[proton007]

 
Quote:

proton: wut? 
 
Higher impedance = higher volts; lower current. 
Lower impedance = lower volts; higher current. 
 
There's a reason 'stats are "current-less," and that orthos are "current hungry."
 
You cannot arbitrarily have "lots of" or even "extra" current, it's dictated by R and V (I = V/R). You get to pick V, the headphones pick R, and I is dependent. And then power = I*V. If you only need X watts based on sensitivity (which is dictated by the headphones), you will only use as much current as Ohm's Law dictates you will. If the amplifier can deliver >X then it will not go into current-limiting, clipping, etc. If it can deliver <X it will have a bad day. For many headphones, X is a very small value relative to what many amplifiers can do. 
 
I feel like I'm missing your point though...
 
 

 
Thanks for the question...it got me thinking, see if this makes sense:
The impedance is fixed by the headphone, i.e. R. (although that also varies acc. to frequency, but for the sake of explanation, assume it to be fixed).
Power and loudness are related in a subjective manner, but the generally accepted rule is that 2x increase in loudness requires 10x increase in Power, and causes 10dB worth of change.
Hence, by P = V^2 / R, for the same R, we need ~3x the Voltage.
However there is also the efficiency of the speaker, so this might be >3.
 
Now P = VI. If P of an amp is fixed, to increase V, it needs to increase I.
Hmmm... So I guess both the V and I can be a limiting factor for an amp. Increasing V beyond the limit will cause the signal to clip, and 'I' will cause the amp to hit the current limit.
 
Going by this logic, a speaker with high R will need higher I to have this corresponding increase in V.
The circuitry's performance is limited by the V and I, whichever one is a limiting factor.
For most modern designs, I guess its the former, and is reached earlier than the latter.
 
 
 

 

[proton007]

Which brings me back to the main point. As long as there is enough Power in the source, your speaker can be driven loud enough.
How to determine if you need an amp? Since power specs are not available in a lot of cases, I think a basic way to test is to set the default earbuds (that came with your device) to a comfortable level.
Now plug in your new headphones. Based on the sensitivity and impedance, you may need to adjust the volume slightly. As long as you find the loudness variation is suitably within the volume setting limits of your device, there's no need for an amp.

 

[obobskivich]

Not really. 
 
Speakers with a high Z will "see" more voltage than current (you can see this via measurements or playing with the math). Power is not "fixed" - V is variable, and I comes along based on what the load demands - which makes power output change (VA apparent).
 
You can't correlate acoustic SPL to figure voltage either. 
 
As an example:
 
Something like the Koss ESP/10, which have an impedance >10k (I forget the exact value; been a while since I measured them) will need almost no current (cheap meters may not even see it), but V is VERY high (figure 100V-ish for 90 dB). 
 
Compare this to something like the HE-6, which has a ~40 ohm impedance, and wants some 20 mW for a reasonably loud output; but that's only 1V input. That's much higher current.
 
A less dramatic example would be looking at the Beyer 600ohms vs the 32 ohms versions of the same headphones (like the DT880). They want roughly the same power, but the 32ohms version wants more current. 
 
Speakers are the same way - for example some Logans swing down to around .7 ohms and demand more current than many amplifiers can deliver. However most amps can provide enough volts to drive them, and that's where you get current limiting and (if the amp is well done) a protection shut down (if the amp is poorly done, probably smoke).  
 
I agree with your second post about picking an amp more or less.  Newer amp designs though seem to have specs published quite readily, which is a nice new change. 
Quote:

Going by this logic, a speaker with high R will need higher I to have this corresponding increase in V.
The circuitry's performance is limited by the V and I, whichever one is a limiting factor.
For most modern designs, I guess its the former, and is reached earlier than the latter.
 
 
 

 
 

 

[proton007]

Afaik most amps do have a max amount of power they can supply, right? Especially battery powered amps.
So even if I go with your statement, I cannot reach to a definite conclusion because the sensitivity also comes into play. So to hit a fixed Peak SPL, the Voltage/Current draw will depend on the SPL/mW or SPL/Vrms of the speaker.
I guess the ultimate factor is the sensitivity then.

 

[obobskivich]

It's not a static value - they increase power output and THD as you go up. Generally the specs you find are THD at 1% or less (where it's inaudible), but a lot of bench-measures will go to 10% THD or something like that (onset clipping) - those outputs are usually higher. Then there's internal limiting with some amplifiers (usually speaker amps though, not headphone amps) because thee PSU can't handle total peak output across all output devices; it may be able to deliver one third or one half of those devices, but not all at once (that's a very unrealistic scenario though). 
 
So yeah, there is a max power out, but it isn't static. In other words, a 50W amplifier isn't always putting out 50W. It's capable of putting 50W into a spec'd R. Power is a dependent variable though; the volume control is variable V. So to make an example the amplifier might be able to drive 20V out (which isn't uncommon for speaker amps), but current is dependent - the impedance will dictate that. So if this amp is able to push 20V out, and spec'd for say, 8 ohms, you hook up a 1 ohm speaker, it's suddenly being asked for TONS of current (from it's view), but it can still provide  that 20V -> current limiting. Contrast this to big-boy amplifiers (Accuphase makes a great example) - they'll hand out 20V into anything from around .5 ohm up to whatever (probably not into multi-k unless there's a transformer). VA apparent (power) is doubling as impedance halves, and this is why lower Z is considered to give the "false sense" of doubling sensitivity. 
 
With headphone amps it's a lot less off an issue, because it's a realm where 1mW is huge. It's also not hard to run full Class A (mondo-inefficient) and have a power supply behind it that doesn't limit; figure 1mW out or even 100mW out and you still only need a few watts from the wall. But that's basically the theory. 
Quote:

Afaik most amps do have a max amount of power they can supply, right? Especially battery powered amps.
So even if I go with your statement, I cannot reach to a definite conclusion because the sensitivity also comes into play. So to hit a fixed Peak SPL, the Voltage/Current draw will depend on the SPL/mW or SPL/Vrms of the speaker.
I guess the ultimate factor is the sensitivity then.

 
 

 

[proton007]

Got it. Glad to clear things up, and thanks for the info!

 

[phantasmor]

 
Quote:

Which brings me back to the main point. As long as there is enough Power in the source, your speaker can be driven loud enough.
How to determine if you need an amp? Since power specs are not available in a lot of cases, I think a basic way to test is to set the default earbuds (that came with your device) to a comfortable level.
Now plug in your new headphones. Based on the sensitivity and impedance, you may need to adjust the volume slightly. As long as you find the loudness variation is suitably within the volume setting limits of your device, there's no need for an amp.

 
Wait.... there are headphones that do not sound loud enough through normal volume settings? I am interested. I want to know what headphones would not sound loud enough and why. The main question being why someone would make a pair of headphones that do not sound loud enough at normal volume settings.

 

[proton007]

 
Quote:

 
 
Wait.... there are headphones that do not sound loud enough through normal volume settings? I am interested. I want to know what headphones would not sound loud enough and why. The main question being why someone would make a pair of headphones that do not sound loud enough at normal volume settings.

 
The discussion in the previous posts explains that. Some sources cannot supply the power required, for example portable devices. And a lot of headphones are designed to work with higher power equipment.

 

[Eric_C]

 
 

 
 
Wait.... there are headphones that do not sound loud enough through normal volume settings? I am interested. I want to know what headphones would not sound loud enough and why. The main question being why someone would make a pair of headphones that do not sound loud enough at normal volume settings.

 
It's not so much that they're made with the purpose of not sounding "loud enough". IIRC pro headphones were manufactured to a 120 ohm standard, at one point--headphones like that just won't get as lou as say, stock earphones on an iPod. Portable devices like an iPod will have more current, therefore less voltage, and therefore be more suited to headphones with lower impedance ratings.
 
"Normal" volume is a relative term. Mixer boards get louder faster than iPods, but they're just built for different purposes.

 

[phantasmor]

 
Quote:

 
 
The discussion in the previous posts explains that. Some sources cannot supply the power required, for example portable devices. And a lot of headphones are designed to work with higher power equipment.

 
 
Quote:

 
 
 
It's not so much that they're made with the purpose of not sounding "loud enough". IIRC pro headphones were manufactured to a 120 ohm standard, at one point--headphones like that just won't get as lou as say, stock earphones on an iPod. Portable devices like an iPod will have more current, therefore less voltage, and therefore be more suited to headphones with lower impedance ratings.
 
"Normal" volume is a relative term. Mixer boards get louder faster than iPods, but they're just built for different purposes.

 
Interesting! I have to say I was wondering about the whole amp thing. Thank you!

 

[obobskivich]

The 120 ohm standard is an IEC spec for Zout. Because of portable devices and their limitations, it doesn't make a lot of sense to adhere to it anymore.  http://en.wikipedia.org/wiki/Headphone_amplifier#Output_Impedance
 
Generally speaking I agree with the IEC, but see my qualification in the first line; due to changes in output devices and their abilities, we've had to change what headphones do as well, and some of them don't play as nice with higher Zout (just review the impedance) due to non-linear attenuation. This is very common with IEMs, but consider what they're designed to plug into - portables that run Zout down around 0. 
 
 
Regarding the "not loud enough" question - we're talking about gain structure between amplifiers and sensitivity of speakers; there is no absolute and there is no right answer. The position of the volume knob does not relate to the output, unless you have to run it "full open" (zero attenuation) and don't have enough output (and that's an insufficient amp but that's rare with many headphones). 
http://audiokarma.org/forums/showthread.php?t=58829
Quote:

 
 
 
It's not so much that they're made with the purpose of not sounding "loud enough". IIRC pro headphones were manufactured to a 120 ohm standard, at one point--headphones like that just won't get as lou as say, stock earphones on an iPod. Portable devices like an iPod will have more current, therefore less voltage, and therefore be more suited to headphones with lower impedance ratings.
 
"Normal" volume is a relative term. Mixer boards get louder faster than iPods, but they're just built for different purposes.