i totally don't know how to read this if an amp spec is put in front of me with a hd650
here is the most simple way I can think of to get concrete information for your hd650:
I think I kept it relativelly understandable and simple, it's long because I explain and repeat stuff several times not to lose people on the way, but there is no rocket science involved at any point in this post(I'm myself just a noob with a few tricks).
I pray the god of all calculators that I didn't mess up somewhere. the science is solid, but the human doing it is rather weak in the fingers when typing numbers ^_^.
when you want to know if your headphone will be well driven, you need the impedance of the headphone of course, and
how loud you want it to sound! this is very important, because the louder you may go, the more power you'll need. for super quiet listening a sansa clip can drive almost anything. so loudness is very important and subjective opinions not helping at all because you never know how loud the guy is going to listen compared to you.
the hd650 is given by sennheiser with those specs:
*
impedance 300ohm (it actually goes higher, but not lower, so using 300ohm is fine here)
*
103db @1v @1khz
meaning that if you send a signal of 1khz reaching 1volt RMS into the hd650, it will create a 1khz sound 103db loud. simple stuff really, in fact that's how they measured it. so you see loudness and volts are directly related.
the threshold of pain in the medium frequencies is about 115db, so if your amp can drive the hd650 that loud at 1khz, you can claim that it can drive it well for any non masochist usage. ^_^
that's one of the reasons why I aim for 115db when I make estimates to be sure it's always good(but realistically 110db is more than enough for loud listening of most albums).
so 1volt gives 103db into the hd650, and we want an amp that's able to push it to 115db. meaning we want to go 115-103=12db louder.
use http://www.sengpielaudio.com/calculator-gainloss.htm for a gain in decibel of 12db it tells you to multiply your voltage by 3.98. so 3.98*1=3.98v I'm so cool I didn't use the calculator here!
now you can face 3 situations:
situation 1:
when amp specs give you a voltage into 300ohm like the project ember of our buddy Zorro, the website gives
Max Output voltage: 15.7Vrms at 300Ohm
we determined that we need up to 3.98v for the hd650 so obviously the ember has more than enough to drive it.
situation 2:
the most common situation, the amp sepcs give you a power value in watt or milliwatt into a given load(impedance of the headphone).
P=U²/Z (it is, just trust me)
P being the power, U the voltage and Z the impedance. how cool is that? we already know the hd650's impedance is 300ohm, and we have decided that we needed a voltage of 3.98v
so P=3.98²/300= 0.0528w or 52.8mw. we want an amp that does at least this much, so now we go look for amp specs.
for example the shiit magni2 gives
Maximum Power, 300 ohms: 260mW RMS per channel
so once again you know that you'll have more than enough.
situation 3:
the manufacturer doesn't give a power output into exactly the impedance value of the headphone.
for example the O2 gives
Max Output (150 Ohms)355 mW
Max Output (600 Ohms)88 mW
there you don't know because you don't have the right impedance.
but because you can see you'll have enough power in both cases surrounding the 300ohm of the hd650, at least here you can be confident that it more than ok.
in fact if you look a little into it, thanks to all the data nwavguy gave, you can learn that the O2 reaches about the same maximum voltage from 80ohm to 600ohm and that's around 7v. so with our desired 3.98v to get 115db out of the hd650, we're now in
situation 1 and we know for sure that we're good.
to recap:
specifically for the hd650, you want an amp that can do at least 3.98v or 52mw into 300ohm. you don't need to know more but I thought it would be nice to explain why those values instead of throwing them at your face.
hope this can help.
edit: some syntax typos etc. /me spik angrish very good
