Schematic Diagram

Including Preamp, Tone Controls, Reg dc Power Supply, 18 Watt into 8 Ohm - 30W into 4 Ohm loads

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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 impedance. 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.

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Circuit diagram:

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Power Amplifier Parts:

R1 = 2K2 1/4W Resistor
R2 = 27K 1/4W Resistor
R3 = 2K2 1/2W Trimmers Cermet
R4 = 2K2 1/2W Trimmers Cermet
R5 = 100R 1/4W Resistor
R6 = 1K 1/4W Resistor
R7 = 330R 1/4W Resistors
R8 = 330R 1/4W Resistors

C1 = 22µF 25V Electrolytic Capacitor
C2 = 47pF 63V Polystyrene or Ceramic Capacitor
C3 = 100µF 50V Electrolytic Capacitors
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)

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Setting up the Power Amplifier:

1. The setup of this amplifier must be done carefully and with no haste:
2. 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.
3. Rotate the cursor of R4 fully towards Q1 Collector.
4. Set the cursor of R3 to about the middle of its travel.
5. Connect a suitable loudspeaker or a 8 Ohm 20W resistor to the amplifier output.
6. Connect a Multimeter, set to measure about 50V fsd, across the positive end of C5 and the negative ground.
7. Switch on the supply and rotate R3 very slowly in order to read about 23V on the Multimeter display.
8. 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).
9. Switch on the supply and rotate R4 very slowly until a reading of about 120mA is displayed.
10. Check again the voltage at the positive end of C5 and readjust R3 if necessary.
11. If R3 was readjusted, R4 will surely require some readjustment.
12. Wait about 15 minutes, watch if the current is varying and readjust if necessary.
13. Please note that R3 and R4 are very sensitive: very small movements will cause rather high voltage or current variations, so be careful.
14. 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.

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Preamplifier Section:

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The Preamp sensitivity and overload margin were designed to cope with most modern music program 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 constructors 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.

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Circuit diagram:

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Preamplifier Parts:

P1 = 50K - Log. Potentiometer
P2 = 100K - Linear Potentiometers
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 = 8K2 - 1/4W Resistors
R5 = 8K2 - 1/4W Resistors
R6 = 4K7 - 1/4W Resistor
R7 = 2K2 - 1/4W Resistors
R8 = 2K2 - 1/4W Resistors
R9 = 2M2 - 1/4W Resistor
R10 = 47K - 1/4W Resistor
R11 = 47K - 1/4W Resistor
R12 = 33K - 1/4W Resistor
R13 = 2K2 - 1/4W Resistors
R14 = 470R - 1/4W Resistor
R15 = 10K - 1/4W Resistor
R16 = 3K3 - 1/4W Resistor (See Notes)

C1 = 470nF - 63V Polyester Capacitors
C2 = 470nF - 63V Polyester Capacitors
C3 = 47nF - 63V Polyester Capacitors
C4 = 47nF - 63V Polyester Capacitors
C5 = 6n8 - 63V Polyester Capacitors
C6 = 6n8 - 63V Polyester Capacitors
C7 = 10µF - 63V Electrolytic Capacitor
C8 = 22µF - 25V Electrolytic Capacitors
C9 = 470nF - 63V Polyester Capacitors
C10 = 22µF - 25V Electrolytic Capacitors
C11 = 470µF - 25V Electrolytic Capacitor (See Notes)

Q1 = BC550C - 45V 100mA Low noise High gain NPN Transistors
Q2 = 2N3819 - General-purpose N-Channel FET
Q3 = BC550C - 45V 100mA Low noise High gain NPN Transistors

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Power Supply Section:

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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.

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Circuit diagram:

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Power Supply Parts:

R1 = 3R9 - 2W Resistor
R2 = 22R - 1/4W Resistor
R3 = 6K8 - 1/4W Resistor
R4 = 220R - 1/4W Resistor
R5 = 4K7 - 1/2W Resistor

C1 = 4700µF - 50V Electrolytic Capacitor
C2 = 100nF - 63V Polyester Capacitors
C3 = 10µF - 63V Electrolytic Capacitor
C4 = 220µF - 50V Electrolytic Capacitor
C5 = 100nF - 63V Polyester Capacitors

D1 = Diode bridge - 100V 4A
D2 = 1N4002 - 200V 1A Diode
D3 = LED - Any type and color

SW2 = SPST - Mains switch
IC1 = LM317T - 3-Terminal Adjustable Regulator
PL1 = Male Mains plug with cord

Q1 = TIP42A - 60V 6A PNP Transistor
T1 = 230V Primary, 35-36V (Center-tapped) Secondary,
50-75VA Mains transformer (See Notes)

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Notes:

• 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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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:

• 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