Hypothetical scenario: an iPhone 3G's headphone jack is used to drive a set of budget-fi headphones (let's say a set of SRH440s, because that is what I own). The typical audiophile response to this given scenario is "noob, get yourself a nice portable amp for that. you are basically driving a Ferrari with a set of $10 tires on it". What exactly does the amp do?
 
Sure you can say 'use a headphone amp and you will understand', but I would rather talk about what is actually going on. I understand the role of a good sound source -- iPhone/iPod amps make use of the dock connector, which provides a digital signals, unadulterated by the iPod amp. Clearly, the same exact signal is used as an input into the iPod's built-in amp, so is it the amplifier that is the problem? Is not the amplifiers entire job to amplify an input and provide an amplified output? If the on-board amp uses the same digital signal that is sent to the dock connector then what is the difference? I will assume that the response will be 'well higher impedance cans like the SRH440s (44 Ω) are harder to drive than a set of XB700s (24 Ω), therefore they need more power so that you get the full sound stage'.
 
Impedance is just resistance. I have seen Ohm's law thrown about to justify the need for amps. Ohm's law specifies that voltage = current x resistance. I believe that headphone amps increase the current, which invariably will increase the voltage at the headphone, which would provide more decibels. Let me say that from my own personal experience that the iPhone's headphone output provides plenty of dbs to my ears through the SRH440s. 
 
To sum it all up: what will a DAC out + headphone amp do that provides a superior 'sound stage' when compared to the standard headphone output of the iPod/iPhone? I am not doubting that a headphone amp would provide better sound -- or at least more responsive bass, but I just do not understand why. I like to understand the mechanics of everything that I delve into -- I just want to know what is happening.
 
Edit: I know I will catch some FLAC for posting this in the full size section as I reference portable devices and amps, but this is not a question specific to portable devices/amps -- it is a general question on amps.

Hello greg
 
Why are headphone amps important? 
 
Well, where would the amp manufacturers be without us buying them and praising them?....lol
 
Just kidding of course.....
 
 
I'll let the engineers do the dirty work on this one which includes voltage swing, ohms, impedance etc...
 
I have acquired a couple of amps in my years here on Head-fi and I like to keep it simple-  Headphones and amps go hand in hand to bring out their full potential.  It's like listening to lossy vs. lossless.  You can hear the music with the former, but with the latter, it shows you how the music is meant to be heard.  Extension, depth, soundstage, imaging.  This can be accomplished with portables or full-sized amps.

I posted a similar question a while back. Check out the thread here: http://www.head-fi.org/forum/thread/506655/ok-why-do-i-need-a-headphone-amp-for-hd650

There's the usual forum bickering but if you ignore that there's also some useful information as I recall.

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alfbaxter said:

I posted a similar question a while back. Check out the thread here: http://www.head-fi.org/forum/thread/506655/ok-why-do-i-need-a-headphone-amp-for-hd650/45#post_6887340

There's the usual forum bickering but if you ignore that there's also some useful information as I recall.

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I am reading through that right now. Definitely a lot of good information and I'm only on page 2. Thank you very much. 

This was perfect for my question on does amp just = higher volumes (db)?
 
http://www.head-fi.org/forum/thread/490485/sufficient-volume-vs-sufficient-headphone-amping-and-critical-listening

I think a good analogy to the relationship between volume, current and audio is that of the relationship between speed(velocity), horsepower and cars. Saying that an amp only allows for greater volumes (db) is like saying that the only difference between higher horsepower cars and lower horsepower cars is top speed. As I have owned a car with 400hp I know that there is an enormous difference in 'feel'.
 
Sorry for opening a new thread on the subject -- I have searched for several days to find out the answer to my question, just couldn't find it. 

the eyefon has a headphone amp built in, it just sucks.

some good links.

Sound stage is something that is impossible to measure with current techniques.  I don't think anyone can tell you why an amps sound stage is like so from any kind of scientific measurements.  Or at least I've never seen these measurements.  But I think most of the change I've noticed using the same source and amp going from SE -> balanced is in the sound staging provided that the setup was powerful enough as it was in SE operation. 
 
In any event, most headphones (besides maybe orthos) have impedances that vary greatly at different frequencies.  So while it may be listed at 44 ohms certain frequencies could be half or double that impedance.  A dap's amplifier output is very likely to be extremely current and voltage limited as the manufacturer has to worry about size and battery life as well as cost and to be quite frank the average consumer uses apple ear buds or the like with them anyhow.  It (a dap) might not be able to do a certain frequency at the same volume or control as other frequencies especially if you need to use the dap near max output for sufficient volume.  Lower impedance is not necessarily easier to drive, to a certain extent yes but high current can be just as demanding, (potentially even more demanding,) as high voltage.  Sensitivity is just as, if not more important than, impedance for determining the volume you are going to be able to reach.  Some amplifiers are only really good with high (or low) impedance loads though.
 
If you are happy with your current headphones / source / amping situation keep your wallet happy  .
 
Also it would be damn near impossible to put a class A amplifier in a portable anything :O  Though it would be pretty awesome and could double as a space heater / testicle burner.

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bebopmcjiggy said:

Also it would be damn near impossible to put a class A amplifier in a portable anything :O  Though it would be pretty awesome and could double as a space heater / testicle burner.

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I listen at work, my goal is to listen directly from my computer as the HDD is bigger than my iPhone's HDD. I use my iPhone because it sounds much better than the computer's headphone output. I am definitely going to get a DAC and amp. I just got a bunch of schematics for hybrid amps, so I'm probably going to build one for fun first then buy a good one. I would like to be able to fully appreciate the engineering that goes into an amp before dropping the money to buy one.

If you're trying to justify whether a headphone amp is really necessary, I would venture to say that if the iPhone is your primary source and some budget-fi buds your choice of output, then I wouldn't really vote for a headphone amp to be used.
 
In the end its the combination of all components of the audio chain that affect what you hear; in this case the iphone/ipod as-is has its own DAC and amplifier which are built for Apple specs and sizing.  Typically some compromises have to be made in order for this to work.  You will hear many say about DACs that it's very important the power section and analog section.  Whether or not the built-in dac and amp is good enough for you only you can answer

I've just acquired an HD650 which is pegged as notoriously hard to drive. I'm a believer in the "crap in, crap out" mantra, but if you took Head-Fi comments as gospel you'd be of the opinion certain "mid-fi/hi-fi" cans sound no better than generic buds when unamped. This is pure hyperbole imo and makes me question the underlying agendas of so called forum experts. Sure there are obvious gains to had (and I'd politely question your hearing if you couldn't notice the improvements provided by "better" gear), but I feel it's not as black and white and absolute as what people make it out to be.

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mr.sneis said:

If you're trying to justify whether a headphone amp is really necessary, I would venture to say that if the iPhone is your primary source and some budget-fi buds your choice of output, then I wouldn't really vote for a headphone amp to be used.
 

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Nothing is really necessary . I am using 440s, not buds.
 
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olor1n said:

I've just acquired an HD650 which is pegged as notoriously hard to drive. I'm a believer in the "crap in, crap out" mantra, but if you took Head-Fi comments as gospel you'd be of the opinion certain "mid-fi/hi-fi" cans sound no better than generic buds when unamped. This is pure hyperbole imo and makes me question the underlying agendas of so called forum experts. Sure there are obvious gains to had (and I'd politely question your hearing if you couldn't notice the improvements provided by "better" gear), but I feel it's not as black and white and absolute as what people make it out to be.

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I am currently acquiring parts to build a hybrid headphone amp. Where I work we have a basically everything that I need sans tube, tube socket and enclosure. Should cost me about $20 maximum to build one. If I hear more 'nuances' and my design is too noisy then I will definitely be buying a retail amp. The idea of having sufficient current to power all frequencies makes sense to me -- especially as the resistance/impedance varies over the frequency spectrum. I do hope that an amp has a noticeable effect. 

audio is a very subjective matter  so why dont you go down any audio store and audit what is the different between amp and no amp ?

 
 
Headphone amps aren't used for more volume, but better control of the headphone drivers they power.  Its really that simple!