[CharlesC]

Hi.  According to Wikipedia the max power that usb can provide a device is 5V, 0.9 amps.  Does this limitation only present an engineering challenge for DAC/Amp designers or is there a definite, line-in-the-sand practical limitation on the ability of a USB powered device to power hard-to-drive headphones? 

 

[mikeaj]

First of all, the limit back in USB 2.0 days was 0.5 A (at 5 V). I think about everybody can expect a USB host to support 2.0 and that level, but proper USB 3.0 or higher ports supporting higher draws than that aren't quite ubiquitous yet. That said, some smaller, more mobile devices may not even support 0.5 A on the ports. Many USB DAC/amps certainly aren't running up into around 0.5 A draws and are at significantly less. However, note that specs aren't necessarily all that strictly adhered to. For example, see the erratic behavior of USB external hard drives on different devices.
 
At a high level, there's clearly a physical limitation at work here: the device can't output more power (on average) to the headphones than it takes in. Due to the presence of all kinds of other supporting electronics, no amplifier stage being 100% efficient, etc. the power output is limited far below that to a level that depends on exactly what's going on in the device.
 
One issue is how you define "ability," "power" (as a verb), and "hard-to-drive." The descriptor of "hard-to-drive" is sometimes ascribed to headphones that by all engineering accounts probably can't reasonably be described that way, in relation to other products. A lot of times, people say that _____ cannot or is deficient in powering _____ when what is meant is some kind of aspect of sound quality (at least as perceived by the listener) and not really meant in a matter of electrical power delivery. Finally, back to "ability." You can run a HiFiMAN HE-6 off pretty much any headphone jack, most with minimal distortion and issues, and depending on the listener's taste, sound good. It really depends on listening volume and program material. The louder you listen, the more power is required.
 
Practical challenges of USB-powered DAC/amps when trying to achieve relatively high power output levels (say hundreds of milliamps max into 32 ohms, which is well above the vast majority of headphones need but perhaps not enough for some users and sets) include having to deal with noisy USB power, only having +5V and ground to deal with so some kind of mechanism to generate a negative rail or otherwise handle the output are required (not to mention boost the +5V to something higher), and board space (size). You could do better if you supposed that weight, form factor, and size were of no consequence, but nobody sells huge bricks that are USB bus powered. If your DAC/amp is past a certain size, people aren't going to take it anywhere, and there's no reason not to use wall power.
 
Looking at existing designs on the market (which I don't do that often, so I may be pretty uninformed), the most powerful these USB-powered DAC/amps get are about 3 V rms max, 300 mW per channel into 32 ohms. You could probably do more but maybe at the cost of some performance. In particular, I think you could probably make a device with somewhat higher voltage rails and be able to put out more than 3 V rms to drive say 300-600 ohms headphones better, even if the max power is limited to something more modest.

 

[esldude]

If you only have 4.5 watts available it does represent a hard limit on power using anything only USB powered.  Though as the post above says, I think it is .5 amps so 2.5 watts.
 
Now there are voltage converters so 5 volts isn't necessarily the limit.  Though with more voltage you will need to use less amperage.  For instance there are USB powered microphone pre-amps that have 48 volt phantom power available.
 
I suppose at an extreme, one could use voltage converters and large capacitors or small batteries to exceed for short periods of time even the 4.5 watt limit. 

 

[PurpleAngel]

  Hi.  According to Wikipedia the max power that USB can provide a device is 5V, 0.9 amps.  Does this limitation only present an engineering challenge for DAC/Amp designers or is there a definite, line-in-the-sand practical limitation on the ability of a USB powered device to power hard-to-drive headphones? 

 
As there are USB/DAC/Amps that can decently drive 250-Ohm headphones, USB power seems to be enough for use with most headphones.
Are you looking at getting certain headphones for use with an USB dac/amp?

 

[CharlesC]

Thanks to you all for the replies.  In the original post I may have been a bit lazy in focusing on hard to drive headphones.  Perhaps I should have framed the question around sound quality instead.  For example,  though USB powered dad/amps tend to come in at under $200, battery powered dac/amps can be substantially more expensive.  Now, I'm assuming that the expensive battery powered headphones sound better because, you know, they can cost a ton.  So, is there any reason that, at some point in the future, a $200 USB powered dad/amp couldn't sound as good as current $600 battery powered dad/amps.  I have the impression that in the last 2 years the USB powered devices have improved quite a bit and perhaps it's reasonable to assume that they can keep on improving ..... at least for a while.
 
Again,  thanks for the very informative posts.

 

[bigshot]

  Now, I'm assuming that the expensive battery powered headphones sound better because, you know, they can cost a ton.

 
Since when did price EVER translate into huge improvements in audio equipment? If you really need a DAC/Amp, just get a nice inexpensive one. There is no reason to pay a lot of money for DACs when you can get a blu-ray player with a built in DAC that has perfect specs and plays every format known to man for $75.

 

[PurpleAngel]

  Thanks to you all for the replies.  In the original post I may have been a bit lazy in focusing on hard to drive headphones.  Perhaps I should have framed the question around sound quality instead.  For example,  though USB powered dad/amps tend to come in at under $200, battery powered dac/amps can be substantially more expensive.  Now, I'm assuming that the expensive battery powered headphones sound better because, you know, they can cost a ton.  So, is there any reason that, at some point in the future, a $200 USB powered dad/amp couldn't sound as good as current $600 battery powered dad/amps.  I have the impression that in the last 2 years the USB powered devices have improved quite a bit and perhaps it's reasonable to assume that they can keep on improving ..... at least for a while.
 
Again,  thanks for the very informative posts.

 
Battery or USB powered I would think is not as big a factor for audio quality,
as a $200 DAC/Amp verses a $600 DAC/amp would be the bigger factor for audio quality.
 
Maybe why the battery powered DAC/Amps cost more is because it costs money to include a (good quality) battery in the design.
I'm also guessing that inside the high priced battery powered DAC/amps, are parts that are designed to run on less power, but the parts cost more.
Where as USB powered DAC/amplifiers in theory have an unlimited access to (low voltage/amps) power, so maybe they can get away with parts that cost less, but require more power to operate.

 

[bigshot]

   
Battery or USB powered I would think is not as big a factor for audio quality, as a $200 DAC/Amp verses a $600 DAC/amp would be the bigger factor for audio quality. Maybe why the battery powered DAC/Amps cost more is because it costs money to include a (good quality) battery in the design. I'm also guessing that inside the high priced battery powered DAC/amps, are parts that are designed to run on less power, but the parts cost more. Where as USB powered DAC/amplifiers in theory have an unlimited access to (low voltage/amps) power, so maybe they can get away with parts that cost less, but require more power to operate.

 
Those are really good guesses, but they aren't the case. Other electronics like iPods, blu-ray players and integrated amps and receivers are all MUCH more complicated and contain MUCH more expensive parts than a simple Amp/DAC. You can get a $40 DVD player at Walmart with audio that sounds just as good as the most expensive portable DAC out there, and you can get a 2 gig iPod shuffle for just $46 that not only has as good sound, it has 2 gigs of solid state storage too.
 
The reason that some amp/dacs are very expensive is simple. It's because people like you assume they are getting their money's worth when they buy it. They listen and say, "Yes it sounds good" and assume it sounds better. But it probably sounds exactly the same as something that costs a fraction of the price.
 
Shop smart. Don't be a sucker for overpriced stuff that is sold as a luxury item.

 

[esldude]

   
Those are really good guesses, but they aren't the case. Other electronics like iPods, blu-ray players and integrated amps and receivers are all MUCH more complicated and contain MUCH more expensive parts than a simple Amp/DAC. You can get a $40 DVD player at Walmart with audio that sounds just as good as the most expensive portable DAC out there, and you can get a 2 gig iPod shuffle for just $46 that not only has as good sound, it has 2 gigs of solid state storage too.
 
The reason that some amp/dacs are very expensive is simple. It's because people like you assume they are getting their money's worth when they buy it. They listen and say, "Yes it sounds good" and assume it sounds better. But it probably sounds exactly the same as something that costs a fraction of the price.
 
Shop smart. Don't be a sucker for overpriced stuff that is sold as a luxury item.

Hey, my Galaxy SIII phone, already long obsolete, sounds pretty darn good whether driving my lightweight little headphones (Creative Aurvana Airs ) or my Beyer DT880s.  Makes the even $46 iPod rather unnecessary.  However, unlike the iPod, it can be a bummer to be groovin' to your favorite tunes, and a call comes through to interrupt. 

 

[castleofargh]

the DAC is no big problem, the amp part sure could benefit from it's own energy source depending on the headphone used (battery or wall outlet).
 
about price, there can be too many reasons why something costs more(production quantity, R&D, architecture choices, more inputs, the guy wants to get rich fast, it's written A&K on the box instead of iriver...).

 

[bigshot]

  about price, there can be too many reasons why something costs more (production quantity, R&D, architecture choices, more inputs, the guy wants to get rich fast, it's written A&K on the box instead of iriver...).

 
I helped clarify your point a bit.

 

[mikeaj]

  So, is there any reason that, at some point in the future, a $200 USB powered dad/amp couldn't sound as good as current $600 battery powered dad/amps.  I have the impression that in the last 2 years the USB powered devices have improved quite a bit and perhaps it's reasonable to assume that they can keep on improving ..... at least for a while.

 
Audio is not moving very quickly. If you look at where most of the money and engineering talent goes, it's not there (and most of it into the chipmakers, people doing the mass-market stuff). Recent technological breakthroughs don't really have much impact on what you can do with regards to sound quality, so I wouldn't read too much into timeframes of a couple years as meaning much. If there are improvements in audiophile-centered products, which there are, it probably has to do with the designers themselves becoming more experienced as they ride the personal audio market. i.e. they're making do with the same kind of stuff that was already available by improving their techniques and somebody could have done the same years ago.
 
The very integrated audio electronics on a phone or most consumer-centric media players are heavily optimized for lower power consumption. You can get better audio performance (say better linearity, lower noise, though not necessarily by any level that's audible in most cases) going with a design with more discrete parts and/or higher power consumption.
 
For concert-level volumes on high-impedance headphones or very insensitive sets (i.e. for headphones only audiophiles use or possibly some in music production who would definitely not be driving this stuff off a mobile or USB-powered device), you'd need your own specialized design.
 
The battery-powered stuff is usually even more power-constrained than USB-powered DAC/amps. I mean, in the latter category you have the CEntrance DACport running a Class-A amp off of internally generated +/- 9 V rails. That would suck your battery dry in no time. On the flip side, a battery gives a very stable (on short time scales), relatively noiseless voltage supply. USB power is considerably dirtier. However, depending on the device and target power output levels, the voltages may need to be converted anyway, so what you get input may not be that relevant. And even if USB power is dirtier, you can use the higher power budget to filter it properly. The other thing to keep in mind is that a lot of carefully designed audio electronics are not very sensitive to power supply noise in the first place, so it could be a moot point.
 
Anyway, there are a lot of factors other than where the power is coming from. It's up to the designer to do things properly. (Of course, none of this makes as much of a difference as most audiophiles think, anyway, so take it as you will.)

 

[CharlesC]

   
Audio is not moving very quickly.......

Thanks for both your posts.  I will probably have to read them each several times.