Sunday, October 19, 2008

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%>

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.


  • Q2 and Q3 in the Power Amplifier must be mounted each on a finned heatsink of at least 80x40x25mm.
  • Q1 and IC1 in the Regulated Power Supply must be mounted on a finned heatsink of at least 45x40x17mm.
  • A power Transformer having a secondary winding rated at 35 - 36V and 50VA (i.e. about 1.4Amp) is required if you intend to use Loudspeaker cabinets of 8 Ohm nominal impedance. To drive 4 Ohm loads at high power levels, a 70 - 75VA Transformer (2Amp at least) will be a better choice. These transformers are usually center tapped: the central lead will be obviously left open.
  • For the stereo version of this project, R16 and C11 in the Preamp will be in common to both channels: therefore, only one item each is necessary. In this case, R16 must be a 1K5 1/2W resistor. The value of C11 will remain unchanged.


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