45 Watt Class B Amplifier

45W into 8 Ohm - 69W into 4 Ohm

Easy to build - No setup required

Circuit diagram:

Class B 45W Amplifier


R1______________18K  1/4W Resistor
R2_______________3K9 1/4W Resistor
R3,R6____________1K  1/4W Resistors
R4_______________2K2 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
C5_____________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__________MJ2955    60V 15A PNP Transistor
Q9__________2N3055    60V 15A NPN Transistor

Power supply circuit diagram:

Class B 45W Amplifier Power Supply


R1_______________3K3  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:

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!


Technical data:

Output power (1KHz sinewave):
45 Watt RMS into 8 Ohms - 69W RMS into 4 Ohms
0.81V RMS input for 45W output
Frequency response @ 1W RMS:
15Hz to 23KHz -0.2dB
Total 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