So, using the spreadsheet and spec sheet of an amp; how would you determine if the amp is up to properly driving a particular headphone?
post #31 of 47
7/9/09 at 9:59pm
So, using the spreadsheet and spec sheet of an amp; how would you determine if the amp is up to properly driving a particular headphone?

If it lists the output power for the given headphone impedance in the spec sheet, you just look in the spreadsheet and see if where it lies wrt the headphone's requirement. If it lists power output at a number of impedances, look at the closest one and then apply I=sqrt(P/R) && E=I*R and look in the correct columns of the spreadsheet (higher is better).
If it only lists an ambiguous max output power rating you are stuffed You will also need to know the amplifier gain and the line out voltage level of your source, when multiplied, these should give a value somewhat higher than the Vpp for the headphone at highest volume (110db is a good highest volume here). 
HI guys,and thank you for the helpful comments, but i think that i didn't quite get it yet. sorry..but it's a little hard for me to understand this...
let's put the sensetivity issue asside for a moment, let's say that all headphones have the same sensetivity...than why a higher impedance phones should be EASIER to drive?? it just doesn't make sence to me. I always thought that: the higher the impedance, the more power you need to drive it. and I know it's true in some cases (many ask about certain amps ability to drive the senn hd650/600 or 800), but in this thread i hear differently. please explain to me like i am a 5 years old.. 
Let me take a stab at it.
A higher impedance is easier to drive because it demands less power from the amplifier than a lower impedance. An amplifier is designed to output a given voltage for a given voltage input to it, based on how much gain the amplifier has. For example, if an amplifier has a gain of 10, if you put 1 volt into it, it will output 10 volts. How much current it is asked to deliver depends on the impedance of the load that it's driving. Ohm's Law for current is I = E/Z where I is current, E is voltage, and Z is the impedance. So let's take our amplifier that's outputting 10 volts as an example. If we connect an 8 ohm load to its output, then the amount of current that load will want to draw is 10/8, or 1.25 amps. If we halve the load impedance and instead connect a 4 ohm load to its output, that load will want to draw 10/4, or 2.5 amps. Twice as much current as the 8 ohm load. Ohm's Law for power is P = I x E where P is power, I is current and E is voltage. So for the case of the 8 ohm load where it's wanting to draw 1.25 amps from the amplifier, the power that the load is demanding from the amplifier is 10 x 1.25, or 12.5 watts. For the 4 ohm load, it is 10 x 2.5 or 25 watts. Twice the power. In other words, as the impedance decreases, it demands more power from the amplifier, not less, and this is what makes lower impedance loads "harder" to drive and higher impedance loads "easier" to drive. Rather like lifting a 100 pound weight is harder and requires more "power" than lifting a 50 pound weight. And this is why higher impedance loads are sometimes referred to as "light loads" and lower impedance loads as "heavy" loads. Does this help? k 
I always thought that: the higher the impedance, the more power you need to drive it. and I know it's true in some cases (many ask about certain amps ability to drive the senn hd650/600 or 800), but in this thread i hear differently. please explain to me like i am a 5 years old..

I my language (hebrew) impedance means "resistance". that means that the higher the impedance the higher is the headphones/speaker's resistance.
and by what these formulas are saying, this term is just the other way around... what's that confuses me the most is that sometimes people taliking on high impedance cans as hard to drive (most of the times actually) and sometimes as easy to drive...why is that? if LOW impedance cans are the harder to drive...than it should always be like that, isn't it? than my RS1 is hard to drive even though i can drive them with my ipod? am i missing something here? 
You really can't talk about the drive requirements based on impedance only. You must also consider the efficency, see above in the thread for an example with K701.

thanks a lot for the detailed explenation mate! i can't argument with the formulas (I = E/Z and P = I x E) ...and i understand what they are saying, but it doesn't make sence to me logically. i mean...how can it be that a high impedance headphone is driven more easely than a low impedance one? but i guess the formulas don't lie!
I my language (hebrew) impedance means "resistance". that means that the higher the impedance the higher is the headphones/speaker's resistance. and by what these formulas are saying, this term is just the other way around... 
what's that confuses me the most is that sometimes people taliking on high impedance cans as hard to drive (most of the times actually) and sometimes as easy to drive...why is that? 
if LOW impedance cans are the harder to drive...than it should always be like that, isn't it? 
than my RS1 is hard to drive even though i can drive them with my ipod? am i missing something here? 
thanks you koyaan for the very well worded explanation, i think i got it.
it's always bothered me for not getting to the buttom of this, and now i understand it more. 
headphones sensetivity is not something new to me, but i always thought that a headphone with a high impedance will be hard to drive no matter of how much is the sensetivity factor of it. I guess the sensitivity has more influence than i thought. 
just one more question than...what headphone will be harder to drive?: A. 300ohm and 117db B. 60 ohm and 106 db what have more influence here...the impedance or the sensetivity? 
thanks. and if you mentioned speakers, i was always confused when i saw the low impedance rating of speakers... (8 or 4 ohm) it was like: how could it be that a speaker has so little impedance and need much more powerfull amp than headphones do?...now i kind of got it.

thanks again. 