Well, I would love to help you here. It turns out that these numbers that they spec. barely tell half of the story.
So, first I am going to bore you to death for a second and scare you a bit. Then I will make you feel all better.
See what they are saying is that your headphone is about 250 ohms (R) at 1 KHz. That is really what a headphone impedance (Z) rating is telling you.
Now audible range is about 20 Hz - 20KHz and the impedance of an inductor (the coil driving the diaphragm) varies with frequency. Depending on the nature of the coil (among other things) driving your cans this impedance can vary slightly or quite a bit. This means that the impedance rating is a useful approximation but one 250 ohm set of cans may require more or less current or voltage across the range of frequencies than another 250 ohm set and here is why.
The Boring Part: (Skip ahead if you like)
To keep it simple an inductor becomes more resistive (goes up in impedance) as frequency goes up. XL = (2 * Pi * f * L). This formula will give you a linear response but the real world is a little more complicated which is why the frequency response graphs for a given headphone can have some interesting shapes. The inductor begins to behave somewhat like a capacitor especially at higher frequencies. The truth is though that there are all sorts of wacky things going on here because frequency is changing rapidly and there is the mass of the driver changing direction, maybe dual diaphragms and so on.
Making sense of mW and ohms and such:
http://www.reuk.co.uk/Ohms-Law.htm
Now if you look at this for a second you will realize it's really very simple. If you have any two numbers. Voltage, Current, Resistance, or Power you can find the others very simply. Now impedance is also simple. The formula above tells you what Inductive reactance (XL) or effective resistance(XL = R) a given Inductance (L) is at a given frequency (f). The problem of course is that the frequency in audio is dynamic, it's always changing.
So, if you look into impedance a bit you will find it's not so cut and dry. At some frequencies inductors begin to behave more like capacitors and all sorts of weird stuff. Which is why when we sweep a coil it isn't linear or why headphone frequency plots look like this:
http://www.innerfidelity.com/content/headphone-measurement-proceedures-frequency-response
OK. OK. So, skip all the B.S. and put it in a nutshell. Your headphone can demand a range of power depending on what sounds it is reproducing to get to a given volume level. What is important is that your amp can provide enough voltage and current across the range of frequencies at a given volume level without clipping or distortion. Or as well as can be expected.
So, saying a headphone is 250 ohms at 1 KHz is like saying that rock over there is kinda sorta heavy. It gives you an idea of what to expect but that's about it.
To make matters worse this is what you are given to work with:
Headphone: AKG Q701
Frequency range: 10Hz to 39.8kHz
Sensitivity: 105dB SPL/V
Input impedance: 62 ohms
Maximum input power: 200mW
Amp: FiiO E09K
Specifications
- Power: 900mW (32Ω); 150mW (300Ω)
- Impedance Range:16 ~ 600Ω
- SNR: >100dB (A weight)
- Distortion: <0.003% (100mW) - Line Out: 2V rms
Oh, well now! That really does clear everything up now doesn't it? No it does not. Yes you can look at these numbers and tell that the E09K is a reasonably powerful amp. What this does exactly squat to tell you is how well it really performs. Still confused? Good.
What they SHOULD tell you is at a given volume level (let's just say 120dB) what was the peak draw of the headphone across several kinds of music playback like say classical, rap, punk rock, trance and pop measured from a source that is way over sized as to provide the headphone with it's actual peak demand.
What the amps SHOULD tell you is what the output voltage and current plots look like into several different impedance headphones at a given volume level (again we would hope for 120dB). They should give you channel balance plots, distortion plots, input impedance, output impedance (many do this) and so on and so forth. Really an amp should have a lot of info. to publish to give the full story.
Continue Here:
So, why am I telling you all of this? The point I am trying to get across is that while this should not be all that complicated between the headphone manufacturer specs. and the amp. manufacturer specs. we are are woefully ill equipped to answer one single simple question. How much power does it take to properly drive my headphone and what amps can do just that?
You should really read the blog by: nwavguy
There is a much more detailed and better written entry about headphone power requirements if you want to dig deeper.
Google is your friend. It is quite educational.
Then you can draw your own conclusions.
Many LOVE the distortion caused by tube amps. Many think this headphone synergies with that amp. In the end it's a matter of taste. This is all about what you like subjectively as to if you will be happy with the end result.
However, both nwavguy's and my opinion are similar in that I believe that an amp you can hear is a bad amp. As for me I had to at least know that my first amp was of good quality and sufficient power and at least his design posts the numbers and inner-fidelity verified them. I built my own with a couple of tweaks and can say that I sleep easy.
Hope this helps as well as makes you wary but in the end have fun out there. Happy hunting and cheers.
EDIT: To clear it up a bit....What I was trying to say is that with the O2 amp at least you have a more complete set of data to help you choose an amp. At least you can say that if this is your first amp (it was mine) you know what you are getting. It will drive those headphones with enough power properly and will do it for a relatively modest price. If you are confused (and you should be) it's a good place to start.
