[Brinston]

Hello guys,
 
I was just wondering what  really happens when you connect an external amp to the hedphone out of either your phone or DAP.
 
I understand how DAC's work. Considering a digital output fed to the DAC, It doesnt lead to any problems.
 
But what I am curious to know is that :
 
In general, behind the headphone out, there already exists a amplifier. So when we connect a external amplifier via this headphone jack, wont this lead to dual amping and lead to issues ?

 

[theveterans]

DACs have a line out that's normally high voltage (2 volts for a single-ended circuitry), but have very low current that headphones needed to be able to drive the transducers properly. The reason for the low current is to feed the amplifier which has a very high input impedance so that it doesn't need to draw a lot of current from the line out of a DAC. The amplifier provides the current thus power to drive headphones and speakers, plus many amps have a very low output impedance (2 ohms or less) so that the current can pass through with less impedance.
 
A DAC with a variable line out such as Chord Mojo have enough current to drive headphones so that it doesn't need a separate amplifier.

 

[Brinston]

  DACs have a line out that's normally high voltage (2 volts for a single-ended circuitry), but have very low current that headphones needed to be able to drive the transducers properly. The reason for the low current is to feed the amplifier which has a very high input impedance so that it doesn't need to draw a lot of current from the line out of a DAC. The amplifier provides the current thus power to drive headphones and speakers, plus many amps have a very low output impedance (2 ohms or less) so that the current can pass through with less impedance.
 
A DAC with a variable line out such as Chord Mojo have enough current to drive headphones so that it doesn't need a separate amplifier.

Nope, that is not what I meant.
 
Consider this setup. A Android device connected to a fiio A3 amp through the 3.5mm headphone jack.
Now the android already has a dac amp inside it. And the android isnt aware of the external amp connected to it. So the sound will be processed in the following stages :
 
Internal Android DAC -->  Internal Android Amp -->  External Amp ( Fiio A3 ).
 
Now wont this lead to dual amping is what I meant to ask.

 

[Music Alchemist]

If you are not using a line out connection, then yes, it is technically double amping, which may or may not result in an audible decrease in sound quality. However, some 3.5 mm outputs can function as line outs.

 

[Brinston]

  If you are not using a line out connection, then yes, it is technically double amping, which may or may not result in an audible decrease in sound quality. However, some 3.5 mm outputs can function as line outs.

Whoa, I dint know that.
 
Do there exist smartphones with line out, except for the likes of the pioneer and onkyo android dap's.
 
Also just curious, In what cases would sound quality change?

 

[Music Alchemist]

  Whoa, I dint know that.
 
Do there exist smartphones with line out, except for the likes of the pioneer and onkyo android dap's.
 
Also just curious, In what cases would sound quality change?

 
I'm not knowledgeable enough about the technical stuff to give you any definite answers. I say just experiment and see how each combination goes. I've discovered so many unexpected things by approaching audio equipment that way. Generally, though, connecting a DAC/amp via line out is the best way to isolate the variables. If you need to use a smartphone and are considering switching to a different one, I guess you should Google which ones have the best line outs for connecting to portable DAC/amps.

 

[Brinston]

   
I'm not knowledgeable enough about the technical stuff to give you any definite answers. I say just experiment and see how each combination goes. I've discovered so many unexpected things by approaching audio equipment that way.

In my home stereo setup, I had made the same mistake of dual amping which had tremendously decremented the audio quality. I could immediately tell the difference when the receiver was given a digital input later. So yes, experience tells me that there is difference in sound quality. 
 
So when I just came across the many different portable amps available for android smartphones, I was just curious to know if the same dual amping causes the sound quality to decrement when used with androids.

 

[Music Alchemist]

  In my home stereo setup, I had made the same mistake of dual amping which had tremendously decremented the audio quality. I could immediately tell the difference when the receiver was given a digital input later. So yes, experience tells me that there is difference in sound quality. 
 
So when I just came across the many different portable amps available for android smartphones, I was just curious to know if the same dual amping causes the sound quality to decrement when used with androids.

 
What I meant about experimenting was that sometimes it can audibly decrease the sound quality, and sometimes it's not audible.
 
I edited my post, by the way. It's best to just use a line out and connect to a DAC/amp.

 

[Brinston]

   
What I meant about experimenting was that sometimes it can audibly decrease the sound quality, and sometimes it's not audible.
 
I edited my post, by the way. It's best to just use a line out and connect to a DAC/amp.

Yes, that is exactly my opinion as well. 
 
Feeding digital signal to a DAC AMP is way to go.

 

[GRUMPYOLDGUY]

 
Also just curious, In what cases would sound quality change?

 
Any distortion present in the passband would be linearly gained up in the second amp stage. For example if a spur at -32dBV is present at the smartphone output, and the second stage gain is +10dB, that spur will be -22dBV at the output of the second stage. You're more likely to pick up noise in the analog path through the smartphone (which was not specifically designed to maximize isolation and signal integrity of the analog audio path) than you are through the preamp stage of an external DAC. Data going to the external DAC is digital, which of course is not going to pick up spurs (though a negligible increase in noise floor due to bit errors is theoretically possible), so you have good "isolation" exiting the smartphone. These external components are (hopefully) designed specifically to minimize signal integrity issues and so will generally produce a cleaner output.