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How to build 45 Watt Class-B Audio Power Amplifier

2015-01-05 20:37  
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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:

Circuit diagram


R1______________18K - 1/4W ResistorR2_______________3.9K - 1/4W ResistorR3,R6____________1K - 1/4W ResistorsR4_______________2.2K - 1/4W ResistorR5______________15K - 1/4W ResistorR7______________22K - 1/4W ResistorR8_____________330R - 1/4W ResistorR9,R10__________10R - 1/4W ResistorsR11,R12_________47R - 1/4W ResistorsR13_____________10R - 1W ResistorC1_______________1μF - 63V Polyester CapacitorC2_____________470pF - 63V Polystyrene or Ceramic CapacitorC3______________47μF - 25V Electrolytic CapacitorC4______________15pF - 63V Polystyrene or Ceramic CapacitorC6_____________220nF - 100V Polyester CapacitorC6_____________100nF - 63V Polyester CapacitorD1,D2,D3,D4___1N4148 - 75V 150mA DiodesQ1,Q2________BC560C - 45V 100mA Low noise High gain PNP TransistorsQ3,Q4________BC556 - 65V 100mA PNP TransistorsQ5___________BC546 - 65V 100mA NPN TransistorQ6___________BD139 - 80V 1.5A NPN TransistorQ7___________BD140 - 80V 1.5A PNP TransistorQ8__________2N3055 - 60V 15A NPN TransistorQ9__________MJ2955 - 60V 15A PNP Transistor

Power supply :

Power supply


R1_______________3.3K - 1/2W ResistorC1,C2_________4700μF - 50V Electrolytic CapacitorsC3,C4__________100nF - 63V Polyester CapacitorsD1_____________200V 8A Diode bridgeD2_____________5mm. Red LEDF1,F2__________4A Fuses with socketsT1_____________230V or 115V Primary, 25 25V Secondary 120VA Mains transformerPL1____________Male Mains plugSW1____________SPST Mains switch


The main design targets for this amplifier were as follows:

Output power in the 40 - 70W rangeSimple circuitryEasy to locate, low cost componentsRugged performanceNo setup


2N3055 and MJ2955 transistors were listed for Q8 and Q9 as the preferred types, but many different output transistors can be used satisfactorily: TIP3055/TIP2955, TIP35/TIP36, MJ802/MJ4502 amongst others.Discrete op-amp output transistors Q6 and Q7 do not require any heatsink as their cases remain at ambient temperature. Power transistors Q8 and Q9 should be mounted on a black, finned heatsink as usual.

Technical data:

Output power (1KHz sinewave): 45 Watt RMS into 8 Ohms - 69W RMS into 4 OhmsSensitivity: 0.81V RMS input for 45W outputFrequency response @ 1W RMS: 15Hz to 23KHz -0.2dBTotal harmonic distortion @ 1KHz: 1W 0.008% 20W 0.008% 45W 0.016%Total harmonic distortion @10KHz: 1W 0.01% 20W 0.015% 45W 0.025%Unconditionally stable on capacitive loads


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.

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