[CharlesC]

Some headphones are considered hard to drive due to their high impedance while other headphones, like most planar magnetics, are considered hard to drive in spite of relatively low impedances.  My question is: do each of these two types of hard to drive headphones require different amp characteristics for top performance or is an amp that is good for one type usually just as good for the other?
 
Thanks.

 

[xnor]

What you're looking for is sensitivity and impedance.
 
Generally speaking:
high impedance means you need higher voltage gain but less current,
low impedance means you need lower voltage gain but more current
 
low sensitivity means you need more of the above to get the headphone playing loud
high sensitivity means you need less
 
 
If, for example, your amp has high gain then you wouldn't want to use a high sensitivity, low impedance headphone. You won't be able to turn up the volume control very far because it will get very loud very soon.
 
The combination high impedance headphone and low impedance, low sensitivity headphone might work well, but without specific models it's hard to tell.

 

[CharlesC]

..... but without specific models it's hard to tell.

 
Thanks for taking up the question.  The distinction between impedance and sensitivity is helpful.
 
Here is a fairly easy to imagine scenario:  Let's say that in a few month's I decide to get some HD-650s (high impedance).  I would likely buy a new amp at the same time because the one I have now just won't do. Let's also say that I buy a Bottlehead Crack and actually get around to building it.  Then, going another year out into the future, I lose my mind completely and buy a pair of Audeze LCD-3s (low sensitivity).  Would it be reasonable to expect than an amp that drives my HD-650s well to also handle the LCD-3s reasonably well? They are both, after all, "hard-to-drive".
 
Thanks again.

 

[SanjiWatsuki]

   
Thanks for taking up the question.  The distinction between impedance and sensitivity is helpful.
 
Here is a fairly easy to imagine scenario:  Let's say that in a few month's I decide to get some HD-650s (high impedance).  I would likely buy a new amp at the same time because the one I have now just won't do. Let's also say that I buy a Bottlehead Crack and actually get around to building it.  Then, going another year out into the future, I lose my mind completely and buy a pair of Audeze LCD-3s (low sensitivity).  Would it be reasonable to expect than an amp that drives my HD-650s well to also handle the LCD-3s reasonably well? They are both, after all, "hard-to-drive".
 
Thanks again.

 
Not always.
 
Amps are commonly limited by two things -- current and voltage. The higher the impedance, the less current is needed to reach the same voltage. 
 
In this case, the LCD-3 is a low impedance, low sensitivity headphone. The HD650 is a relatively high impedance headphone. Thus, the amplifier will need more current to reach the same voltage with the LCD-3 compared to the HD650.
 
So, suppose I had two different headphones. #1 has a resistance of 16 ohms and #2 has a resistance of 300 ohms. I need to reach, say, 2V of power to drive the headphones properly. My amp can put out 3V of power, but it can't handle high current. In this situation, I could drive headphone #2, but possibly not headphone #1, despite both of them requiring the same amount of voltage.

 

[xnor]

Bottlehead Crack doesn't seem to be a good choice for low-impedance headphones, because it has a ~120 ohm output impedance.
 
 
edit: the manufacturer even says on his site: "designed specifically for high impedance headphones (best above 120 ohms)"

 

[CharlesC]

   
Not always.
 

Thanks.  I'm getting the gist of this.  The harder I think about it the more sense it makes, which is good.

 

[CharlesC]

  Bottlehead Crack doesn't seem to be a good choice for low-impedance headphones, because it has a ~120 ohm output impedance.
 
 
edit: the manufacturer even says on his site: "designed specifically for high impedance headphones (best above 120 ohms)"

Thanks.  This brings up something.  A certain enthusiast was talking about how well the highest powered (1w) Geek Out works with his Audeze headphones.  Looking at the Geek Out preorder page ( mustgeekout.com ) it says that this level dac/amp is for headphones of greater than 300 ohms which the Audeze is not. While mentioning this amp, the enthusiast referred to the amps wattage more than the amp's headphone impedance recommendations.  There seems to be a certain amount of "what's good for one is good for the other" going on.
 
At any rate, I'm pretty sure that I will need to get beyond my surficial understanding of electronics to fully understand all this.  Right now it is enough to know that I will need to be careful about these things.

 

[xnor]

Well it is supposed to output about 11.3V with a 2V source.
 
Into 300 ohms this leaves about 8V, into 50 ohms about 3V, into 30 ohms about 2V.
 
If the amp can handle those "low" impedance loads then there is no problem. It's just not gonna go extremely loud.

 

[manbear]

Thanks.  This brings up something.  A certain enthusiast was talking about how well the highest powered (1w) Geek Out works with his Audeze headphones.  Looking at the Geek Out preorder page ( mustgeekout.com ) it says that this level dac/amp is for headphones of greater than 300 ohms which the Audeze is not. While mentioning this amp, the enthusiast referred to the amps wattage more than the amp's headphone impedance recommendations.  There seems to be a certain amount of "what's good for one is good for the other" going on.
 
At any rate, I'm pretty sure that I will need to get beyond my surficial understanding of electronics to fully understand all this.  Right now it is enough to know that I will need to be careful about these things.

Impedance matching is only relevant for dynamic headphones. As long as you have enough power, you can use whatever output impedance you want for planar headphones like Audezes. Planars have a constant impedance with respect to frequency, so they are not affected by damping factor. If your friend had low impedance dynamic headphones, it would be a different story.