W resistor to the amplifier output.
  5. Connect a Multimeter, set to measure about 50V fsd, across the positive end of C5 and the negative ground.
  6. Switch on the supply and rotate R3 very slowly in order to read about 23V on the Multimeter display.
  7. Switch off the supply, disconnect the Multimeter and reconnect it, set to measure at least 1Amp fsd, in series to the positive supply (the possible use of a second Multimeter in this place will be very welcomed).
  8. Switch on the supply and rotate R4 very slowly until a reading of about 120mA is displayed.
  9. Check again the voltage at the positive end of C5 and readjust R3 if necessary.
  10. If R3 was readjusted, R4 will surely require some readjustment.
  11. Wait about 15 minutes, watch if the current is varying and readjust if necessary.
  12. Please note that R3 and R4 are very sensitive: very small movements will cause rather high voltage or current variations, so be careful.
  13. Those lucky enough to reach an oscilloscope and a 1KHz sine wave generator, can drive the amplifier to the maximum output power and adjust R3 in order to obtain a symmetrical clipping of the sine wave displayed.

Preamp Circuit diagram:

Preamp for the Mini-MosFet Amplifier

Preamp Parts:

P1______________50K  Log. Potentiometer (or 47K)
                     (twin concentric-spindle dual gang for stereo)
P2,P3__________100K  Linear Potentiometers
                     (twin concentric-spindle dual gang for stereo)

R1_____________220K  1/4W Resistor
R2_____________100K  1/4W Resistor
R3_______________2K7 1/4W Resistor
R4,R5____________8K2 1/4W Resistors
R6_______________4K7 1/4W Resistor
R7,R8,R13________2K2 1/4W Resistors
R9_______________2M2 1/4W Resistor
R10,R11_________47K  1/4W Resistor
R12_____________33K  1/4W Resistor
R14____________470R  1/4W Resistor
R15_____________10K  1/4W Resistor
R16______________3K3 1/4W Resistor (See Notes)

C1,C2,C9_______470nF  63V Polyester Capacitors
C3,C4___________47nF  63V Polyester Capacitors
C5,C6____________6n8  63V Polyester Capacitors
C7______________10µF  63V Electrolytic Capacitor
C8,C10__________22µF  25V Electrolytic Capacitors
C11____________470µF  25V Electrolytic Capacitor (See Notes)

Q1,Q3_________BC550C  45V 100mA Low noise High gain NPN Transistors
Q2___________2N3819   General-purpose N-Channel FET


The Preamp sensitivity and overload margin were designed to cope with most modern music programme sources like CD players, Tape recorders, iPods, Computer audio outputs, Tuners etc. The source selecting switches and input connectors are not shown and their number and arrangement are left to the constructor's choice.
To obtain a very high input overload margin, the volume control was placed at the preamp input. After a unity gain, impedance converter stage (Q1) a negative-feedback Baxandall-type Bass and Treble tone control stage was added. As this stage must provide some gain (about 5.6 times) a very low noise, "bootstrapped" two-transistors circuitry with FET-input was implemented. This stage features also excellent THD figures up to 4V RMS output and a low output impedance, necessary to drive properly the Mini-MosFet Power Amplifier, but can also be used for other purposes.

Regulated Power Supply Circuit diagram:

Regulated Power Supply for the Mini-MosFet Amplifier

Regulated Power Supply Parts:

R1_______________3R9 1 or 2W Resistor
R2______________22R  1/4W Resistor
R3_______________6K8 1/4W Resistor
R4_____________220R  1/4W Resistor
R5_______________4K7 1/2W Resistor

C1____________3300µF  50V Electrolytic Capacitor (or 4700µF 50V)
C2,C5__________100nF  63V Polyester Capacitors
C3______________10µF  63V Electrolytic Capacitor
C4_____________220µF  50V Electrolytic Capacitor

D1_____Diode bridge  100V 4A 
D2___________1N4002  200V 1A Diode
D3______________LED  Any type and color

IC1___________LM317T 3-Terminal Adjustable Regulator

Q1____________TIP42A  60V 6A PNP Transistor

SW2_____________SPST  Mains switch

T1_____________230V Primary, 35-36V (Center-tapped) Secondary,
                    50-75VA Mains transformer (See Notes)

PL1____________Male Mains plug with cord


A very good and powerful Regulated Power Supply section was implemented by simply adding a PNP power transistor to the excellent LM317T adjustable regulator chip. In this way this circuit was able to deliver much more than the power required to drive two Mini-MosFet amplifiers to full output (at least 2Amp @ 40V into 4 Ohm load) without any appreciable effort.


Technical data:

Output power:
18 Watt RMS into 8 Ohm (1KHz sine wave) - 30 Watt RMS into 4 Ohm
Input sensitivity of the complete Amplifier:
160mV RMS for full output
Power Amplifier Input sensitivity:
900mV RMS for full output
Power Amplifier Frequency response @ 1W RMS:
flat from 40Hz to 20KHz, -0.7dB @ 30Hz, -1.7dB @ 20Hz
Power Amplifier Total harmonic distortion @ 1KHz:
100mW 0.04% 1W 0.04% 10W 0.06% 18W 0.08%
Power Amplifier Total harmonic distortion @10KHz:
100mW 0.02% 1W 0.02% 10W 0.05% 18W 0.12%
Unconditionally stable on capacitive loads

Preamp Maximum output voltage:
Preamp Frequency response:
flat from 20Hz to 20KHz
Preamp Total harmonic distortion @ 1KHz:
1V RMS 0.007% 3V RMS 0.035%
Preamp Total harmonic distortion @10KHz:
1V RMS 0.007% 3V RMS 0.02%
Bass control frequency range referred to 1KHz:
±20dB @ 40Hz
Treble control frequency range referred to 1KHz:
+18dB/-20dB @ 20KHz