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:


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 :
45 Watt Class-B Audio Power Amplifier


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
  • 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
  • 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
..:: 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.
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USB Powered Mobile Phone Battery Charger

Now you can charge your Mobile Phone from the USB outlet of PC

This simple circuit can give regulated 4.7 volts for charging a mobile phone. USB outlet can give 5 volts DC at 100mA current which is sufficient for the slow charging of mobile phones. Most of the Mobile Phone batteries are rated 3.6 volts at 1000 to 1300 mAh. These battery packs have 3 NiMh or Lithium cells having 1.2 volt rating. Usually the battery pack requires 4.5 volts at 300-500 mA current for fast charging.

But low current charging is better to increase the efficiency of the battery. The circuit described here provides 4.7 regulated voltage and sufficient current for the slow charging of the mobile phone. Transistor Q1 is used to give the regulated output. Any medium power NPN transistor like CL100, BD139, TIP122 can be used. Zener diode D2 controls the output voltage and D1 protects the polarity of the output supply. Front end of the circuit should be connected to a A type USB plug.

Connect a red wire to pin1 and black wire to pin 4 of the plug for easy polarity identification. Connect the output to a suitable charger pin to connect it with the mobile phone. After assembling the circuit, insert the USB plug into the socket and measure the output from the circuit. If the output is OK and polarity is correct, connect it with the mobile phone.

Circuit diagram:
USB Powered Mobile Phone Battery Charger


Q1 = BD139
D1 = 1N4001
D2 = 4.7V - 1/2W
R1 = 560R - 1/2W
C1 = 16V - 100uF

  • If the polarity is incorrect, it will destroy the mobile battery. So take extreme care.
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Automatic Loudness Control

Simple add-on module, Switchable "Control-flat" option

In order to obtain a good audio reproduction at different listening levels, a different tone-controls setting should be necessary to suit the well known behavior of the human ear. In fact, the human ear sensitivity varies in a non-linear manner through the entire audible frequency band, as shown by Fletcher-Munson curves.

A simple approach to this problem can be done inserting a circuit in the preamplifier stage, capable of varying automatically the frequency response of the entire audio chain in respect to the position of the control knob, in order to keep ideal listening conditions under different listening levels.

Fortunately, the human ear is not too critical, so a rather simple circuit can provide a satisfactory performance through a 40dB range. The circuit is shown with SW1 in the "Control-flat" position, i.e. without the Automatic Loudness Control. In this position the circuit acts as a linear preamplifier stage, with the voltage gain set by means of Trimmer R7.

Switching SW1 in the opposite position the circuit becomes an Automatic Loudness Control and its frequency response varies in respect to the position of the control knob by the amount shown in the table below. C1 boosts the low frequencies and C4 boosts the higher ones. Maximum boost at low frequencies is limited by R2; R5 do the same at high frequencies.

Circuit diagram:
Automatic Loudness Control


P1_________________10K Linear Potentiometer (Dual-gang for stereo)
R1,R6,R8__________100K 1/4W Resistors
R2_________________27K 1/4W Resistor
R3,R5_______________1K 1/4W Resistors
R4__________________1M 1/4W Resistor
R7_________________20K 1/2W Trimmer Cermet
C1________________100nF 63V Polyester Capacitor
C2_________________47nF 63V Polyester Capacitor
C3________________470nF 63V Polyester Capacitor
C4_________________15nF 63V Polyester Capacitor
C5,C9_______________1µF 63V Electrolytic or Polyester Capacitors
C6,C8______________47µF 63V Electrolytic Capacitors
C7________________100pF 63V Ceramic Capacitor
IC1_______________TL072 Dual BIFET Op-Amp
SW1________________DPDT Switch (four poles for stereo)

Technical data:

Frequency response referred to 1KHz and different control knob positions:
knob adjustment for automatic loudness controller
Total harmonic distortion at all frequencies and 1V RMS output: <0.01%

  • SW1 is shown in "Control flat" position.
  • Schematic shows left channel only, therefore for stereo operation all parts must be doubled except IC1, C6 and C8.
  • Numbers in parentheses show IC1 right channel pin connections.
  • R7 should be set to obtain maximum undistorted output power from the amplifier with a standard music program source and P1 rotated fully clockwise.
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NiMh and NiCd Battery Charger Circuit

This automatic NiCd charger for 9V NiCd batteries is using 555 timer properties and is very easy to build. Why is an automatic 9 volts NiCd battery charger? Because you can leave the battery for charging as much as you like: it will be always completely charged and ready for use when is needed. It wont be overcharged and it will not discharge. With the values presented in the circuit diagram, the battery charger NiCd circuit is suitable for 6V and 9V batteries.

9 volt types with 6 and 7 cells are charging with 20mA; P1 must be adjusted so that the NiCd charger disconnects after 14 hours. Window inferior level is set at 1V below this value with P2. 5V battery type with 4 or 5 cells are charged at 55mA. Again, with P1 adjust the NiCd charger circuit so it disconnects after 14 hours. Window inferior level must be set at 0.8V below this value.
9 volt at 200mA NiMh battery picture
Circuit diagram:
NiMh and NiCd Battery Charger Circuit


P1 = 50K
P2 = 50K
R1 = 820R
R2 = 820R
R3 = 1K
R4 = 10K
R5 = 10K
R6 = 100R
D1 = 4.7V Zener
D2 = 1N4001
D4 = 1N4148
D5 = B40C1500 Diode Bridge
C1 = 220uF - 25V
Q1 = BC547B
IC = 555
B1 = 9V Ni-Cad Battery
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NiCd Battery Charger With Reverse Polarity Protection

Small and portable unit, Can charge multiple batteries at once

This NiCd battery Charger can charge up to 7 NiCd batteries connected in series. This number can be increased if the power supply is increased with 1.65V for each supplementary battery. If Q2 is mounted on a proper heatsink, the input voltage can be increased at a maximum of 25V. Unlike most of comercial NiCd chargers available on the market, this charger has a reverse polarity protection. Another great quality is that it does not discharge the battery if the charger is disconnected from the power supply.

Usually , NiCd batteries must be charged in 14 hours at a charging current equal with a tenth percent from battery capacity. For example, a 500 mAh is charged at 50 mA for 14 hours. If the charging current is too high this will damage the battery. The level of charging current is controlled with P1 between 0 mA – 1000 mA. Q1 is opened when the NiCd battery is connected with the right polarity or if the output terminals are empty. Q2 must be mounted on a heatsink. If you cannot obtain a BD679, then replace it with any NPN medium power Darlington transistor having the output parameters at 30V and 2A. By lowering R3 value the maximum output current can be increased up to 1A.

Circuit diagram:
NiCd battery charger circuit diagram with reverse polarity protection

P1 = 1K
R1 = 680R
R2 = 47K
R3 = 1R-3W
Q1 = BC557
Q2 = BD679 (Darlington)
D1-D5 = 1N4148
D6 = 1N4001
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Adjustable 1.3-22V Regulated Power Supply Circuit Diagram

Want a regulated voltage that can be adjusted to suit your application? This Adjustable Power Supply is small, easy to build and can be adapted to produce a fully regulated voltage ranging from 1.3V to 22V at currents up to 1A. This circuit come from SiliconChip MagazineThere are many fixed-voltage IC regulators available and these can be had with 5V, 6V 8V, 9V, 12V & 15V outputs. But what if you want a voltage output that does not fit into one of the standard ranges or if you want to be able to easily adjust this output voltage? An adjustable regulator is the answer – one that can be set to provide the exact voltage you require.

This Adjustable Power Supply comprises a small PC board that utilises a 3-terminal regulator. It does not have too many other components – in fact, there are just three diodes, three capacitors, a resistor and a trimpot to set the output voltage from the regulator. The circuit is based on an LM317T adjustable voltage regulator. D1 provides reverse polarity protection while P1 sets the output voltage.

Project looks like:
picture of the project
Parts layout:
Parts layout of regulated power supply
PCB layout:
PCB layout for regulated power supply
Circuit diagram:

Parts list:

IC = LM317T adjustable 3-terminal regulator
P1 = 2k horizontal trimpot
R1 = 110R-0.25W
C1 = 100uF-25V
C2 = 10uF-25V
C3 = 100uF-25V
D1 = 1N4004
D2 = 1N4004
D3 = 1N4004
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3-30V 3A Adjustable Regulated DC Power Supply Circuit Diagram

A power supply for all general circuits, Based on a stablized DC voltage of 30 volt

This power supply is meant as an auxiliary or as a permanent power supply for all common circuits based on a stabilized DC voltage between 3 and 30V provided that the consumption does not exceed 3A. Of course this power supply unit can also be used for other purposes. Be replacing the trimmer by a potentiometer, it may even be used as an adjustable power supply unit. A good quality heatsink must be used.

Picture of project:
 3 -30 volt dc 3 ampere power supply schematic circuit diagram
Circuit diagram:
 3 -30 volt dc 3 ampere power supply schematic circuit diagram
Parts list:
R1 = 8.2K
R2 = 2.2K
R3 = 680R
R4 = 1K
R5 = 82K
R6 = 0.18R/5W
C1 = 470p
C2 = 100nF-63V
C3 = 100nF-63V
C4 = 100uF-63V
C5 = 10KuF-60V
D1-D6 = 6.6A
Q1 = MJ3001 (Darligton)
IC1 = UA723D


  • Overload protected
  • Sshort-circuit stable
  • Output current: max. 3A
  • Output ripple voltage: 0.5mV
  • Output voltage: adjustable from 3 to 30V, stabilized
  • Input voltage: 9 to 30V AC (depending on the desired output voltage)
Assembling into a housing:
  • Depending on the transformer used, one may chose one of two housings
  • If a metal housing is used, it must be earthed for security purposes.
  • Make sure the cooling body does not touch the housing. This might cause a short circuit.
  • When mounting a toroidal transformer, it must be seen to that the fixation bolt does not touch the cover. This might cause the burning of the transformer
  • If the circuit is to be integrated into another housing, it must be provided with ventilation holes (one may make these holes oneself), necessary for the release of the heat developed.
Notes :
  1. Connect a voltage meter to the points ‘GND’ and ‘+OUT’ and adjust ‘RV1’ until the desired output voltage is reached.
  2. Apply some thermo-conducting pasta to the bottom side of the transistor and mount it on the heatsink.
  3. If you need 3-8 volt then R2 will be 5.6K
  4. If you need more than 8 volts then the R2 will be 2.2K
  5. Suitable transformer 30vAC at 120VA
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