Power Amplifier Circuit diagram: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)
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:
- 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.
- Rotate the cursor of R4 fully towards Q1 Collector.
- Set the cursor of R3 to about the middle of its travel.
- Connect a suitable loudspeaker or a 8 Ohm 20W resistor to the amplifier output.
- Connect a Multimeter, set to measure about 50V fsd, across the positive end of C5 and the negative ground.
- Switch on the supply and rotate R3 very slowly in order to read about 23V on the Multimeter display.
- 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).
- Switch on the supply and rotate R4 very slowly until a reading of about 120mA is displayed.
- Check again the voltage at the positive end of C5 and readjust R3 if necessary.
- If R3 was readjusted, R4 will surely require some readjustment.
- Wait about 15 minutes, watch if the current is varying and readjust if necessary.
- Please note that R3 and R4 are very sensitive: very small movements will cause rather high voltage or current variations, so be careful.
- 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 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
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 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, R11 must be a 1K5 1/2W resistor. The value of C11 will remain unchanged.
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:4V RMS
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