Every audio interface needs power for its preamps, converters and 48 V phantom supply. Small ones take it from the USB cable, which is called bus power. Bigger ones bring their own, either a brick on a cable or a supply built into the case with a mains lead. For a used buyer, this matters mostly because a box that needs its own supply is only as complete as the supply in the listing.
What bus power gets you
A bus-powered interface has one cable to the computer and nothing else to lose. Focusrite lists the Scarlett 2i2 4th Gen as drawing up to 900 mA over the bus, and its 2nd Gen Solo manual says any compatible Mac or PC port should power it. The limits show up on laptops and tablets. Focusrite warns that some laptops deliver less current to USB devices on battery, and recommends running the laptop from the wall while you record. MOTU says its M-series works with an iPad through Apple's adapter kit and a powered USB hub. UA's advice for its USB Apollos is to plug straight into the computer and avoid hubs and docks.
Some newer boxes give you a second option. Several Scarlett 4th Gen models have a 5 V USB-C power input for when the computer can't supply enough, or for running from a phone, and Focusrite suggests a charger rated for 5 W with the 2i2 4th Gen.
How each model is powered
Here's how the interfaces we cover get their power, from the makers' manuals and support pages.
USB bus power, no supply needed
- Focusrite Scarlett Solo 1st, 2nd and 3rd Gen
- Focusrite Scarlett 2i2 1st, 2nd, 3rd and 4th Gen
- Focusrite Scarlett 4i4 3rd Gen
- MOTU M2 and M4
- PreSonus AudioBox USB 96, Studio 24c and Studio 26c
External power brick
- UA Apollo Solo USB, 12 V DC at 1.6 A, centre positive
- UA Apollo Twin USB and Apollo Twin MkII (Duo and Quad), 12 V DC at 1.5 A, centre positive
- UA Apollo Twin X Duo and Apollo x4, 12 V DC at 4.2 A, centre positive
- UA Apollo x6 and Apollo x8, a 12 V external supply on a locking 4-pin XLR plug
- MOTU UltraLite-mk5, 15 V DC at 1 A, tip positive
- PreSonus Studio 68c, Studio 1810c and Studio 192
Supply built in, mains cable
- Focusrite Scarlett 18i20 3rd Gen, with a standard IEC mains socket
- MOTU 828es, 8pre-es and 1248, each with an auto-switching internal supply
The pattern is simple. Once a box has more than a couple of mic preamps, DSP on board, or a desktop flagship's worth of outputs, it brings its own power. That's part of what you're paying for when you move up to the prosumer and pro tiers.
What a missing or wrong supply means
An externally powered interface won't run at all without its supply, so a listing without one is an incomplete unit. Factor in a factory replacement from the maker or a dealer before you commit, or pick a listing that shows the original.
If you do use a generic adapter, UA spells out what has to match, and the same logic applies to any brand. The voltage has to be right. The current rating has to be right. The polarity has to match, which for UA's desktop Apollos means centre positive, and the barrel has to fit snugly, which for those Apollos means 5.5 mm outside and 2.1 mm inside. UA also says it isn't responsible for damage from a non-factory supply, and its rack manuals tell you to use only the factory supply to avoid circuit damage. A cheap adapter with the right plug and the wrong voltage, or with a reversed centre pin, can damage an interface.
MOTU is more relaxed about the UltraLite-mk5. Its manual says the DC jack accepts any standard 15 V, 1 A, tip-positive supply, and the manual rates the DC input at 10 to 24 V.
Two more things trip people up. UA's desktop Apollos, from the Solo USB to the x4, use a locking barrel plug, and UA says one that isn't twisted home can cause the unit to lose power now and then or not power on at all. And imports are less of a worry than people think. Focusrite's DC adaptors switch automatically between 100 and 240 V, so a unit from another country only needs the right plug, while UA's Twin X and x4 bricks take a standard C7 mains lead you can swap.
Phantom power on bus power
Bus-powered interfaces still give you 48 V phantom power for condenser mics. PreSonus says a condenser doesn't need an inline preamp on a bus-powered box, because the interface supplies its phantom power. Dynamic mics are the harder case, since they need more gain than some bus-powered preamps provide.
How phantom is switched varies. The AudioBox USB 96 uses one button for both inputs, while the MOTU M2 and UltraLite-mk5 switch it per input. A Scarlett 4th Gen mutes the input for three seconds after you turn 48 V on, to protect your speakers, and Auto Gain waits until the mute ends. None of this means a fault, so don't let a seller's video of that pause put you off.
When an external supply is worth it
You don't choose external power for its own sake. It arrives with features that need it, like the extra preamps on an 18i20 or a 1810c, the UAD DSP inside an Apollo, or the 18 inputs and 22 outputs of an UltraLite-mk5. If a bus-powered box covers everything you record at once, it's the simpler buy, and our first interface guide covers those. If you're weighing an Apollo for its DSP, read do you need an Apollo first.
What to check in the listing
- The supply in the photos. For any box in the brick or built-in lists above, look for the original supply or mains lead in the photos. Ask the seller if it isn't shown.
- The label. Ask for a close photo of the brick's label and check the voltage, current and polarity against the maker's spec.
- The locking plug. On a desktop Apollo, ask whether the plug locks with a twist and holds.
- How it was run. For a bus-powered box, ask whether the seller used a hub or a direct port. Dropouts on a hub don't always mean a faulty interface.
- Rack ears. The Scarlett 18i20 3rd Gen comes with a set. If you'll rack it, look for them in the photos.
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