[Llloyd]

   
What if I wanted to drive a headphone like HE-6? Would the O2 be up the to task? If not, what's a solid and reasonably priced amp that will?

 
It would not be up to the task. I don't know of any low priced amplifiers that are considered good enough to make the HE-6 do really well, probably a speaker amp or a vintage receiver, hooking the HE-6 up to the taps.  I also don't have experience with the headphone so I can't say. I know a lot of people run the HE-6 off of speaker taps though. I was simply stating there's a difference between power and volume, and the quality of the amplifier and hte power ends up being really important, compared to just being able to make the headphone loud enough to listen to it.

 

[stv014]

Power and volume are not unrelated. The maximum power the amplifier can output into that particular load, together with the efficiency of the headphones, determines the maximum peak SPL that is possible without audible distortion. This is simple physics, for every 10 dB increase of SPL, you need 10 times more power; at some point, the amplifier will no longer be able to supply the required power, and it will start to clip (or, if there is no excess gain, you just reach 100% volume, which will still not be clipped, but simply not loud enough). I have yet to see any evidence that significant amount of extra power that is never used improves sound quality.
 
In short, there is no universal hard limit on how much power is not enough. It always depends on the dynamic range of the music, and personal preferences (of course, snobs will never admit that they are listening loud). One person listening to loud classical music may need 100 times more power than another listening to dynamically compressed pop music at moderate loudness.

 

[DefQon]

Almost any low-end headphone amp can drive the HE-6, whether or not you can get it loud enough to meet your needs or enough current (+synergy) to make them sound there best is another story.

 

[jaddie]

  Almost any low-end headphone amp can drive the HE-6, whether or not you can get it loud enough to meet your needs or enough current (+synergy) to make them sound there best is another story.

Here's the problem with the HE-6.  It's not a hard load for an amp to drive, it's 50 ohms.  But it's sensitivity is 83dB @ 1mw.  1mW into 50 ohms is just over .2 volts.  The O2 has a maximum output voltage of 4.5 volts, which means it clips 27dB above 83dB, about 110dB.  That sounds like it would be pretty loud, but that's the maximum/peak it can do. Average levels are between 10 and 20dB below that depending on the music and how loud it was mastered.  That puts our average at 90dB to 100dB, which is loud, but not "damage your hearing" loud.  
 
By comparison, the EF-6 amp will do 5 watts into 50 ohms, which is just shy of 15 volts max.  Skipping the math, that puts the maximum at 120dB, averages between 100 and 110, which is enough to satisfy anyone and permanently hurt a few who overdo it. 
 
So the O2 will drive the HE-6 to loud and safe levels, but it won't satisfy the loud-freak, and users will have to get used to turning the volume up all the way, which can be psychologically disappointing.  Classical music fans may not care for the result.
 
The 02 has higher than average maximum output voltage, and that means that "almost any low-end" amp is going to have a bit of trouble.

 

[xnor]

  I have yet to see any evidence that significant amount of extra power that is never used improves sound quality.

Exactly, and we have evidence for the opposite.
 
High gain increases noise and distortion. It reduces usable volume control range, often resulting in channel imbalance. With DC-coupled amps it can cause high DC offset potentially damaging the headphone drivers or at least degrading their performance.
...
 
edit: This amp is actually a good example for that:

  By comparison, the EF-6 amp will do 5 watts into 50 ohms, which is just shy of 15 volts max.

I think this is shocking: Sound&Vision measured an A-weighted signal to noise ratio of 60 dB. They could not reproduce the 5 watts output power, that is btw specified at 10% THD. Output impedance is 23 ohms.
Another measurement (dunno, but guess unloaded) by German Audio shows that the third harmonic rises strongly and lines up with the 2nd harmonic at 1V of output voltage.
 
 
 
As I use to say, if more power were always better we'd all be driving our headphones with 1000W mono blocks.

 

[jaddie]

  As I use to say, otherwise we'd all be driving our headphones with 1000W mono blocks

Anecdote:
 
In another "life", I was Chief Engineer at a radio station that had 120W power amps installed for headphone amps.  I didn't even do the math on that one, but did turn the gains down (they were Crown pro amps) so the announcers couldn't hurt themselves, and my facility could actually meet OSHA standards. 
 
You know what? They complained!  So I tried it back up loud where they wanted it, and stood next to one of the jocks when they were on the air, mic on, speakers muted, closed full-ear headphones (sorry, don't recall what they were, Sony something or others).  There was so much sound coming out of their headphones I could hear everything perfectly well, and they would get feedback into the announce mic.  
 
I turned the amps back down, told them it was a legal issue.  When I rebuilt the studios I installed real headphone amps that topped out a a couple of watts. 

 

[xnor]

Are we talking about one such amp per headphone or headphone distribution amps?
 
Anyway, 120W sounds like you have to turn the volume all the way down and hardly touch it... When you reduced the gain, did they use the full volume control range? Maybe the cans were high ohm (2k) ones requiring lots of voltage?

 

[jaddie]

  Are we talking about one such amp per headphone or headphone distribution amps?

One 120W stereo amp per headphone.  The amps were driven from a volume-controlled headphone output of the console, each user had his own amp and volume control. 
 
Quote:

  Anyway, 120W sounds like you have to turn the volume all the way down and hardly touch it... When you reduced the gain, did they use the full volume control range? 

The controls I pre-set were front panel gain controls on a unit like this:
 
Speaker outputs wired to the headphone jacks directly.  Yikes.  It wasn't me, I didn't do it!
 
The controls were set to the user's volume control could go full up and be dangerously loud, but the amp controls were turned down quite a ways.  Sorry, that one's out of my head.  I just know to turn it all down and make it right I had to build fixed pads to put in front of the amp, something like 15-20dB.  
 

  Maybe the cans were high ohm (2k) ones requiring lots of voltage?

Well, nothing requires THAT much voltage

 !!  They were some kind of Sony headphones, late 1990s, about $30/set, not high Z at all, probably like 20-30 ohms, would have worked in a late 90s Walman.  They would destroy headphones, no point in getting anything good, they wouldn't last any longer.  All mechanical failures like broken earpiece yokes, internal wiring breakage (not worth my time to fix) etc., no blown drivers though  Amazingly.  We had one announcer with a set of Sennheiser HD-414, I got her those bright yellow pads, and she loved them.  Didn't run her cans hot at all because open design cans would be an instant feedback problem into an announce mic.  
 
By comparison, at another station I built in the early 1980s (classical), we built our own headphone amps, doubt they even did 1 watt, but they fit in a little metal box just below the desk top. The headphones were Yamaha YH-1 and YH-2 (I'm actually surprised I still remember that!), we had a bunch of them, even for remote recording.  They lasted forever, nobody cranked them loud, and they sounded pretty nice.  We had a set of early Stax Lambda in the music recording studio.   I owned a set of YHD-2 back then, they sounded great, and folded flat in my brief case.

 

[xnor]

  The controls were set to the user's volume control could go full up and be dangerously loud, but the amp controls were turned down quite a ways.  Sorry, that one's out of my head.  I just know to turn it all down and make it right I had to build fixed pads to put in front of the amp, something like 15-20dB.

So the amp effectively acted more like a buffer I guess? Turning the volume control down plus a pad should actually reduce gain to far less than unity.

 

[jaddie]

  So the amp effectively acted more like a buffer I guess? Turning the volume control down plus a pad should actually reduce gain to far less than unity.

The pad was so I could put the gain controls on the power amp in a more or less normal position and actually adjust to match channels.  The problem with just doing it all with the controls was no real ability to gain match the channels with the controls near their minimum settings.  Hence the pads.  The voltage gain was probably less than unity through the system, but since the headphone output of the console was a line out, not capable of safely driving 30 ohms, there was a power gain of greater than unity involved.  And lots of headroom.  I'm not saying it made sense, that's why I removed that mess and did something proper.  The entire thing was quite dangerous.  The line output would clip at about 8.5Vrms unloaded, the power amp clipped at something like 25Vrms.  With average line out levels at about 1.4 volts, a 20dB pad would have put the controls in the right general range for really loud headphone levels.  And the headphones needed probably a volt to be loud.  Even with all levels adjusted, there was a potential for hearing damage due to errors, failures, etc.  

 

[Skavoovie]

  Here's the problem with the HE-6.  It's not a hard load for an amp to drive, it's 50 ohms.  But it's sensitivity is 83dB @ 1mw.

For what it's worth, based on Tyll's measurements, HE-6 sensitivity (at least his measured example) seems to be more in the 77dB/mW range.

 

[jaddie]

  For what it's worth, based on Tyll's measurements, HE-6 sensitivity (at least his measured example) seems to be more in the 77dB/mW range.

 
Ah.  More's the pity. Sure hope they're worth the trouble.