Why A voltage regulator is the stage in a power supply that holds the output voltage at a set value despite changes in input voltage or load current. Common examples are Zener diode shunt regulators and three-terminal ICs like the 7805, which delivers a steady 5 V. Without regulation, the DC output would sag as more current is drawn and rise as the AC line voltage climbs.
Watch out A filter is tempting because it also follows the rectifier, but it only smooths ripple into steadier DC; it does not set or hold the voltage level. An oscillator generates AC signals, and a phase inverter reverses signal polarity.
Regulator regulates the volts; filter just smooths the ripple.