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I'm planning to buy an audio interface and it being exclusively bus powered is a requirement. However I also use condenser mics which require phantom power. Now I'm a bit confused about why some bus-powered interfaces require additional PSU for phantom power and some not. For example, audio interfaces lke Audient ID4 require connecting external power supply adapter when using phantom power, which seems logical. What I can't understand is how another USB 2.0 bus-powered interfaces like SSL2/SSL2+ provide phantom power on USB voltage alone - phantom power is 48v and USB 2.0 only provides 5v, if I'm not mistaken. The reason I'm asking is that I am choosing the audio interface and want to avoid any nasty surprises in the future. Any insights would be greatly appreciated.

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    @ChrisH It is an assumption, certainly, and if the OP intends to be doing a lot of field recording then I'd agree with bus-powering being needed. But if he's doing field recording then he's likely better off with something like a Zoom recorder than a fragile laptop and USB interface. And if he's working from a van with a 12V battery but no mains, then he can use a 12V-to-whatever-other-voltage adaptor instead of the wall wart. The subset of having no external power source and being safe to set up a laptop, USB adaptor and condensor mics is rather small, so it's worth asking the question. – Graham Sep 25 at 12:33
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    @AndreiSch., btw, I have a USB-C power meter and have measured the extra draw that 48V phantom power requires on my Zoom H6 and can look that up, or run it again on the 2+, if you're curious. That said, because it's 48V at a very low amperage, that extra draw isn't much -- remember, the Zoom can run phantom power on four XLR ports at once using only USB bus power (from an older version of the USB spec, at that); the SSL is driving fewer ports, and it's using USB-C, so if it needed to draw more power than the baseline standard requests it could just leverage the PD extension to the standard. – Charles Duffy Sep 26 at 15:40
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    Oh -- no, I'm not saying it does use PD, I'm saying that if it needed that much power, its designers could have done so. Anyhow -- let me find the meter, and I'll add an answer once I've taken some measurements. – Charles Duffy Sep 26 at 20:04
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    @AndreiSch. USB-C can theoretically do more, but you have the practical problem that you need the user's PC to have USB-C ports. Until most PCs are, it's not in any manufacturer's interest to design their interfaces for it. But even then, most interfaces are older designs, and they certainly aren't going to go back and redesign their old kit. I don't disagree it's convenient to not need the wall wart (although these days they're much smaller anyway), but it is going to limit your choices. – Graham Sep 26 at 22:27
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    @CharlesDuffy Oh sure, they're nothing like the same recording quality. But if the OP was doing field recordings, survivability is a big deal. I'd happily put a Zoom recorder in a ziplock bag in a rucksack and expect it to just work at the other end. I couldn't say the same about a laptop, USB interface and separate mics, without some serious packaging. It turns out that's not it, but still, it was worth checking. – Graham Sep 26 at 22:31
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Phantom power has very little current requirements, so a step-up voltage converter can easily provide +48V from USB power.

One reason not to do it (a choice some interfaces make) may be that designing a step-up converter in a manner where it doesn't interfere with high-sensitivity audio circuitry in the same enclosure is non-trivial. If you need phantom power for one input and another input is a high-impedance unbalanced instrument input, having the latter not pick up any converter whine in the range of the A/D converters requires careful design of internal shielding and operating frequencies.

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    I'd considered that too - but so many cheap wall-wart switch-mode power supplies are hideously noisy, I don't see how they're gaining anything, unless it's simply physical separation, hoping the distancing will outweigh the cheapskating on circuitry ;) – Tetsujin Sep 24 at 10:46
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    What they're gaining is the convenience of doing it all with one cable and hoping the user isn't that concerned about noise. And yes, stepping up voltages is common. That's what the 120V inverters that plug into your cigarette lighter plug do. – Duston Sep 24 at 13:11
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    @Tetsujin: physical distancing is a significant, as well as being able to generously throw in some ferrite beads. – whatsisname Sep 25 at 4:08
  • @Tetsujin ... as well as being able to throw the blame over the bad noise performance to the PSU – fraxinus Sep 25 at 9:30
  • many of the cheap wall supplies do not regulate voltage properly as well; this is the case with the multi-voltage ones where the voltage advertised is delivered for a specific load and anything else is approximate. Whereas the USB voltage is stable. That being said, I've had ground issues with USB and laptop (with MacBook Pro) when it was charging – Thomas Sep 25 at 11:03
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Some work, some not. Creating 48V from 5V is done by a converter. Well-known technology but can be tricky to not induce noise in sensitive audio signals.

Back of the envelope calculation:

P48 should promise 10mA to the mic. If we calculate on the 10ma x 48V it comes out as 0.48W. Two mics and a voltage converter efficience of, say, 80%, the mics will require a power budget of 1,2W. Many mics draw less, say 4 mA, so we could might cheap out a bit on the power budget and most users will not notice.

Maximum current from USB2 is 0.5 A, times 5V gives 2.5W. In reality a bit less, as there are some cable losses.

This would leave a power budget of, say, 1.2W for the AD and DA and headphone amp, say 50mW output, and rest of the circuits. Can be done but may require either very good circuits or some "shortcuts".

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    The most common "shortcut" is to just ignore the USB current limit. USB hard drives typically require around 5 watts to spin up, so it's common for USB2 ports to actually be able to provide 1A or more. – Mark Sep 24 at 22:14
  • @ghellquist thank you, based on the reviews I think I can consider SSL2 as 'having very good circuits', so I may give it a go. – Andrei Sch. Sep 26 at 14:33
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It's possibly a measure of the unit's internal power-efficiency. If the interface needs most of the power for itself, then it doesn't have enough to spare for phantom too. I have never had any issues powering 2 high-end condensers from a single USB interface.

As mentioned in comments power transformers have no problem adapting voltage upwards as well as downwards, so long as the overall wattage/amperage is preserved.
You can even have 240v in your car, google 'power inverter'.

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Others have described the theoretical basis -- that one can convert to a higher voltage with a corresponding drop in amperage. To go into the specifics for this hardware, I own a SSL 2+ and a USB-C power meter, so I'm able to produce actual measurements of the power draw with and without phantom power turned on. :)

These are instantanious snapshots -- if we wanted more solid numbers we'd want to actually record usage change over time, but eyeballing the numbers, they look pretty stable. Note that while I rounded the amperage and voltage numbers to two decimal points, the wattage numbers are based on the full instantaneous measurements.

  • Running without phantom power turned on on either input: ~0.56A @ 4.96V (~2.79W)
  • Running with phantom power turned on for one input: ~0.62A @ 4.95V (~3.05W)
  • Running with phantom power turned on for both inputs: ~0.65A @ 4.94V (~3.22W)
  • Actively recording both inputs with phantom power on: ~0.65A @ 4.93V (~3.20W)

...so:

  • Going from no use of phantom power at all to phantom power on one input, we have an increase in power usage of about 0.26W; adding the second input, that increases by an additional 0.17W; and whether we're actively recording or just monitoring appears to have no impact on power draw.

  • The USB 2.0 battery charging extension allows 1.5A@5V on a standard USB-A port; if your laptop supports this extension -- or you upgrade to a laptop with USB 3.0 support that supports the 4.5W "high-power SuperSpeed" profile -- you'll be more than fine. On the other hand, if your laptop tops out at the .5A@5V "high-power device" profile, you might have trouble.


This test was run with two microphones plugged in: An Aston Origin on the first input, and a Movo LV8-C on the second. The power source (relevant to the voltage drop numbers) was a Dell XPS 13 9370 plugged into its preferred 20V upstream power supply.

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  • Straight to the point, informative and helpful, you have my sincere thanks. OTOH, it seems my 10 year old PC that only has USB 2.0 may be in need of an upgrade. I will be getting SSL2 from ebay soon. – Andrei Sch. Sep 26 at 23:11
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The problem is not the voltage (it can be converted from 5 to 48 inside the interface) but the total amount of power available. If the current supplied by the computer to the USB is enough, the thing may work. But you're right to be concerned, I think this is a borderline situation, and although things may work with just USB power, it could be wise to get an interface with an external power unit, to be sure you'll never have this problem.

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    The SSL 2 uses USB-C -- the 5V/3A that the baseline standard (without the power delivery extension) offers is quite enough. – Charles Duffy Sep 26 at 15:45
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    @Charles, the question is how much current is delivered by the computer. A new computer's USB-C connector will deliver enough current. An older computer or an older laptop with a USB-2 connector, as mentioned in the question, may or may not be enough. – MMazzon Sep 26 at 18:02

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