Anyone with a pair of high end headphones will have learnt from experience that smartphones simply aren't powerful enough for multiple driver in - ears or oversized one ear cans. If you're determined to use your phone as a playback device, however, an external amplifier like the TEAC HA-P50 will get the job done. With an on-board DAC, it also supports high-resolution audio playback – even on iPhones , which can't play 24bit files natively.

No larger than the average smartphone but a slightly thicker 21mm, the HA-P50 will still slip into a pocket – although you may need to keep it in a different pocket to the phone itself if you're a fan of skinny jeans. There are much smaller portable DACs and amplifiers out there, but it's reassuringly sturdy; made from aluminium and feeling like a suitably premium product, the raised guards help keep the connected cables safe from damage and to prevent accidental volume changes when you're on the move. Two rubber bands can be used to attach your phone to the amp, with two rubber strips running down the length of the unit to prevent scratches or scrapes, but it's not particularly convenient if you find yourself constantly pulling your phone out of a pocket to check notifications.

The aluminium volume dial is chunky, with plenty of room to the side for the 3.5mm headphone jack and 3.55 optical mini-Toslink input. There's also a high/low gain selector for adjusting the output for high impedance headphones, ensuring you get the best possible quality. You'll need to use an adaptor for headphones with a 1/4in jack; although less common than they used to be, there are still plenty of pairs of high-end cans that use the larger plug rather than 3.5mm.

On the bottom of the unit, there are full-size USB and Micro USB connections for attaching the amp to your smartphone, using either the bundled Micro USB cable if you have an Android phone or your own Apple-supplied Lightning cable. There's also an input switch and power port. The HA-P50 doesn't draw power from your smartphone; instead it has its own internal 2,100mAh battery, which is good for eight hours of continuous use. We managed slightly longer during our testing, by making sure to switch it off when we'd finished listening, but it will automatically power down after 30 minutes with no sound signal. Thankfully you can continue listening when charging up the amp.

Inside, an OPA 1652 amplifier is capable of dual 160mW output at a load of 32ohms, meaning it can drive a pair of 600ohm headphones at full power. These kinds of studio-ready cans certainly aren't cheap, and with most over-ears requiring as little as 16ohms, all but the most discerning of audiophiles should be able to use their on-ears or over-ears with the HA-P50. There's no protection for low sensitivity headphones, however, so if you aren't careful you could blow out the drivers if you turn the volume too high.

The Texas Instruments Burr-Brown PCM5102 digital-to-analogue converter, one of our favourite DAC chips, will happily play high quality files. Using the TEAC HR Audio Player app, which is free to download from the iOS app store, you can play high-resolution 24bit/96kHz audio from an iPhone, iPad or iPod Touch – despite Apple not officially supporting the file format on iOS. It will also let you play hi-res tracks on an Android device, whether it natively supports 24bit playback or not; it worked pefectly on a Nexus 5 , and although Samsung's stock Music app crashed on our Galaxy S5 , switching to PowerAmp resulted in flawless playback.

Whether listening to CD-quality audio, lossless FLAC or low bit rate YouTube and internet radio streams, the HA-P50 sounded fantastic; it produced more powerful bass without creeping into the mid-range, which remained clear and precise. The entire soundstage feels wider compared to listening through the built-in audio circuitry, even when played at very low volumes. It has power to spare to reach very high volumes, or push more powerful drivers closer to their maximum, so you certainly won't be left wanting for louder music.

Naturally, sound quality will depend on the quality of your headphones or speakers, with our music sounding fantastic through a pair of Sennheiser CX3.00s but still able to get the best from the more expensive Shure SE425 in-ears. There was a welcome step up in clarity when playing high quality tracks, and although it can't work miracles on low bit rate audio, it ensures your playback device isn't the weak link when listening on the move.

Whether it's possible to justify £300 on a portable headphone amplifier will depend entirely on what source material you'll be playing and what headphones you'll be using to listen to it with. If you're still using the basic buds that shipped with your smartphone, the HA-P50 certainly isn't for you. If you've spend hundreds of pounds on a premium pair of on- or over-ear headphones, have a growing library of 24-bit music tracks, but don't fancy spending thousands on an Astell and Kern portable audio player however, the TEAC could be the step up you need.