What About a DC Power Supply?
As I'm sure you'll know already an AC power normally uses voltage from a wall outlet. It can be used to convert to DC with the help of a rectifier that converts the AC voltage (alternating current) into a pulsating direct voltage and with the help of built-in filters, resistors and capacitors the current is smooth and more efficient.
In the old day, well maybe not that long ago before solid-state electronic lots of equipment used valves. Generally many homes would have has these in TV's but they require very high voltage and will to step up transformers, filters and rectifiers you sometimes also found that because of the different voltages required that you would get to him. I was at various valves.
AC Power Supplies
AC voltage requirements can be different depending on what part of the world you are in. If you live in Europe for example generally hey will use 230v. For example you will find many AC PSU's used in musical equipment like certain guitar effects pedals. Many types of pedals will also use DC.
Switch Mode Power Supply
This will take its power from the AC mains supply and will then rectify the this voltage and current so it can obtain a DC voltage. It's called a switch mode PSU because it can switch on and off at very high speeds by using electronic circuitry. You get an increase in duty cycle and because these are always regulator they can supply constant and clean power.
If you are looking to purchase a power supply its important that you have all the vital information before you start looking around. If you're looking for a replacement adapter insurance that you know the manufacturer, model number and other information like volts and amps that are required.
Many websites have all this vital information anyway once you know the manufacturer and model number. There are many alternatives to purchasing the PSU is supplied by major manufacturers, which will do the same job for you.