Power amplifiers are typically used with musical instruments such as electric guitars and keyboards. However, if you want to keep the music down to avoid disturbing others it is possible to connect a pair of headphones to a power amplifier. This does require that you use a cable adaptor--power amplifiers have quarter inch cable connectors while most headphones only use 3.5mm jacks. Once you have converted the cable connection it only takes a few seconds to hook up the headphones to the power amp.
Insert the 3.5mm jack running out of your headphones into the 3.5mm to quarter inch adaptor.
Plug the quarter inch adaptor into the "Line-Out" port located (usually) on the front of the power amplifier. This is for easy access and adjustments.
Power on the power amplifier and the electrical instrument you have connected. Place the headphones into (or over) your hears and begin playing. The audio now runs through the power amp into the headphones and projects into your ears.