Here is a simple circuit which can provide an adjustable voltage source of 0 to 330 Volt.The supply is short-circuit proof: the current is limited to about 100mA.source

This power supply does not really excel in power efficiency, but it is the cheapest and most compact solution for a small power supply.Note that the low voltage output is directly connected to the mains; it can not be used when the low voltage part can be touched (e.g. chargers).With the component values of this circuit, the circuit can supply 12V /15mA max.source

If you want a 9V to 3V Step-Down Converter here is a simple circuit using LT1073 from Linear technology.The LT®1073 is a versatile micro-power DC/DC converter.The device requires only three external components to deliver a fixed output of 5V or 12V. The very low minimum supply voltage of 1V allows the use of the LT1073 in applications where the primary power source is a single cell. An on-chip auxiliary gain block can function as a low battery detector or linear post-regulator.The LT1073 is good idea,it used for dc to dc converter.Average current drain of the LT1073-5 used as shown in the Typical Application circuit below is just 135mA unloaded,making it ideal for applications where long battery life is important. The circuit shown can deliver 5V at 40mA from an input as low as 1.25V and 5V at 10mA from a 1V input.source

This is a basic circuit dual power supply which fixed output voltage.This circuit used widely in general OCL power amplifiers.The output voltage calculated by:
Vo = 1.414 x Vac
When Vac = Secondary transformer voltage .
for exaple if transformer is 24-0-24 V the output voltage is
1.414 x 24 = 34V
The capacitor should have rated voltage greater than 34V.For diode rated voltage should be greater than 2×1.414xVac.source

This circuit can generates 100V from 25V. The circuit is a typical flyback regulator that uses a couple of well-established circuit techniques to handle the high voltage. The first technique is to insert an n-channel MOSFET (Si4480) in series with IC1′s internal power transistor. The cascoded FET stands off the large switch voltage that arises when the switch turns off. The large switch voltage represents the input voltage summed with the reflected output voltage of the transformer’s primary. source