[DjAmTraX]

When an amp or source with headphones out specs show the following, what does it mean to me and how do I use the information on pairing up headphones?
 
Lehmann Audio Black Cube Linear:
- Output power: 200mW/300 Ohms, 400mW/60 Ohms 
 
Apogee Mini-DAC:
- 300mW into 30 Ohms
 
Sony CD Player:
-15mW into 32 Ohms
 
What is considered high power?

 

[DeadEars]

OK, here's a few things to think about.  Headphones and speakers are mechanical devices that move air to produce sound.  Moving air requires power.  Some of these devices have motors that are more efficient than others to move air.  Given the same amount of power, the more efficient motors move more air = louder & more dynamic range.
 
The measurement of power is watts.  Electrically, power is measured by voltage *times* amperage.  An amplifier with a higher voltage at the same amperage produces more power.  Or if the voltage is held constant, the higher the amperage the amp produces, the more power it generates.
 
Headphones that are not efficient require more watts to produce the same sound output (measured in decibles = db).  The very tricky part of this is that headphones are often not linear in their power requirements at different audio frequencies.  That is, they may be more efficient at some frequencies than at others.  So an amplifier might be able to drive them very well at treble frequencies -- which require small-but-fast movement of the air pump --  but fail to provide enough power for low frequencies which require large movements of the diaphram.   Or vice-versa.
 
Headphone amp specifications are usually insufficient to tell you whether they will be a good match for your headphones, since they reflect an average amount of power delivery into a given impedence across the frequency range.  Just as headphones are not equally efficient at all frequencies, amplifiers are not equally powerful at all frequencies.  A good example of this is OTL amps, where the headphones are connected directly to the output transistors/tubes, with no transformer.  Many OTL amps produce much more power at higher impedence than they do at lower impedence.  When matched to a headphone that happens to have greater impedence at lower frequencies than it does at higher frequencies, the result can be sonic bliss!  The same headphones on another amp might sound like crap.  Or the same amp with a set of headphones that have a more level impedence curve could also sound like crap.
 
The general rule of thumb is that if you know how much power your headphones require, you want an ampflifier that will give you at least 10-50% more power than that, for musical peaks and headroom.  Except that the specs never tell you what the power delivery really is at different frequencies and across different impedences.  On paper, it may look like the amp would do a great job with your headphones, but when you actually listen, you might discover the bass to be MIA.  Or because the treble delivery is so strong, you find the amp to be shrill and fatiguing to listen to.
 
That's why this forum is so useful.  You can usually find other people who have broad experience with specific amps with specific headphones in which you are interested.  If a particular combination has worked will for a variety of people whose ears you trust, it is definitely worth your time to audition.  But reading about other people's experience is no substitute for hearing things yourself.  Getting to a meet and hearing the combination that you are seeking is the best way to make a decision.
 
Hope this helps!
 
Frank

 

[DjAmTraX]

Frank,

Thank you for the detailed response. It's a bit too deep for me to fully understand. So the specs listed on headphones amp is in terms useless unless we know how it was tested to get the results?

 

[DeadEars]

 
Quote:

Frank,

Thank you for the detailed response. It's a bit too deep for me to fully understand. So the specs listed on headphones amp is in terms useless unless we know how it was tested to get the results?

Sorry, my answer was kinda like telling you how to build a watch, when all  you wanted was the time.  But the real answer is...  It Depends.
 
The same amount of power (e.g. 20mw) can blow you away with IEM's but barely move a planar magnetic headphone.  So you need to know the load as well as the power.  High power is relative.
 
First you need to know your headphone's characteristic impedence   Like if you have 300-ohm Sennheisers or 32-ohm Grados or whatever.  Plus it helps to know their sensitivity as well.  That will help you figure out whether an amp has enough power or not.
 
Next, you need to know the amp's ability to deliver enough watts (or milliwatts) to that load.  Some headphone amps are designed for a particular load and only work within a narrow impedence band.  Others might have separate taps for high impedence headphones versus low impedence headphones.  With an easy load, a low-power amp can deliver fantastic performance and all the volume you can stand.  A high-power amp would be useless above that threshold, and probably would sound worse than an amp with just the right amount of power.  
 
Some manufacturers publish the sensitivity of their cans (decibles per watt), others do not.  Insensitive cans need more power.  Listening to music at headbanging levels also requires more power.  How many decibles can you stand?
 
As a general rule, any headphone amp delivering more than one watt is high power for almost all headphones, and much too high for many of them.  On the other hand, I'm currently playing with a pair of Audeze LCD-2 planar magnetic headphones and they have low nominal impedence at 50 ohms, and are insensitive as well.  They might require a couple of Watts (not milliwatts) to sound their best.  The manufacturer says they will handle as much as 15 watts!  Suddenly you are in the realm of speaker amplifiers, which has got to be "high power" right?
 
My major point was power ratings are just averages, and only a weak predictor of whether or not you get a good match between a particular amp and your headphones.  Since both the headphone load AND the amp's power delivery are not constant at all frequencies, power ratings are not all that useful. 
 
Which is why I suggested you should let your ears should be your guide.  Specs on paper are only a general guide, and the amp makers are going to try to make their amps appear as powerful/capable as possible.  But if you know the specs of your headphones (impedence and sensitivity) and the specs of the amp (power delivery at the same impedence as your cans), you can get a fairly good idea whether the amp will have enough power for your cans.
 
 
Hope this helps!
 

 

[temporaryname]

Thank you DeadEars for the informative posts. I, for one, prefer to know how to build a watch, as the more I know the better I can understand.