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Learning Electronics

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45 Watt Class-B Audio Power Amplifier

45W into 8 Ohm - 69W into 4 Ohm, Easy to build - No setup required

These goals were achieved by using a discrete-components op-amp driving a BJT complementary common-emitter output stage into Class B operation. In this way, for small output currents, the output transistors are turned off, and the op-amp provides all of the output current. At higher output currents, the power transistors conduct, and the contribution of the op-amp is limited to approximately 0.7/R11. The quiescent current of the op-amp biases the external transistors, and hence greatly reduces the range of crossover.

The idea sprang up from a letter published on Wireless World, December 1982, page 65 written by N. M. Allinson, then at the University of Keele, Staffordshire. In this letter, op-amp ICs were intended as drivers but, as supply voltages up to +/- 35V are required for an amplifier of about 50W, the use of an op-amp made of discrete-components was then considered and the choice proved rewarding.

The discrete-components op-amp is based on a Douglas Self design. Nevertheless, his circuit featured quite obviously a Class A output stage. As for proper operation of this amplifier a Class B output stage op-amp is required, the original circuit was modified accordingly. Using a mains transformer with a secondary winding rated at the common value of 25 + 25V (or 24 + 24V) and 100/120VA power, two amplifiers can be driven at 45W and 69W output power into 8 and 4 Ohms respectively, with very low distortion (less than 0.01% @ 1kHz and 20W into 8 Ohms).

This simple, straightforward but rugged circuit, though intended for any high quality audio application and, above all, to complete the recently started series of articles forming the Modular Preamplifier Control Center, is also well suited to make a very good Guitar or Bass amplifier. Enjoy!

Circuit diagram:
45 watt audio power amplifier circuit diagram
45W Class-B Amplifier Circuit Diagram

R1______________18K - 1/4W Resistor
R2_______________3.9K - 1/4W Resistor
R3,R6____________1K - 1/4W Resistors
R4_______________2.2K - 1/4W Resistor
R5______________15K - 1/4W Resistor
R7______________22K - 1/4W Resistor
R8_____________330R - 1/4W Resistor
R9,R10__________10R - 1/4W Resistors
R11,R12_________47R - 1/4W Resistors
R13_____________10R - 1W Resistor

C1_______________1µF - 63V Polyester Capacitor
C2_____________470pF - 63V Polystyrene or Ceramic Capacitor
C3______________47µF - 25V Electrolytic Capacitor
C4______________15pF - 63V Polystyrene or Ceramic Capacitor
C6_____________220nF - 100V Polyester Capacitor
C6_____________100nF - 63V Polyester Capacitor

D1,D2,D3,D4___1N4148 - 75V 150mA Diodes

Q1,Q2________BC560C - 45V 100mA Low noise High gain PNP Transistors
Q3,Q4________BC556 - 65V 100mA PNP Transistors
Q5___________BC546 - 65V 100mA NPN Transistor
Q6___________BD139 - 80V 1.5A NPN Transistor
Q7___________BD140 - 80V 1.5A PNP Transistor
Q8__________2N3055 - 60V 15A NPN Transistor
Q9__________MJ2955 - 60V 15A PNP Transistor

Power supply :
power supply circuit diagram for 45 watt amplifier
Power Supply Circuit Diagram

R1_______________3.3K - 1/2W Resistor
C1,C2_________4700µF - 50V Electrolytic Capacitors
C3,C4__________100nF - 63V Polyester Capacitors
D1_____________200V 8A Diode bridge
D2_____________5mm. Red LED
F1,F2__________4A Fuses with sockets
T1_____________230V or 115V Primary, 25+25V Secondary 120VA Mains transformer
PL1____________Male Mains plug
SW1____________SPST Mains switch

The main design targets for this amplifier were as follows:
  1. Output power in the 40 - 70W range
  2. Simple circuitry
  3. Easy to locate, low cost components
  4. Rugged performance
  5. No setup
Technical data:

Output power (1KHz sinewave):
Frequency response @ 1W RMS:
Total harmonic distortion @ 1KHz:
Total harmonic distortion @10KHz:
Unconditionally stable on capacitive loads
..:: UPDATE ::..
There is a little mistake in circuit diagram. Q8 and Q9 are displayed on wrong places. Please replace Q8's place wiht Q9's. Q8 is NPN Transistor 2N3055 and Q9 is MJ2955 PNP Transistor. Please update your notes.