Smartphones are starting to sound pretty good, in fact, the new LG V10 and HTC 10 (I am a long term owner) sound downright great. But there`s a reason why Apple is supposedly removing the 3.5mm headphone jack on their new iPhone 7 (and why Motorolla removed it from their Z and Z-Force).
You might have read that the new HTC 10 has a 24bit DAC or that Bang & Olufson have partnered with LG to create a 32bit DAC module for the G5. Unlike what most reviewers or even the suppliers themselves might have you believe, every digital device that can play audio has a DAC, or a “digital to analogue converter” that converts your digital files (your MP3’s, FLAC’s, etc) into an electrical signal. This analogue signal is sent to the amp that, as its name suggest, amplifies the current to produce volume through your earphones/headphones. This process is somewhat like converting a video; every time you convert something from one format to another there will be a loss of some sort. So what the 10 and G5 really have are high quality DACs that will convert with minimal information loss (or less information loss) and a good AMP that will similarly supply a “clean” current with minimal interference or distortion; or perhaps more current (for low impedance earphones) and high voltage (for high impedance headphones) and hence more volume. So it can thus be determined that both the Amplifier and DAC will have a substantial effect on the way your music sounds, as the saying goes garbage in, garbage out, if your headphones are revealing enough you`ll easily be able to perceive the benefit of a proper source.
Apart from output power, a big benefit of amplification is output impedance. The rule of the thumb is that output impedance should be as low as possible, less than 1/8th of the impedance of your headphone/earphone or else the sound will be affected. This is especially prevalent on multi-driver setups such as the Sony XBA-4, as it can affect the cross-over (circuit that determines what sound is delivered to what driver). For example, my HTC M8 has a relatively high output impedance and my Westone W30`s have a relatively low impedance. When comparing the sound of the W30`s running through my M8 to my Fiio e17k, that has an extremely low output impedance, the W30`s sound much more rolled off in the bass, losing a lot of sub-bass and rumble, and sound considerably darker in the upper midrange, somewhat truncating female vocals. However adding an amp to my M8 reveals that its DAC is quite strong and although the stock amp delivers plenty of current, the output impedance is negatively affecting the sound. When adding an external amplifier, the output impedance of the source is irrelevant and becomes that of the external amplifier instead. If you find your headphones sound vastly different from different sources, then investing in an amp, even a cheap one such as the Fiio A1, is something you should consider. If you actually prefer the sound of your earphones with a high impedance source then impedance adapters will help you achieve that sound (Same concept as the Etymotic ER4 P to S converter).
Beyond what a DAC actually is and the benefits of amping, I`ll talk a bit about why you should actually purchase a dedicated DAC/AMP at all. Just like designing speakers, where the subsystems can have a larger impact on the sound than the drivers themselves, using quality components is irrelevant if they aren`t properly implemented; that is to say, well isolated from the interference in the environment and between components themselves. Phones are getting a lot better, every iteration sounds a little closer to dedicated equipment and many manufacturers such as Samsung, Lg and Apple even employ DACs from Wolfson, Saber and Cirrus to provide better audio experiences. However smartphone audio is a niche market, the vast majority of buyers won`t even hear the difference and with the new fixation on slimness and design, quality audio sections are becoming harder and harder to implement. A dedicated DAC will always sound better simply because there is a lot more space to house what is essentially the same circuit. The components will be better isolated, the manufacturer doesn`t have to worry about power constraints and they are willing to dedicate all their resources to quality audio components because they are providing an audio product! It really comes down to the fact that whenever you convert anything, there is loss, so a higher quality dedicated DAC will sound better, it`s not debatable like cable upgrades, it`s as scientific as it gets.
But does this really make a perceivable difference? At first, I didn`t believe in the need for a discrete DAC, I was perfectly happy using my phone and iPod along with my JDS Labs Cmoyy BB. Many sources on the internet stated how high-end android phones/iPhones have “good enough” DAC`s and that phones and dedicated audio players all sound very similar. This was not helped by the fact that most portable DAC`s cost upwards of $250 AUD, I was always under the assumption that that money was better spent on some good earphones or headphones; after all, how much difference could the source make? So when the Fiio Q1 was released with the same DAC as the e17k at less than $100 AUD, I was intrigued and found time to visit Minidisc for an audition. And wow, what a difference! When I first started using an amp, it made a small difference, but mainly it got rid of the source discrepancy with output impedance and hiss. The Q1 made a real impact on the sound quality and listening from my ie800`s, mids were cleaner, details were more apparent and the sound was more extended, sub-bass response in particular was much improved. The dynamic range was far greater than my laptop and smartphone which made well-mastered songs a treat to listen to. In addition, you still get to interface through the UI of your favourite device/music app but get to reap the sound quality benefits of a dedicated DAC/AMP, much better than a dedicated DAP in my opinion.
So my subjective impressions were quite positive on a whole, I do think that really audio focused smartphones such as the 10 and V10 are about as good as a budget DAC but the amplifier will always be weaker on the smartphone due to simple size and power constraints. In addition, I found that a DAC/AMP made a bigger difference to my listening than a simple AMP for most of my gear (mainly iems) but my planar magnetic Oppo PM3`s and more output impedance sensitive earphones did benefit quite a bit from a proper amplifier.
Ultimately, If you have a great sounding smartphone you should look into a decent amp and a line out cable (bypass the stock amp, signal straight from the DAC), but if you have a more acoustically mediocre phone, or even a high-end one with poor sound quality, then a solid DAC/AMP can now be had for less than $100. The purchase is worthy if your headphones are greater than that in value, but I would hesitate to spend as much on the source as I would on my headphones; I wouldn’t amp something like the Shure SE215 for example. I still think that source should come second, at least as long as your headphones or earphones were intended to be used portably and have a decent sensitivity rating.