Regarding the AC interface power supply creating noise and/or ground loops, that's typically a non-issue.
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. 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. 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. Most are optimized for driving headphones around 65 ohms. That makes the phones output perform poorly when driving both low (less than 50 ohms) and high (more than 100 ohms) impedance headphones.
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. Most USB interface headphone output ports won't produce more than 30 to 50 mW per channel. There is a very popular 2-in/2-out interface with a headphone output that is limited to only 6 mW per channel.
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.įinally, power is required for the analog line-level output drivers (amplifiers) and the headphone amplifiers.īecause 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. Power is then required for the analog circuitry: mic pre-amplifiers, line-level analog input amplifiers. Power is first required for the A/D and D/A converters, data buffers, and the actual USB connection chips. With the severe power constraints imposed by the 2.5 watt power limit of USB powering, designs must limit certain interface performance specifications. 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). The AC mains power supply can be as large as the interface designer wants it to be or can afford within his/her budget.
That's only 2.5 watts of power (total) available for all functions of the interface.Īn AC mains (wall outlet) powered device (an interface) is not "power-limited". Not more! The computer will switch to current-limiting if the external device (the interface) tries to draw more than 500 mA.
USB ports supply 5.0 volts nominal (5.2 volts maximum) at up to 500 mA. What else?There ARE major differences between USB bus-powered audio interfaces and AC-mains powered interfaces.Ī computer USB port is strictly "power limited" by the USB standard.
And that they can run more of a load?īut I hear there are loopback and ground problems, what exactly are those?īus powered is probably more portable for laptops on the road, and less clutter from not having a big plug power adapter.īut I suppose it can handle larger/more demanding/task driving loads? Really a issue for the average engineer or musician? I heard power adapter ones can better power headphone pres to better run higher ohm cans. Was just wondering if each of these had pros and cons on a audio interface: