The better amp?

joser, god knows. With any op amp it depends on the application. Most audio aficionados are going to say discrete components are always best. All I can tell you is that you will have to listen to examples and decide on your own.   

sorry, what means discrete in amps? thanks

Ok, I already read it, but according to what I read in the description of Texas Instruments for the TPA6120A2 is this: 

In applications requiring a high-power output, very high fidelity headphone amplifier, the TPA6120A2 replaces a costly discrete design and allows music, not the amplifier, to be heard. The TPA6120A2’s current-feedback AB amplifier architecture delivers high bandwidth, extremely low noise, and up to 128dB of dynamic range.

Three key features make current-feedback amplifiers outstanding for audio. The first feature is the high slew rate that prevents odd order distortion anomalies. The second feature is current-on-demand at the output that enables the amplifier to respond quickly and linearly when necessary without risk of output distortion. When large amounts of output power are suddenly needed, the amplifier can respond extremely quickly without raising the noise floor of the system and degrading the signal-to-noise ratio. The third feature is the gain-independent frequency response that allows the full bandwidth of the amplifier to be used over a wide range of gain settings.

joser - TPA6120A2 is a monolothic "chip" type op amp headphone driver.  Like mijostyn said, discrete analog stages will almost always sound better than monotholic op amps.  Also, Roland is driven by a wal-wart switching power supply, which is never as good as linear power supply (like in Emotiva). I have said this in your other thread.

@auxinput, thanks, but I just want to know between those 2 amps and the Roland uses USB power, is a audio interface via USB.

@auxinput: Which of the specifications of both amps do you see best for audio quality? I wrote the link for each one of the Texas Instruments page.thanks.

@joser9616 
Problem is,  there is not straight answer. I have several different op amps at home now, including the 21#4.  I've listened to the 6120A2 driving headphones with good results,  but I've never used it.  The 2134 is a versatile op amp,  it works and sounds good in many circuits.  
Depending you what you are using it for either one might be better than the other.  You need a host of spec's of what it is being used with to get a decent handle on paper to give you an answer and in some cases,  it could end up being only your taste that tells which is better. In most cases the manufacturers do a good job of selecting op amps.  If you want to roll op amps,  I would look at which op amp is in the unit and look for op amps with similar input impedences making sure minimum and maximum voltage capabilities are within the operating limits needed. There are many other things to consider,  signal to noise, gain, fet/ bipolar etc. 
There isn't a straight forward answer,  You are going to have a hard time to get an answer without posting a lot more info and even then.  
If you really have no idea of what or why,  I suggest to leave it alone or be prepared to purchase several to try.  Good luck,  Tim 

USB contains 2 wires for digital signal and 2 wires for "USB Power".  This is just 5V DC and it's driven from the computer/laptop (which is basically the same or worse than a wal-wart switching power supply). 

Also agree completley with timlub.  You can just compare op amp specs and expect it to tell you exactly how good a DAC/preamp is.

In comparison, the Emotiva DAC has extremely high current power supply that has a +12V and -12V lines to the op amp / audio circuits.  This is much better than another DAC that uses switching wal-wart or only a 5V USB power supply.

@auxin @timlub, thank you guys for the info. I'm just learning here... 

I made a comparison between the Emotiva XDA-1 and the Roland Mobile UA and I am very serious about it, the Mobile UA is much better than the Emotiva, a lot more detail, more clear ... that interface can be connected to studio monitors In a balanced way, it is exactly the same as connecting it to my two Crown D-75 amps, the active studio monitors(active), as you should know, have their own amp, I have the amps alone with speakers connected.