A DAC is always needed yes, but you're not offering any other answer then shiny marketing snake oil as to why sometimes and more often in modern devices its not. That says to me you know what?
"A DAC is needed because a DAC is needed and I have no other reason of saying a DAC is needed because everyone else has told me a DAC is needed and I have no idea what I'm talking about."
Until such time that you elaborate your views further that's exactly what I'm going to think. Provide me with some objective evidence that the average device today actually has a poor DAC and we'll come back and talk about this. Otherwise you're just another snake oil marketing clown trying to get people to buy things unescessarily. Frequency response is about as objective as it gets, this measures the natural response of the iPhone and what it does, you will see the ruler flat response rate, what that means if you understand how to read it, which I'm doubting at this point, is that what you will get before plugging headphones in is a perfectly equal response between lows, mids and highs. I'm assuming at least some knowledge of what sound does, and looks like when its measured here.
What you get by adding a DAC in line is an unknown, you might enjoy the flavor it adds, but objective, it may even lead to a remarkable degradation in both frequency response and SQ because you believe some snake oil nonsense from marketing companies for these DACs on these websites telling you that you need a DAC... because it makes "things sound awesome" when you objectively don't understand what those "things" are.
And so we have the current state of Head Fi, people spending hundreds, or even thousands of dollars on things that might not make any difference, or in fact might make the sound they're listening to worse. Of course this is Head-Fi so this type of response quickly gets corralled as per usual and closed down because of the sponsors that be here... People buy things they don't need and the cycle continues of people buying expensive things that objectively add nothing to their sound stage, or in some, if not many cases, degrade it. Unfortunately you can't help some people though.
Well, I've studied physics at university, I've measured plenty of headphones and other electronic devices including amplifiers, and amplifiers coupled with DACs in the past. I've heard many different setups, including turntable setups and speaker setups. I've made blind testing many times. I've been at concert halls many times, rock shows, hundred coral concerts, and I've listened to music critically for hours a day since at least ten years ago. On top of that, I normally spend some time a day here helping newcomers, 90% of my 3300 posts here took place on the Recommendation forum and you can check them, you'll find math formulas on a bunch of them. Beside Head-Fi, you can find me on much more technical forums as well.
If that's your definition of a clown, then yep, I'm a clown.
This is what I've said:
A DAC is always needed to play digital files. An external DAC is not needed, but it could be a good addition if happen to hear (and enjoy) the difference.
You can use your phone and that's perfectly fine since it has a built in DAC then it works. If you have a chance to try a separate DAC then why not...
As I've said, I had the chance to try a lot of DACs and Amps and also measure some of them, and they don't sound the same, even when most of them measure perfectly flat.
There are other factors involved such as harmonic distortion, intermodulation distortion, jitter; USB receivers on DACs can make a slight difference, power supplies can make a slight difference. and so on...
Sometimes the typical measurements (FR/THD/IMD) can point out these subtle differences and sometimes don't. Sometimes you have to find out a proper test to expose why certain DAC is perceived as warmer than the other despite measuring very much alike. Part of the complexity comes from the fact that our hearing involves our brain which is an extremely complex device that's not fully understood. On top of that we can only do blind testing or sighted comparisons and both depend strongly on our memory so they are not that great either.
I think we are a bit laggy with regards to measuring very subtle audible differences on DACs and Amps, and to a lesser extent headphones and speakers.
I'm normally the one who's pushing experiments to get a better understanding of what we hear, and that's why I enjoy technical forums.
Feel free to join more technical forums and start measuring things yourself and develop new techniques as well. That's much more useful and enlighting than doing nothing more than comparing FR charts.
It's true that people often pay thousands of dollars for things that could be priced 200usd, or even 20 in some cases. But it's also true that today there's plenty of affordable yet properly engineered gear costing less than 100usd, adding functionalities and lasting many years. More so, you can get most of these devices, use it for a week or two, decide if you enjoy it or not, and then resell it if you feel the need, and that would probably cost you 20usd if not less.
