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I've seen some vague claims that an XLR microphone-to-usb that's powered solely by USB will necessarily sound worse than one powered by a separate power source (a wall wart) because the preamp will necessarily be of lower quality?

Is there any objective reasoning behind that?

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3 Answers

It won't necessarily be lower quality, but it could be.

USB ports can quite happily deliver 5v at 2 amps - but if your PC/laptop/mac is under heavy load you may find that it drops a bit, or the 5v isn't very pure so you may get fluctuations in the power supply to your mic adaptor.

The same thing happens with a mains supply, but it can be easier to filter out the 50/60Hz mains hum than to filter out the wide range of high pitched interference you can get from components within your computer. You can also get a higher voltage easily from rectifying and smoothing mains current, as opposed to having to upscale from your USB's 5V with an inverter.

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High-quality linear components generally need more than 5V supply, insofar the claim is reasonable.

However, it is nowadays not a big deal to include a DC-DC converter to attain a higher voltage. Of course this needs more current, but USB offers rather plenty of Amps. Then you also have enough headroom to smoothen out most interference with fixed-voltage regulators etc.. Some people will claim that this never achieves as clean results as a traditional 60Hz AC trafo, but there is no real, scientifically sound reason why this should be so.

Most USB interfaces apparently go the DC-DC conv. route, otherwise they also couldn't provide 48V phantom power.

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The USB standards guarantees just 0.5 A. –  chirlu Sep 6 '13 at 6:23
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In theory, there are electro-physical reasons as mentioned by Dr Mayhem.

In practice, you will find AC power supplies almost only on larger multi-channel USB interfaces and hardly ever on a one-channel or two-channel device. If you do some research and explore reviews about audio quality of USB interfaces, you will find out that the quality of semi-professional USB-powered audio devices is notably good nowadays. Renowned manufacturers use the same microphone pre-amps as in large multi-channel devices and thus deliver comparable audio quality.

So in theory, there is a difference, but in practice, the only difference is that small one-channel or two-channel devices are USB-powered for mobility reasons without a notable loss of quality.

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