Telefunky,
ich zitiere mal von "Gearslutz":
"A computer USB port is strictly "power limited" by the USB standard. USB ports supply 5.0 volts nominal (5.2 volts maximum) at up to 500 mA. Not more! The computer will switch to current-limiting if the external device (the interface) tries to draw more than 500 mA. That's only 2.5 watts of power (total) available for
all functions of the interface.
An AC mains (wall outlet) powered device (an interface) is not "power-limited". The AC mains power supply can be as large as the interface designer wants it to be or can afford within his/her budget. It's not uncommon to find power supplies that can supply 15 volts at 1.5 amperes or more powering small (2-in/2-out interfaces (that's over 22 watts of power).
With the severe power constraints imposed by the 2.5 watt power limit of USB powering, designs must limit certain interface performance specifications.
Power is first required for the A/D and D/A converters, data buffers, and the actual USB connection chips. Power is then required for the analog circuitry: mic pre-amplifiers, line-level analog input amplifiers.
Then, power is required for the DC voltage multiplier that provides +48 volts for the phantom power that almost all small interfaces are expected to have to power capacitor mics that need it.
Finally, power is required for the analog line-level output drivers (amplifiers) and the headphone amplifiers.
Because of the severe power constraints of USB bus powering, the headphone outputs, and often the phantom mic power (mic current capability) are limited to power levels that are far less than ideal.
There is a very popular 2-in/2-out interface with a headphone output that is limited to only 6 mW per channel. Most USB interface headphone output ports won't produce more than 30 to 50 mW per channel. Without getting into the technical details, the output impedance and the voltage swing limits of these "wimpy" headphone driver circuits are also poor (high output impedance and low voltage swing) because of the limited USB power. That makes the phones output perform poorly when driving both low (less than 50 ohms) and high (more than 100 ohms) impedance headphones. Most are optimized for driving headphones around 65 ohms.
With an AC mains-powered interface, there are no real power constraints, so it's easy to include a headphone amp that can provide 100 mW or even more per channel and can drive very low as well as higher impedance headphones.
With line-powered devices, designers can use better, lower-noise mic pres, better output line drivers, and better (higher current capability) phantom power supplies in their designs.
The downside is that the interface must have an additional AC line connection and also include the cost of the internal or external power supply itself.
Regarding the AC interface power supply creating noise and/or ground loops, that's typically a non-issue. Unless you are running your audio on a laptop computer running on its internal battery, you already have a AC power supply connected to the "system". If it's a typical "switch-mode" universal power supply, it may generate some electrical noise. Ground loops may be an issue with desktop computers using grounded mains connections (IEC 3-wire power cables), but usually are not issues with laptop power supplies with 2-wire plugs. Adding the interface power supply will usually not change any power supply noise or ground loop issues that don't already exist due to the computer's own power supply. Proper input/output connections (fully balanced are always better) are essential.
Even with the USB power limitations, there are a
few bus-powered interfaces that actually work pretty well. The
Sound Devices USB Pre2, the
Apogee Duet-2, and the
RME Babyface all sound great and the SD USB Pre2 has a decent headphone output. By using custom designed integrated circuits (instead of "off-the -shelf" parts) better engineering, and higher-quality components, those companies are able to make fairly good performing interfaces within the USB bus-power limitations."
http://www.gearslutz.com/board/newb...os-cons-usb-bus-powered-vs-power-adapter.html
Kurz zusammengefaßt kann man wohl sagen: es KANN funktionieren, muß aber nicht.