High-quality audio amplifiers need a power supply that is very efficient and has almost no ripple, hum, or buzz. Normal bridge-type power supplies have a problem in that when the load is connected, the voltage drops to 3 to 4 volts. A power supply that is controlled by a transistor or IC is a much better choice, but it still has some ripple and hum that can be annoying to people with good hearing. Here is the circuit for a power supply with regulation that uses a center- tapped transformer and is very efficient. After going through the rectifier bridge, the 15V AC output from the secondary drops by 0.6V x 4 = 2.4V. Also, the resistance of the transformer coil drops to 0.5V, and the ripple is 1V. So the drop is about 4V DC as a whole. Because of this, the secondary output of the transformer should be at least 4 V higher than the regulated output. As can be seen in the diagram, the regulators consist of two pairs of transistors connected in a Darlington configuration: T1-T2 and T3-T4. The

current gain goes up because of the Darlington pairs of transistors. The 470F capacitors connected to the bases of transistors T1 and T3 reduce hum and also lower higher-order harmonics. The 12-kilohm resistors (R9 and R10) make up for any changes in the gain of the transistors. The peak current at power-on is limited by putting resistors R1 through R4 (0.25- ohm, 5W) in series with diodes. The internal thermal noise of diodes C3 through C6 is quieted by the 22nF, 50V capacitors C3 through C6. To cut down on RF noise, the mains filter capacitors C7 and C8 (4700F) are switched by capacitors C9 and C10 (100nF, 50V). When capacitors C1 and C2 (47nF, 250V) are placed across the double-pole, double-throw switch S1, they stop the switch from bouncing. Resistors R5 through R8 (220 ohm) and R9 and R10 (12 kilo ohm) are either 0.25W metal-film or 0.5W carbon resistors. If 10,000F capacitors are used instead of 4700F capacitors C7 and C8, the DC output will not be affected by changes in the mains (200V-260V AC). All of the electrolytic capacitors should be grounded at the same point. As the common ground, use a copper bar or a small copper plate. Heat sinks must be used with transistors T1 through T4. Use copper wire that is at least 16 SWG to connect the other parts of the circuit to the common ground. EFY note. In the EFY lab, a 230V AC primary to 15V-0-15V, 3A secondary transformer was utilized to get 15V DC, 2A. Choose the voltage and current ratings of the secondary coil between 18V and 22V without modifying any components.