[AndrewG]

in all the portable amps I've tried, I was able to drive my cans more than loud enough.  so are these amps with 20 + volts output necessary? 

is there another benefit to high volt power different from just how loud they can go? or are they only for high ohm cans like beyers/sennheisers/he-6 ….

Does higher output/voltage power of these amps still benefit cans which are pretty easy to drive low to mid gain?

 

 
Then desktop amps can go as high as 100-200 volts, why is all that power necessary?

 

[BlackbeardBen]

This thread over at Hydrogen Audio explains the interaction between headphones and amplifier quite well - much better than I can do.
 
 
 
High voltage amps are needed for high impedance, low sensitivity headphones.  Think of them as "easy to drive, hard to make loud".
 
A high headphone impedance is not demanding on the capabilities of an amplifier, so such headphones usually can't get as loud on lesser amplifiers (say, a poor DAP) - but they will run out of voltage (volume) before driving the amp into current clipping.
 
I'm not sure how much voltage is really needed - it depends on the headphones of course.  My uDAC gets plenty "loud" with my HD 600s, so +/- 2.5 V is enough for them.  Less efficient or higher impedance headphones are going to need more voltage than that.  Of course, how a given amplifier responds to the load of a headphone across the frequency range has an impact on the sound as well.
 
 
 
For low impedance, low sensitivity headphones, lots of current capability is needed - that's why the AKG K701 and HiFiMAN planar headphones need such a beefy amp.  They still need decent amounts of voltage, but because their impedances are so low they need a whole lot of current to maintain a given voltage.
 
Think of them as "hard to drive, (somewhat) hard to make loud" - they often make DAPs and other insufficient amps run out of current, causing clipping that prevents the amp from reaching its actual maximum voltage peaks (and can cause damage to the headphones).  Thus, they might reach a high maximum average volume, but peaks will end up mangled.
 
Lower impedance headphones also "interact" more with an amplifier since the load impedance isn't large in comparison to the output impedance of the amplifier.
 
So that's why lower impedance headphones are actually a more difficult load for an amplifier (again, that link explains more of it better than I could...).

 

[justin w.]

5V is enough for most headphones to go "loud", but if you are listening to very dynamic music such as from an orchestra, to be realistic there will be loud peaks well over 100dB.  You'd be surprised how many volts this requires for some headphones.  If you listen to compressed pop CDs where the range is only a few dB you won't have this problem.  Plenty of headroom is a good thing to have and this can require 30, 40 volts for some full-size headphones (to go really, really loud).
 
There is also the issue of amplification quality.  A portable amp outputting its max power is likely going to have significantly more distortion than a home amp utilizing 20% of its power for the same output level.
 
When researching amps, remember that sometimes the max output voltage will be listed in peak-to-peak or rms.  And is that the output voltage with no load?  a certain impedance load?  Are the numbers even correct?  Make sure to research this properly

 

[wuwhere]

Yup, those voltages are usually quoted at 1kHz with a resistive load not at 20Hz to 20kHz bandwidth. Total power has to be calculated across the entire audio spectrum.
 
Quote:

5V is enough for most headphones to go "loud", but if you are listening to very dynamic music such as from an orchestra, to be realistic there will be loud peaks well over 100dB.  You'd be surprised how many volts this requires for some headphones.  If you listen to compressed pop CDs where the range is only a few dB you won't have this problem.  Plenty of headroom is a good thing to have and this can require 30, 40 volts for some full-size headphones (to go really, really loud).
 
There is also the issue of amplification quality.  A portable amp outputting its max power is likely going to have significantly more distortion than a home amp utilizing 20% of its power for the same output level.
 
When researching amps, remember that sometimes the max output voltage will be listed in peak-to-peak or rms.  And is that the output voltage with no load?  a certain impedance load?  Are the numbers even correct?  Make sure to research this properly