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1) don't own usb sound card, so don't know. dac + amp gives more flexibility for upgrades, because it is almost inevitable that you will want to upgrade once you hear improvements in sound from better equipment. combined amp/dacs can be a good value.
2) they're inexpensive and the best bang for the buck for a dac. My udac/D10 dacs are MUCH better than my computer sound card/ipod and a tiny bit worse than my non-portable dacmagic. And the udac cost just $100. Don't use portables as amps for most full sized headphones tho (even a stereo jack will sound better).
3) for dacs & amps (particularly for amps), compatibility with particular headphones, relative performance for $$.
Try the Apogee Mini DAC, or the Duet.
It's not useful to discuss something based on its form factor, a sound card is just a DAC without a case and on a PCI or PCI-E interface and sometimes with a chipamp inside.
Plus, where is it that everyone is suggesting portables for home use? I wouldn't do that for sure.
The people recommending portable DACs are most of the time people who have never owned anything else and are just excited at the improvements they've gained from the thing they bought after reading one of the reviews by a couple of people on here.
Basically though, a DAC is more than just digital to analogue conversion, it has a power supply, the quality of which is critical to the performance of the DAC, and the "output stage" which is some or other degree of amplification between the DA chip and whatever the DAC is connected to. If a DAC is combined with, say, a headphone amplifier, there is usually some degree of compromise which works against each component, mainly the shared power supply. This is why, as you go higher up the latter of quality, things become more separate, to the point that even the power supply for an amp or DAC is in a separate box. A computer environment, where sound cards reside, is, not surprisingly, the worst choice, as many electrically noisy components are sharing a power supply designed for purposes different to quality audio reproduction.
I hope that helps.
The damn search function, that's what.
I've been around here for years and I can't answer this one, your observations are correct and unless you need the portability stay away from them.
I keep hearing the same old stories about soundcards being prone to noise etc etc etc,what utter rubbish.
I have an RME Digi PST that is 10 years old has lived inside 5 different pc's and has never suffered any type of noise etc etc etc problems at all.
Seperate dac's amps etc are not the be all to end all that they are touted as.
Never ever overlook a GOOD audio card just on hearsay.
I'm thinking that it has to do something with the pricing of said amps. Portable amps are usually cheaper. Because of this, they have a much larger population of owners to support the use of them. More people to recommend them and the population spreads more. Just a thought.
Amps don't improve SQ, they amplify the signal they're given. So if that signal is poor, you'll amplify the flaws and make them more noticeable. Some headphones require a lot of power to sound the way they were designed too, and that requires a decent full size desktop amp. Those that require a little power will run fine from portable amps. Power is the main purpose of amping. Added volume is a side effect.
A sound card is a better value option if you never intend portable use and/or using power hungry phones. Although bandwidth has been shredding the ozone with people running such phones 'brilliantly' from sound cards with built in amps, who argue others are amp snobs, when they haven't actually heard those phones amped properly.
External DAC/Amps will cost more than equivalent quality soundcards because they have casings, and portable convenience.
I'm using a Xonar ST soundcard atm and I'm very satisfied about the SQ it produces.
Not to get into semantics, but in that sense full-sided amps do make some headphones sound better than they do with portable amps (because they sound as they were designed as you put it). So I'll go on a limb and say that the net effect of full-sized amps is to make some headphones produce better quality sound than portable amps at the same volume. The biggest effect is in the bass, which requires the greatest force to swing the speaker membrane. The higher electrical current creates stronger magnetic force, which is capable of making faster changes (of the first derivative) of motion of the speaker, reducing lag in the initiation of movement and producing tighter, stronger bass.
Range and timing are two factors, I understand.
I had a Xonar STX and wasn't unsatisfied, though it sounded a bit harsh. Bought an amp (M-Stage Matrix) and it still sounded harsh. Bought an old DAC (Arcam Delta) and it suddenly sounded a lot better.
All with K701, HD 580...