Whether you have rectified and smoothed an AC supply, or just want to set a fixed (or variable) voltage, you will need some sort of regulator.
Below is a simple fixed regulator
But what if the regulator you have doesn't quite give the voltage you want, for example you may have some 5 Volt regulators lying around, but want at least 6 Volts, there's a simple solution, add a diode in series with the ground connection (this adds about 0.7 volts per diode depending on type), or add more if you want a higher voltage as in the diagrams
So, 0.7 volt steps may be a bit much if all you have is a 12 volt reg, and want a 48 volt supply, if you also have some 12 volt zeners, this is another way.
That's fine if the regulator will withstand the current to power your project, so how do you increase the current without having to buy a higher power one ?
Add a power transistor, i'm sure any geek on a budget can find one in some old equipment, this is how to do it
You can if course add diodes as above to alter the voltage.
All the diagrams above are simplified, I have not shown any filter caps across the outputs.