Different condenser microphones specify different amounts of required phantom power. As a rule these ratings can range from 48 volts all the way down to 9 volts. Fortunately there is a certain amount of flexibility in terms of matching phantom voltage to your mic. A rough rule of thumb would be "more is better". For example, a 24-volt condenser mic will run perfectly on 48 volts. In fact some mics rated at as little as 9 volts can operate on 48 volts, but you might want to check with the manufacturer first. Conversely, a mic will generally perform best driven by not less than its rated voltage so, for a 48V mic, you would obtain best results with 48 volts of phantom power.
A three-way 12V/OFF/48Vselector switch lets you choose 48V for mics rated at 24 to 48V, 12V for lower-voltage mics or OFF to conserve battery life. Power can be supplied by two 9V batteries (included) or an optional AC adapter. The power LED goes out when the unit is running on batteries and switched OFF, but remains on while an AC adapter is in use.