Connecting Multiple Devices to an iPhone or iPad: Hubs, Mixers, Sync and the One-Interface Rule

Here is the fact that clears up most of the confusion, and almost nobody states it plainly: on iOS and iPadOS you can only use one USB audio interface at a time. There is no aggregate device like there is on a Mac, where you can bundle two interfaces into one. Plug in a second audio interface and iOS simply switches to it, dropping the first. Apps like AUM follow whatever iOS hands them, so AUM jumps to the last interface you connected.

MIDI is a different story, and this is the part that trips people up. Over USB you can address many MIDI devices at once through a hub, sequencing and controlling a whole rack of gear. Audio and MIDI travel down the same USB cable but they play by different rules. Many MIDI devices, one audio device. Hold onto that sentence, because the rest of this guide is really just working around it.

This is the question I see asked more than any other, usually with a note that nobody will give a straight answer. Here it is. You can plug a USB-C hub into your iPad, connect several devices to it, and use all of them for MIDI in AUM at the same time. What you cannot do is get audio from more than one of those interfaces at once. Only one is the live audio device.

So the dream of a hub with five instruments all streaming audio into AUM as separate channels does not work the way people picture it. For MIDI, sequencing and clock, a hub is great. For audio, you still have one interface.

Two practical notes. Use a powered USB-C hub. iOS is stingy with how much current it gives to bus-powered gear, and a powered hub keeps hungry devices stable. Unpowered hubs are fine for light MIDI use but fall over when you hang real instruments off them. None of this changed in iPadOS 17 or 18. The one-audio-interface rule is still the rule.

If iOS only takes one audio interface, the trick is to make one device carry everyone's audio. You mix your instruments together first, in hardware, then send that single mixed signal into the iPad or iPhone through one interface. The mixer becomes your audio hub. This is why small mixers with USB audio built in, like the Teenage Engineering TX-6 or the far cheaper K.O. Sidekick, have become the centre of so many iOS setups. They solve the exact problem iOS creates.

So which device should be the hub? You have a few options, each with a catch.

• A plain USB-C hub. Perfect for MIDI, sequencing and clock across many devices. Useless for combining audio, since only one interface is ever live.
• A small mixer with USB audio (TX-6, K.O. Sidekick or any class-compliant USB mixer). This is the real answer for audio. Everything sums into one box. One cable goes to the phone.
• A do-it-all instrument as the hub. An OP-XY or OP-1 Field can sequence other gear, pass audio and act as the USB device into iOS, so for a two or three piece rig you may not need a separate mixer at all.
• A computer. A Mac can aggregate interfaces and do anything, but then you are not really on iOS anymore, which defeats the point of a phone or tablet setup.

The best way to plan a setup is backwards. Do not start from the gear. Start from what you are trying to make, then build the smallest chain that gets you there. Here are the common ones.

You want to jam with a few devices and record the whole thing. This is the everyday setup. Your instruments run into a small USB mixer, the mixer runs one USB-C cable into your iPhone or iPad, and a recorder or AUM catches the performance. Pick one device to be the clock so everything stays in time. You play, you capture, you move on.

You want everything locked in time. Choose one device as the clock master, an OP-XY or the TX-6's beat tracking both work well, and send MIDI clock out to the rest. A USB-C hub is fine here because clock is MIDI, not audio. Pocket Operators join the timing through their own analog sync rather than MIDI. When something drifts it is almost always two devices both trying to be the master.

You want each device on its own track on the iPad. This is where channel count matters. A TX-6 presents up to twelve channels to iOS, so every source can land on its own track in a multitrack app. A K.O. Sidekick does a smaller version, up to six mono channels or three stereo pairs plus the mix. Worth knowing: a dedicated recorder like Reel is a four-track, so it captures your mix or a handful of stems rather than twelve separate feeds. Match the tool to how many tracks you really need.

You want a portable DJ-style rig with samplers. This is the K.O. Sidekick's home turf. Snap it between two EP-133 K.O. II samplers using the EP Pegs connectors, use the performance effects for transitions, and run the master out to a speaker or into your phone to record the set.

You want the smallest possible rig. Sometimes the answer is no mixer at all. A single do-it-all box like an OP-XY or a K.O. II can sequence, make sound and act as the USB audio device straight into your phone. One cable, one instrument, nothing to manage. The best setup is usually the fewest boxes that do the job.

The two obvious Teenage Engineering choices for that hub sit at very different prices. The TX-6 is around $1,199. The EP-136 K.O. Sidekick is $179. They are not the same tool at two prices. They are built for different jobs.

Both are class-compliant USB mixers that sum your gear into the one interface iOS allows, both have per-channel EQ and compression, and both multitrack into iOS. The TX-6 buys you more channels, twelve to the Sidekick's six, plus a machined-aluminium build, beat-tracking clock mastering and TP-7 integration, though it has famously tiny knobs. The Sidekick is plastic and far cheaper, with bigger, more playable controls, hands-on performance effects and the ability to chain for more channels. For most iPhone and iPad rigs the Sidekick does the core job for a seventh of the price. The TX-6 is for bigger, gig-focused setups.

For the full breakdown, the effects, the build and whether three Sidekicks really beat one TX-6, see the dedicated TX-6 vs K.O. Sidekick comparison in the related reading below.

Before you plan any of this, it helps to know which of your instruments can send audio over USB at all, because the ones that cannot have to go in through a mixer's line input instead.

• OP-1 Field: yes, and generously. It works as an eight-channel USB audio interface, so its tracks can arrive separately.
• OP-XY: yes. It acts as a USB audio host and device, so it passes audio over USB-C.
• EP-133 K.O. II: yes, as of the 2.5 firmware update in 2026, which added USB audio to a device that previously only had analog out. Update it and it becomes a USB audio device.
• Pocket Operators and the PO-33: no. These are analog-out only. They join a setup through a mixer channel, and they sync through their own audio-based sync rather than USB.

Syncing just means keeping every device's tempo and playback locked together so they do not drift. You need it the moment you have two or more things playing patterns at once. There are three common ways it happens.

MIDI clock is the usual one. A master device sends a steady tempo signal, over USB or over a 3.5mm TRS MIDI cable, and everything else follows. This is where a USB hub earns its keep, since clock is MIDI and MIDI is happy to fan out to many devices.

The TX-6 has its own twist with beat tracking, reading tempo from an incoming signal and generating clock for the rest of your gear, which is handy when your source is something that does not send clock itself.

Pocket Operators use analog sync, a click signal carried on an audio channel rather than MIDI. It is simple and reliable but lives in its own little world, so you often need an adapter or a spare channel to bridge it to a MIDI setup.

One honest clarification, because it confuses people: Ableton Link is not a hardware feature on these Teenage Engineering boxes. Link lives on the app and computer side, in things like AUM and my own recorder, syncing iOS and desktop apps to each other over wifi. Your hardware still syncs by MIDI clock or analog sync. When sync goes wrong it is nearly always one of two things: two devices both set as master, or a Pocket Operator's analog sync fighting a MIDI clock. Decide on one master and one clock language and most problems disappear.

Almost every Teenage Engineering box uses 3.5mm jacks, and this is where a surprising number of setups quietly fail. The plug looks the same but the wiring is not.

• TRS, the one with two black rings, carries a stereo signal or a balanced mono one. This is what you want for most audio connections between these devices.
• TS, with a single ring, is mono. Fine for a mono instrument, but use it where a stereo signal is expected and you lose a channel.
• TRRS, with three rings, is the headphone-plus-mic type from phone headsets. It looks almost identical and people grab it by mistake constantly. In an audio chain it will usually give you silence or a broken signal.

Once your devices are summed through a mixer and feeding one clean signal into your phone, something has to catch it. On a computer that is your DAW. On iOS it is an app like AUM for routing and mixing, or a dedicated recorder if you just want to hit record and keep the take.

This is the one place I will mention my own app, because it is genuinely the job it does. Reel is a four-track recorder for iPhone that takes a class-compliant USB input, so the summed output of your TX-6 or Sidekick lands straight into it in clean 32-bit float, with no clipping when the jam gets loud. You sum in hardware, you capture on the phone, you keep the performance. If you want a fuller walkthrough of that side, there is a separate guide on recording a dawless setup on iPhone.

Two deliberate omissions, so you are not left waiting for them. This is a guide about connection and signal flow, not about the deep insides of each instrument, so the engines, effects and undo behaviour of individual boxes are their own topic. Sample management, swapping and loading without a laptop, is a genuinely separate rabbit hole that deserves its own guide rather than a paragraph here. Get the plumbing right first. Everything else is easier once the signal actually reaches your phone.