45 watt class b amplifier - red - page150

45 Watt Class B Amplifier

45W into 8 Ohm - 69W into 4 Ohm

Easy to build - No setup required

Circuit diagram:

Parts:

R1______________18K 1/4W ResistorR2_______________3K9 1/4W ResistorR3,R6____________1K 1/4W ResistorsR4_______________2K2 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 Resistor

C1_______________1µF 63V Polyester CapacitorC2_____________470pF 63V Polystyrene or Ceramic CapacitorC3______________47µF 25V Electrolytic CapacitorC4______________15pF 63V Polystyrene or Ceramic CapacitorC5_____________220nF 100V Polyester CapacitorC6_____________100nF 63V Polyester Capacitor

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

Q1,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__________MJ2955 60V 15A PNP TransistorQ9__________2N3055 60V 15A NPN Transistor

45 Watt Class B Amplifier - RED - Page150 http://www.redcircuits.com/Page150.htm

1 din 3 18.02.2011 20:55

Power supply circuit diagram:

Parts:

R1_______________3K3 1/2W Resistor

C1,C2_________4700µF 50V Electrolytic CapacitorsC3,C4__________100nF 63V Polyester Capacitors

D1_____________200V 8A Diode bridgeD2_____________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

Comments:

The main design targets for this amplifier were as follows:

Output power in the 40 - 70W range

Simple circuitry

Easy to locate, low cost components

Rugged performance

No setup

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


2 din 3 18.02.2011 20:55

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!

Notes:

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 Ohms

Sensitivity:

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


3 din 3 18.02.2011 20:55

45 watt class b amplifier - red - page150

Documents