A 3.5mm headphone amplifier which is charged by USB power like FiiO E5 Headphone Amplifier is great for increasing the sound volume and quality of headphone jacks on Tablets, eReaders, MP3 players, and other devices which uses the 3.5mm headphone jack type for plugging in 3.5mm headphones and speakers which are not very sensitive to your sound players headphone jack's sound signal.
A headphone amplifier works by increasing the electrical power of your sound signal to power your headphone or external speakers with better amplified sound.
Headphone Amplifiers are also very easy to use. You just plug your headphones into the amplifier jack for the headphones, and plug the amplifier into your MP3 player, and turn it on.
You can also save some electricity and battery power when using a headphone amplifier to plug in between your music player and your headphones or speaker since you won't need to turn up your player's volume as high to hear louder sound. The headphone amplifier also has volume controls which can make adjusting your volume easier.
Since the headphone amplifier's battery is charged via USB, it means you won't need to search for a battery charger to charge it, and you can charge it from any powered USB port like on a PC, laptop, game console, or DVD player with USB ports, and you also save money in buying batteries and chargers for the batteries, and you don't have to remove the battery to charge or replace, and reinsert a fresh pair of batteries like AA or AAA batteries. Plus, you know the battery will last 20 hours since you are using the same battery instead of different battery like with AA or AAA batteries which can have worse or better battery life depending on brand and quality.
The size of a USB charged headphone amplifier are also small, so you can easily carry them around in your pocket or jacket easily. There is also a metal clip for clipping the headphone amplifier to your pocket, or back pack strap.