bus powered interface question

The bus power is iffy and it depends on what power is available on the USB port from the PC.  It can vary from one PC to another.  I would rather rely on an external power supply.  Just like external USB hubs, the self powered are needed as there is not enough power available to properly run an external USB Hub. 
 
I have an Octa-Capture and perfectly happy with it.  I like the built in limiters, although I don't use them all the time.  Low Latency, too.  Music audio electronics guru Craig Anderton has a glowing review of the Octa-Capture on the harmony central site. 

Thanks for the replies.  I'm really leaning to spend the extra on the octa unit.  Just read through almost 17 pages of comments on the Anderton review and seems like there's no way to go wrong with this one.  More than I set out to spend, but also don't want to end up limited by buying short now.
 
As for the salesman, I was in Guitar Center and just like at Best Buy, you have to be extremely skeptical of everything they say.  
 
thx again.

Something to think about:
 
A bus-powered USB interface can draw 5V DC at a max 0.5A (= 2.5W).
 
From a 5V DC bus-power supply, an interface must somehow supply 48V DC for phantom power.
 
At 5V DC supplied power, the "+" and "-" power supply rail voltages (and thus, maximum output voltage levels) for an interface's mic preamps (and headphone amp) would be +2.5V DC and -2.5V DC (referenced to a "virtual ground 0V DC level = 1/2 the supply voltage) or 5V rail-to-rail (although in actual practice lower than that). Yet many bus-powered interfaces have their preamp power supply rails at rather higher/lower voltages (e.g. +/-15V DC rails) .
 
Hmm...

No, getting necessary higher voltages from 5V USB bus power isn't really a problem.
 
But it does require designing for, and it does incur expense for the necessary components. And the supply current available from a USB port is finite (100~500 milliAmps) and must power not only the mic/inst preamp(s) and headphone driver amp(s) but also line input/output buffers, AD and DA converters (codecs), onboard DSP/mixer logic, optical I/O, metering and indicators,  and a USB interface, each of which can consume substantially more supply current than the amount required for supplying phantom power bias voltage (also possibly used for powering a microphone's onboard drive electronics).
 
Consequently, apart from being dictated by cost, when confined to using USB bus-power, compromises must often be made in an interface's features and capabilities, such as limiting the number of mic preamps and headphone outs and total number of I/O channels provided, the available preamp gain and headphone driver output power and possibly even the complete absence of onboard DSP/mixing, compared to a mains-powered interface where the available supply power is not so constrained (or even to bus-powered Firewire interfaces where the supply current is typically significantly higher than for USB although still severely constrained compared to mains-powering).
 
Regarding a bus-powered Scarlett, "how it sounds" might be affected by how capable it is of driving the particular headphones (of whatever particular impedance they might be) plugged into its headphone jack or of driving its main (line) outputs, rather than anything to do with its mic preamps.
 
Point being: there is a very good reason why the Octa-Capture (or Scarlett 18i20) requires mains power and can't be USB bus-powered like the Scarlett 2i2 or 2i4 can.
 

Well, the solution is to get an external pre.  If he didn't suggest that then he might be dumb, but honest.  And pushing most IC pres, esp. built-ins built to cost, isn't a way to improve your tone.
 
A well designed, well powered pre can make a difference - but not that much, depending upon your recording techniques.  Either of those pres, bus powered or not, should be fine 90+% of the time.  So if you don't like the sound you are getting from the unit, it is most likely due to the room, mic or technique (or a combination of all three).  It takes time to develop and ear for recording - even if you spend all kinds of time on mic placement.  And the mic choice will have much more impact (usually) than the preamp choice.  And if you have a crappy room, guess what recording in it sounds like?
 
I've used the roland units (the 100 and 700 anyway) and the pres/conversion are good.  The dsp - not so much.  I'd never buy a unit based on the dsp comps/gates.  Sonar or any daw is going to do it as well since the dsp is post conversion.  And most interface dsp reverb is for the talent, not recording (tho some like the TC Konnekt 48 are better than good).
 
Buy your interface for the conversion but most importantly if it works out of the box w/ your pc system. Then learn on that as you add software/outboard.  Then upgrade it when you outgrow it - which you might not.  Rinse, repeat.

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.

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.
 
@

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.

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)!

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
 

Balanced power is an interesting subject. They are not cheap from what I can see though.
 
Slightly off topic but important none the less is the differences between expensive Mic preamps (with hefty power supplies) and less expensive preamps with so called inferior power supplies.
 
http://www.soundonsound.com/sos/oct12/articles/preamps.htm
 
And the wrap up here as well.
 
http://www.soundonsound.com/sos/dec12/articles/preamp-results.htm
 
To be able to hear the effect of the power supply within a preamp for example may be only be possible in some circumstances but not so in many others. These articles go a long way to debunk a few myths. As one engineer said in the wrap up he would happily record a piano through a Mackie Preamp. (fiddling the gain pots might be another story as he mentions. I have produced a few very nice sounding acoustic albums using Mackie pres without any issue so it can be done!) Note we are not pushing into saturation here with any of these either. (differences may be more apparent when done so)
 
Modern power supplies are quite capable of doing a very good job. (not USB powered though ) They do not exhibit their sound into the audio in such a bad way at all. A very controlled blind A/B test would prove that and people who say they can hear so much in something would very likely fail under these conditions.
 
As quoted in the article:
We hope, though, that this test will have done something to restore a sense of perspective. Preamps are not the be-all and end-all of recording, and contrary to the advice that is sometimes dished out on Internet forums, those on a limited budget would often do better to spend their money on microphones or acoustic treatment, and to experiment with mic positioning, to achieve the sound they seek, than on expensive preamps. Don't be bullied into thinking that you can't make good recordings on affordable equipment. You can.

Aye, Mike, there's the rub.  You make money.  So it makes sense to spend it so you don't have to redo everything and you can be sure that the problem is not at your front end.  90% of the time a good ole mackie preamp will suffice.  But for the other 10% ....  If you don't want to spend 100% of your time on it and need the confidence that your tools won't betray you, spend some money.  If you are doing in the box stuff and only an acoutisc track at a time, you might not need the big studio tools (I'm in that category here at home but I do have some nice front end stuff and still ain't divorced) and the impact on your music can be minimal.  If you are making money or have disposable money it can be worth your while to have good stuff.  Besides, large power supplies and transformers are sexy :-)
 
@

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.

 
You need to do the math correctly.  The current draw numbers on microphone stats are estimated for 48 volts, not 5 volts.  You need to draw a lot more current to step up the voltage.  
 
Not a lot of people realize that the power supply is probably the most important component of a high quality audio device.  A flat, clean, and ample DC source is crucial for the system's performance.  Yes most makers of multichannel interfaces do an adequate job of providing enough power for the system to be used without problems.  It is only when you push it with power hungry mics that you may hear problems.  A good 2 channel stand-alone preamp will have voltage rails of 18-24 volts and supply at least 1amp of current.  Arguably overkill, but the benefit is sharper transients, lower noise, and cleaner response.  

It is all interesting though and I am older like Mike and do appreciate a well designed power supply. I am just from that era too! (I still think big hefty power supplies sound good in power amps!) I just think the differences are less than we think at certain levels of music production. I would be interested Mike in what you did not like about the SOS test. I thought it was pretty reasonable.
 
For those who do have any DSP after the MIc pre but before the DAW I also have this feature in my Yamaha mixer but rarely use it as well. But the other day I was recording a very dynamic male singer. I did not know what to do (other than patch in an external device but there was no time for that)  so I found using the compressor built in to the mixer set for limiting (if you can do it that is) with a very high threshold, fast attack and release did a great job of catching very loud bits end preventing them from clipping the input to the DAW. There was no recorded distortion anywhere (shows how clean the Mic Pre was in the Yamha mixer too) and the vocal track was easier to edit later on for sure.
 
That is about the only DSP I have ever used on the way in. But you do have to be able to put any compressor into limiting mode and set it up accordingly. Can you do that with the Roland Pre?
 
 

One more thing... the mere mention of Etta James makes my knees buckle.
 
Off for a listen!!!