Good advice - you buy your pre for the conversion. DSP's - well they're mostly for tracking with real-time effects on monitor (which is useful for some folks), unless you have a unit with a lot of channels and you're using it live and doing some sort of poor man's mixer with it.

Good advice - you buy your pre for the conversion. DSP's - well they're mostly for tracking with real-time effects on monitor (which is useful for some folks), unless you have a unit with a lot of channels and you're using it live and doing some sort of poor man's mixer with it.

In the case of the Focusrite USB bus-powered interfaces to which the OP was referring, Focusrite very obligingly reveal their tech specs for all to see:
 
http://global.focusrite.c...ett-2i2/specifications
 
http://global.focusrite.c...ett-2i4/specifications
 
as do Roland for their bus-powered interfaces also, e.g.:
 
http://www.rolandus.com/products/details/1166/specs/
 
So, no speculation necessary, as the specs reveal, among other things, that on the USB bus-powered Scarlett 2i2 and 2i4 interfaces the mic preamps offer up to 55dB of gain (along with their headphone outputs' rated power and impedance),  and that the USB bus-powered Roland Quad-Capture interface can require up to 480 mA of current supply from the USB port to which it is connected (along with revealing its headphone output's designed-for load impedance).
 
Quite possibly a search for reputable reviews of the interfaces would reveal whether their performance was found to be in accordance with their published specifications.
 
It's always wise when shopping for something to know more about it than the salesperson, so before going to the store it might be helpful to go to school first.
 

Right, Goddard.  Read and memorize the specs, even if you are not sure what they mean, although the OP had no reason to think that the USB power was an issue.  As the name implies, a salesman is supposed to sale you stuff - hopefully the item w/ the largest profit.  It ain't bad, just the way things are.  Whether for audio or cars or whatever, if you act like you know what you want and sound like you know the basics, you can cut a lot of the crap out of buying.  I love the kids at Guitar Center - but I don't deal w/ them much since I had a friend at the local shop.  If he ain't there I just ask for what I need.  And I love the young guys selling cars - they always hit you first.  I just smile and say I want that car, smile and no thanks to undercoating (mainly a yankee sell) or extended warranty or a waffle-bot w/ that SUV.
 
Still, I would be wary of using a USB powered mic preamp as my only choice.  I like stuff I can plug into the wall.
 
@

Goddard is correct in that USB power is rarely sufficient to power a preamp well.  The limited current usually results in much higher noise specs at higher gain levels and lagging response.  Two standard preamp chips will draw about 80mA from a 5 volt supply.  Add two phantom powered mics at 90mA from 5 volts, a headphone amp only draws about 200 pico amps but there is the onboard DSP and and converters which will draw nearly 100mA each and various opamps for I/O...well you get the picture.  Once you run out of power to draw on, the voltage drops to compensate, and the performance of each device suffers.  
 
If you are only recording loud sources with sensitive mics, you should be fine.  Anything that requires significant gain and detail, you need a separate preamp for good results.   

I think it needs to be pointed out the the preamplification needs of an A/D aren't all that much, we're talking about 1v or 2v P-P at full blast, and not much of a current draw. Where there are issues, it will be in in line-outs and headphones. 
 
As for phantom powered condensers, unless it's an (expensive oldy), current should be more like about 5ma per mic, about a tenth of what you're quoting. I doubt that voltage drop is an issue at all. Most advice on issues of sufficient current, voltage and the like usually based on discrete or esoteric designs. Modern designs, using asics and cmos where possible, are very efficient.

The older transformer based 48vDC mics usually use less current than their newer servo balanced counter parts.
 
best regards,
mike

A major appeal of a bus-powered USB interface, just like that of a laptop computer, is mobility and portability, since it can allow recording and mixing to be performed away from a source of mains power.
 
But the basic reality is that the power available from each USB2 port on a computer is limited, and may be especially so in the case of using a laptop computer running on battery power where battery life is of primary concern.
 
(Btw, this limitation affects not only USB bus-powered interfaces but has also affected the usability of Firewire bus-powered interfaces with laptop Cardbus/Expresscard FW cards which require connecting a power cable to a USB port on the laptop for supplying bus-power, which accounts for why many Cardbus/Expresscard interfaces include or even require a mains adapter.)
 
Now, an obvious way of coping with the power supply limitation of USB ports is to simply limit the features and capabilities of a USB bus-powered interface so as not to exceed the available supply power, as I'd already mentioned, and this has pretty much been the norm especially at the lower cost end.
 
Fortunately, as the USB audio specification has evolved and lower power components have become available (and at lower cost), manufacturers have been able to offer more capabilities in their bus-powered USB2 interfaces including, more recently, more I/O and an onboard DSP mixer/fx, features previously only available on mains-powered units. But even so, the mic/inst preamp gain/headroom and headphone/line output power of bus-powered interfaces may still be limited compared to mains-powered offerings.
 
Bearing this in mind, when auditioning a bus-powered interface, be aware that the power output of its headphone output will very likely be rather limited and that it is therefore essential that headphones of the proper impedance (referring to the interface's specifications) should be employed. But even so, the headphone output level of a bus-powered interface may seem low/weak when compared to a mains-powered interface (because it very likely is, although on the upside it's not as likely to pose a hearing loss risk).
 
Btw, another way of coping with the power supply limitation of USB ports is to draw power from an additional USB port(s), which is precisely what RME do, supplying their USB bus-powered Babyface interface with a special USB cable having a second USB connector for taking power from an additional USB port.
 
Another reality is that connected USB peripheral devices must negotiate for the amount of current to be supplied over the USB interface, during device enumeration (configuration), as well as abide with system power management regimes. This has complicated the design and slowed the availability of more capable bus-powered interfaces with onboard DSP/microcontrollers even from larger companies like Roland and Yamaha which can develop and produce their own dedicated logic and signal processing chips for their interfaces unlike the smaller manufacturers which must typically buy-in such components and solutions. Which probably goes a long way towards explaining why we are still seeing new USB2 interfaces being launched these days when USB3 ports abound on every new computer.
 
Returning again to the question raised in the OP, as to whether any bus-powered units will have issues, consider that the Roland Quad-Capture can provide a maximum phantom power current of 6mA at 48V.
 
Then consider that the vast majority of phantom-powered condenser microphones on the market were designed for connection to mains-powered mixing consoles, not to the mic inputs of bus-powered audio interfaces, and that besides requiring a small amount of phantom power for polarizing their diaphragm capsule some mic's may also have onboard drive electronics which also must draw phantom power (the Rode NT2a comes to mind).
 
Once one considers these factors, it becomes easier to understand why people might become frustrated when unable to use more than a single condenser mic at a time with their bus-powered interface (the NI KA6 comes to mind).
 
If you don't understand what the specifications mean, then learn (try a glossary). Besides possibly saving you from making a bad purchase decision, you're more than likely going to need to know some of that stuff anyway if you ever hope to get serious work done in your studio.
 
Some bus-powered study material:
 
http://tascam.com/product/us-366/specifications/
 
http://www.native-instruments.com/en/products/komplete/audio-interfaces/komplete-audio-6/specifications/
 
Note that sometimes it's a spec which you won't find listed which may be important (and may raise a question as to why it's not been given).
 
Ok, enough on this.
 
Good luck with your purchase (especially if you try to B-S your way through it)!

A far simpler explanation. If you are serious about making recordings with multiple mics and you want the better converter sound then stick to an interface that is either mains or power supply powered.
 
Bus powered devices are OK for limited applications such as one or two mics and basic conversion duties and portability. You will get OK results with a bus powered converters. The better quality devices or higher end devices will be powered. I am sure you can get very good results with a bus powered device. If I were doing a serious recording or production I wouldn't use one though. More so for the Mic Pres than the converters.
 
For those who may be wondering about internal voltages, step up circuits can be used to create higher voltage rails but you don't get anything for nothing. Lower current with the higher rails. (and step up circuits are not perfectly efficient either) The 5V rail was never intended to do what we are trying to do with audio though. Separate power supply at higher volatge and current solves a lot of problems.
 
The bulit in DSP in bus powered devices is usually limited and not that great and it shouldn't be a deal breaker.
 
My information is always (mostly) correct and based on 40 years practical experience as both a recording engineer and electronics engineer. The OP was only after opinions anyway.

Jeff Evans
A far simpler explanation. If you are serious about making recordings with multiple mics and you want the better converter sound then stick to an interface that is either mains or power supply powered.
 
Bus powered devices are OK for limited applications such as one or two mics and basic conversion duties and portability. You will get OK results with a bus powered converters. Better sound more than liekly with a powered device.

Yes, that's simpler, but it's also incorrect and explains nothing, only states your own opinions without any actual foundation.
 
Want "better converter sound"? Try an E-MU 0404 USB 2.0 bus-powered interface. Some really nice AKM converters in there:
 
http://www.creative.com/emu/products/product.aspx?pid=15185
 
http://www.soundonsound.com/sos/jan07/articles/emu0404.htm