Choosing an audio interface for guitar on iPhone
What audio interface do I need for guitar on an iPhone?
One that is class compliant, has an instrument or Hi-Z input, and does not demand more power than the phone can supply. Connection is USB-C on current devices and Lightning on older ones. Most small guitar interfaces qualify, but check power draw before buying.
Why you need one at all
A guitar pickup puts out a very quiet, high-impedance signal. A phone has a microphone input designed for a very different job, and nothing that will receive a guitar properly. An audio interface sits between the two: it takes the guitar at the right impedance, amplifies it to a usable level, converts it, and hands it to the phone.
It also solves the other half of the problem, which is getting sound back out. The interface's output is what you plug headphones, monitors or a desk into, and it is the reason latency is manageable at all.
What actually matters
Class compliance
iOS cannot install drivers. An interface that needs a manufacturer's driver on a computer will not work on a phone, full stop. A class-compliant one follows the standard USB audio specification and simply works when connected. Most small interfaces are class compliant, and the ones that are usually say so.
Power
This is where money gets wasted. A phone supplies limited power over its port, and an interface that draws more than it can give will either refuse to start or drop out under load. Interfaces that can take external power, or that are designed with mobile use in mind, avoid the problem entirely. Check power draw before you check anything else.
An instrument input
Sometimes labelled Hi-Z or instrument, as opposed to line or mic. Plugging a guitar into a line input gives you a thin, weak signal that no capture will rescue, and it is a common cause of "the amp models sound bad" complaints that have nothing to do with the models.
The connector
Current iPhones and iPads use USB-C, older ones Lightning. Adapters exist and generally work, but each one is another thing to carry and another thing to fail at a bad moment. If you are buying now, buy for the port you have.
What does not matter much
Converter specifications, sample rates beyond the ordinary, and channel counts you will never use. For playing a captured amp through headphones, the difference between a competent interface and an expensive one is small, and it is dwarfed by whether you set your input level correctly.
A good headphone output is worth more than a good spec sheet, because that is the part you actually listen through.
How do I know my level is right?
Play as hard as you ever play and watch the input meter. You want the loudest playing to sit comfortably below the top with room to spare, not brushing it. Captures are trained at a particular level, and feeding one a signal that is too hot or too quiet makes it sound wrong in ways people usually blame on the capture.