[ccbass]

I've been attempting over the past few weeks to jump into the high quality audio world.  I'm a bit lost on how things work, why components are used or not used, etc.  Hoping some of my science-y questions can be answered.
 
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What exactly is volume?
From what I can tell, it is how much the amplitude of a signal is changed.  Now is that "boosting" of the signal measured in volts or amps or what?  I head that for a +3dB change, it requires twice as much power as before?  Lets say an amp is rated for 500W RMS.  Does this mean that as you turn the volume knob up that the amount of Voltage being outputted is becoming greater?  Or is it the amps that changes?
 
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What is the difference between voltage and amperage in terms of amps and headphones and how does it relate to impedance?  I realize that voltage is the "pressure," amps is the "flow," and ohms is resistance.  Now before you jump down my throat about this, I know the basics about them:
R=V/I   or   I= V/R   or   V=R/I
W = I x V
Now how does this relate to "drawing current" or amps?  I see the term, "drawing current" a lot around the forum.  Lets say an amp is outputting 1 volt at 10ohms and it connected to two different headphones.  If one headphone has an impedance of 50ohms and the other 100ohms, would the amount of current being drawn in the 100ohm headphone be 1/2 of that of the other headphone.  Assume the sensitivity and Efficiency of the headphones is the same. 
Now how does all of this relate to output power?
 
What values change and do not change in an amp? [ie. volts, amps, resistance]
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What outputs are typically amp'ed?  It seems that normal 3.5mm are most common.  Is that why when iPods are used as the DAP people bypass the internal amp by connecting through the dock connector?
 
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It seems as though many "sensitive" IEMS still need to to amped.  What is the reason for this?  If they have such a high sensitivity, why do they need to be amped?  Impedance? 
 
http://www.head-fi.org/forum/thread/17179/please-explain-headphone-sensitivity
 
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What is a channel?  Does this just mean left vs right, etc?
 
 
I feel like I need to be a physicist to understand all of this.  I love learning all of this kind of stuff, but I've hit a wall.

 

[xnor]

Let's take a headphone with a sensitivity of 100 dB at 1mW and impedance of 50 Ω.
P = 0.001 W
R = 50 Ω
 
P = V*V/R
-> V = 0.2236 V
 
+6 dB corresponds to approx. twice the voltage, so for 106 dB you'd need 0.447 V.
Of course the amp also needs to provide I=V/50 amperes.
 
That makes about 4mW which is 1mW doubled (+3 dB) and then doubled again (+3 dB).
 
With a 100 Ω headphones the current drawn would be 1/2, power also would be 1/2 (-> 103 dB).
 
The volume control of your output device controls the output voltage.
 
 
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You should be able to combine this pieces of information and figure out the answers to a lot of questions yourself.

 

[Ypoknons]

My understanding is that outputs are amplified to some degree or other, even so called line-outs have something to bring the signal to line level, so the differences are a matter of degree. 

 

[xnor]

About headphone impedance:
As you've seen, low impedance means high current and high impedance means high voltage - both is limited on any device.
 
 
Quote:

Higher impedances, such as 600 ohms, are more useful in studio installations where many units may be wired in parallel for studio monitoring applications.

 
On the other hand, if you connect two 16 ohm headphones in parallel the amp will see a 8 ohm load -> ouch.
 
 
edit: Headphone impedance changes with frequency and amps also have a source impedance. related thread
 

 

[ccbass]

Quote:

Let's take a headphone with a sensitivity of 100 dB at 1mW and impedance of 50 Ω.
P = 0.001 W
R = 50 Ω
 
P = V*V/R
-> V = 0.2236 V
 
+6 dB corresponds to approx. twice the voltage, so for 106 dB you'd need 0.447 V.
Of course the amp also needs to provide I=V/50 amperes.
 
That makes about 4mW which is 1mW doubled (+3 dB) and then doubled again (+3 dB).
 
With a 100 Ω headphones the current drawn would be 1/2, power also would be 1/2 (-> 103 dB).
 
The volume control of your output device controls the output voltage.
 
 
--
You should be able to combine this pieces of information and figure out the answers to a lot of questions yourself.

 
Now how does all of this relate to the output power of the amp? 
 
And how do you determine if an amp can provide enough current to the phones?
 
And it seems like many manufacturers are mixing up sensitivity and efficiency.  How can you calculate between dB/mW and dB/V?

 

[xnor]

Quote:

Now how does all of this relate to the output power of the amp? 
 
And how do you determine if an amp can provide enough current to the phones?
 
And it seems like many manufacturers are mixing up sensitivity and efficiency.  How can you calculate between dB/mW and dB/V?

 
1) Well, lets say an amp can output max. 100 mW (before clipping) into a 50 ohm load, or 50mW into 100 ohms, or ... you can use that information to do calculations I've shown above to get the max. volume.
 
2) See above.
 
3) Ratings given as x SPL at a given input are sensitivity ratings. Input often is 1mW, but could also be 1V (some manufacturers use whichever results in higher SPL numbers, heh).
Easiest way would be to double the 0.2236 V from above over and over again until you're close to 1V. That's 2 times, or roughly +12 dB from the 1mW rating.
 
Or use the formula
    from Wikipedia.

 

[ccbass]

I have 1/4" TRS inputs, L/R, on my AV40's.  What balanced outputs can be used with these?  I haven't seen any DAC's with these outputs. 
 

 

[xnor]

emu0404 usb for example

 

[ccbass]

Can RCA be balanced?

 

[Skylab]

They are not in their normal configuration, no.

 

[ccbass]

Quote:

They are not in their normal configuration, no.

What allows them to be balanced?  Is it just the DAC that outputs balanced RCA or do you need something special?

 

[ccbass]

Quote:

About headphone impedance:
As you've seen, low impedance means high current and high impedance means high voltage - both is limited on any device.
 
 
 
On the other hand, if you connect two 16 ohm headphones in parallel the amp will see a 8 ohm load -> ouch.
 
 
edit: Headphone impedance changes with frequency and amps also have a source impedance. related thread
 

 
Why is it ouch for the amp to see an 8 ohm load?  Because it needs to supply a heck of a lot of current because of the low resistance?
 

 

 

[Ypoknons]

See dampening factor.

 

[xnor]

Quote:

Why is it ouch for the amp to see an 8 ohm load?  Because it needs to supply a heck of a lot of current because of the low resistance?

Yes, I'd say that many headphone devices would struggle with such a load, because they're not designed for something like that. 16 or  >30 ohms would be "safer" to use. Some headphone amps/devices specify the min. impedance of the load or the range (for example: 30 - 300 ohms), take a look at some spec sheets. Dampening factor would be another issue, there's a thread about it in this subforum.
 
If we're talking about loudspeakers 8 ohms is nothing unusual. The impedance of some speakers is as low as 2 ohms! But same as above applies here: don't connect stuff to an amp which it wasn't designed for. 

 

[ccbass]

When dealing with multiple devices, how do you deal with all the different volumes.
 
Lets say I have:
 
macbook pro  to usb dac to speakers. 
 
All three have volume controls, what do I use to change the volume?