Why, from a technical perspective, is an amp necessary?

[MinorPrelude]

Can anybody explain to me exactly why a headphone amp is necessary to drive high-end cans properly? I know it sounds like a naïve thing to ask in such a place, but as many posts and articles as I've read, I can't seem to get a grasp on what a good amp actually does that the amps built into consumer electronics cannot do, aside from the obvious such as producing coloration or greater volume.

To put things in perspective, I've got Sennheiser 700s and 800s, as well as an entry-level tube amp, the Bravo Audio Ocean. The Ocean sounds nice and adds some pleasant warmth, and in particular it makes the 800s less fatiguing, but it's nothing special and doesn't exactly add a "wow" factor to the already superb sound of either pair of headphones.

I've been considering buying a quality amp that's known to pair well with the 800s, and in that regard I'm leaning towards something transparent rather than colored, but before I spend many hundreds of dollars in that pursuit I'd really like to know the nitty-gritty of what benefit I'll attain from the investment.

My understanding is that truly transparent DACs are now so cheap that nearly everything of decent quality has them—plug your headphones into an iPhone and maybe you'll get a bit of unwanted noise, but as far as the fidelity of the audio signal coming out of that little jack goes it should be nearly perfect, right? If you're just driving a pair of earbuds, everyone seems to say, you're not going to reap a significant benefit from a fancy-pants amp or DAC because there's almost nothing you can do to better that signal except improve the signal-to-noise ratio, which nowadays is already so high on cheap electronics as to be insignificant under most listening conditions.

But then there seems to be this nebulous notion that such low-powered outputs are not capable of driving good phones for whatever mysterious technical reasons. I'm wondering, if the outputs on my iPhone and MacBook and ASUS motherboard can produce more than enough volume for an enjoyable listening experience on the 800s, what's the element that's missing from that signal that an appropriate amp would afford me? All I can seem to find are subjective claims about the superiority of a good amp-headphone combination, but very little in the way of details about why this is—let alone, say, ABX results demonstrating the audibility of a high-end transparent amp over built-in DACs.

Your responses will be appreciated, thanks in advance.

 

[ProtegeManiac]

Can anybody explain to me exactly why a headphone amp is necessary to drive high-end cans properly? I know it sounds like a naïve thing to ask in such a place, but as many posts and articles as I've read, I can't seem to get a grasp on what a good amp actually does that the amps built into consumer electronics cannot do, aside from the obvious such as producing coloration or greater volume.

The primary determinant isn't that they're high end headphones, but if the sensitivity is low or the high impedance offsets some of the sensitivity, you could end up with whatever it's plugged into already piling on distortion and/or noise, if not actually clipping. For example, an integrated audio chip in common devices rated at 5mW at 32ohms would have trouble getting a 62ohm, 93dB/1mW (note that you do not literally get 93dB at 1/5 of the volume setting) or a 300ohm, 97dB headphone to a good listening level without piling on noise and distortion. These aren't uniform - in many cases just hearing more of a distinct "slam" on the bass on a better am isn't due to that amp "boosting" it (though not always the case of course) but because it is actually already playing louder but not piling on distortion nor noise. A good example of a high end headphone that barely needs an amp would be the Grado RS1e. Going frm a smartphone to a portable DAC-HPamp has an appreciable enough difference, but going from that portable DAC-HPamp to a Violectric or Meier isn't likely going to help (unless that's one noisy portable DAC-HPamp).

In some cases the impedance can be too low that some of the things you might plug them into end up EQ-ing the sound, either making them sound like molasses, or like tin cans.

The next determinant is how well they isolate. If the headphones are practically just protected by chicken wire so you don't poke the drivers with your fingers everytime you put them on, you need to play them louder (and hence require more power, without piling on distortion and noise), or have less noise around you. Even a quiet room in the evening can register over 30dB - to match that in a acoustically sealed room you might as well put a gaming PC test bench in there.

If anything, what makes high end headphone need an amp that non-reference cheap headphones don't do is a smoother response curve. Some headphones intentionally boost the upper bass and midrange, maybe also some part of the treble, making the sound more appealing (that includes some of the more expensive ones). A headphone with a flatter response by contrast when played at the same volume level will be perceived as "boring" by a lot of people (note that none of them are absolutely flat yet). Drive them both loud howeverand the peaks in the first become fatiguing, while the other finally has that "exciting" aspect of sound presentation, ie the foot-tapping effect you get out of a playback system.