[SilverEars]

What is the difference?  If you use an amp to headphone out, is it double amping?  What is the consequences of double amping?

 

[mikeaj]

Headphones out implies passing through some electronics capable of enough output power and handling low enough impedance to be in the reasonable range for headphones. Think of it as yet another stage in the electronics. You can call it double amping, but it's usually not such a big deal.
 
Usually it would be overkill in terms of output specs to be connected to the input of yet another amp and you could expect the extra distortion, frequency response deviation, etc. to be measurable but insignificant. I guess the thing to look out for is if the amp has high gain and you need to turn the volume down elsewhere; you could be bringing up the level of the noise floor in a potentially audible way in some setups. If you're not hearing background hiss, don't worry about it.

 

[SilverEars]

Well when I measured a RF amp way back, when I increase the gain of the amp, it would intruduce harmonic distortions(or was it when I increase the power of the signal coming in?).  Would the headphones out act the same if I have the volume level at a high level?  Would it introduce increase in THD?  I thought headphone out was an attenuator?  
 
I've heard of line-level being as high as 2V or less.  Does this mean there is an amp before headphone out for headphones of low sensitivity?  I hear headphone volume control as attenuator so I'm confused.  

 
 
What components are actually there for it to be called headphone out?
 
What does the signal have to be to called line level?
 
If the headphone out introduce distortion at high volumes, would it be necessary to reduce to a certain volume if you are amping from the headphone out? 
 
Also why do some setup use pre-amp?

 

[mikeaj]

Virtually all headphone amps should increase the level (not be attenuating) on max volume. The volume control itself (the knob) is usually a potentiometer or similar that at most positions attenuates the level, yees.
 
There are various line level standards, but these are rarely followed. In fact, in audiophile gear, over 2V rms is common.
 
You could call pretty much anything a headphone out. It's kind of arbitrary, hence the vague first statement of the previous reply. There are just various levels of performance.
 
Something line level is pretty much whatever you want to call line level. As mentioned earlier, people aren't sticking to the (multiple) standards of what voltage level that should mean.
 
Headphone amps can sometimes introduce non-insignificant distortion at high volumes with certain loads. Sometimes the electronics aren't capable of enough power, run out of current, etc. If you are connecting to another amp, power consumption is much lower than with actual headphones (2V into 50 ohms headphones is 80 mW, while 2V into 10000 ohms input of another amplifier is 0.4 mW and is relatively a joke) and generally there shouldn't be any significant distortion at any volume unless the headphone amp designer intentionally set the levels too hot for whatever reason. Note that if you're outputting some high level into an amp, it could possibly not handle that and clip the input.
 
You might want a pre-amp if dealing with multiple input sources with different levels, if the source's output level is say unusually low, if the amp has some weird input requirements, or the amp doesn't have a volume control itself or you don't want to use that volume control, etc. Usually it's more common on speakers setups for the above reasons.

 

[SilverEars]

Quote:

  Virtually all headphone amps should increase the level (not be attenuating) on max volume. The volume control itself (the knob) is usually a potentiometer or similar that at most positions attenuates the level, yees.
 
There are various line level standards, but these are rarely followed. In fact, in audiophile gear, over 2V rms is common.
 
You could call pretty much anything a headphone out. It's kind of arbitrary, hence the vague first statement of the previous reply. There are just various levels of performance.
 
Something line level is pretty much whatever you want to call line level. As mentioned earlier, people aren't sticking to the (multiple) standards of what voltage level that should mean.
 
Headphone amps can sometimes introduce non-insignificant distortion at high volumes with certain loads. Sometimes the electronics aren't capable of enough power, run out of current, etc. If you are connecting to another amp, power consumption is much lower than with actual headphones (2V into 50 ohms headphones is 80 mW, while 2V into 10000 ohms input of another amplifier is 0.4 mW and is relatively a joke) and generally there shouldn't be any significant distortion at any volume unless the headphone amp designer intentionally set the levels too hot for whatever reason. Note that if you're outputting some high level into an amp, it could possibly not handle that and clip the input.
 
You might want a pre-amp if dealing with multiple input sources with different levels, if the source's output level is say unusually low, if the amp has some weird input requirements, or the amp doesn't have a volume control itself or you don't want to use that volume control, etc. Usually it's more common on speakers setups for the above reasons.

Thank you for your responses.  I'm guessing that the attenuator is a pot, and it changes the gain of the amp before it.  Gain can be greater than unity or less.  And headphone have this gain adjusting stage(maybe this is what sets headphone out apart from line-out).  How would this aspect adversely affect the signal if amping after headphone out?
 
What takes it to be an output to be called line-out?
 
Why is line-out favored when using a dedicated amp?  Why would it be chosen over headphone out?  

 

[mikeaj]

Nah, pot would usually be part of some kind of voltage divider I'd think, not actually changing the gain itself.
 
Gain is almost always unity or greater (prior to any attenuation from volume controls). Overall through a device that means that you could be decreasing rather than increasing the signal level, yes. Other times you may be increasing the signal level.
 
Often a plain line out isn't affected by or doesn't have a volume control, but again, there's no strong distinction. Sometimes an output is labeled as both a line output and headphone output.
 
Line out is favored when connecting to another amplifier because you don't want to pass the signal through the additional electronics of another headphone amp if you don't need to because of the very small negative impact on fidelity, as described earlier. Again, none of the electronics are perfect and if you're running through extra stages you could be adding some slight extra noise or whatever else. Usually it's not a big deal.