By Tug·July 10, 2026·7 min read

How to Record an EP-133 K.O. II into Your iPhone

The EP-133 K.O. II records straight into an iPhone over USB with no computer and no interface, but only if it is on the right firmware. Teenage Engineering added class-compliant USB audio in the OS 2.5 update, so once your K.O. II is up to date it shows up on the phone like any audio interface. Here is the firmware check that trips people up, the cable for USB-C and Lightning iPhones and how to keep the signal clean.

Update to firmware 2.5 first

This is the step that catches everyone. USB audio was added to the EP-133 K.O. II in the OS 2.5 update, which landed in June 2026. If your unit is on 2.4 or earlier its USB-C port only does data and charging, so the phone gets no audio at all no matter what you plug in.

Update it first. The firmware is a free download from Teenage Engineering's EP-133 downloads page, installed over USB. Once it is on 2.5 or later, the USB audio is there and you are ready.

What you need

Not much and no interface.

An EP-133 K.O. II on firmware 2.5 or later.
Fresh AAA batteries for it. More on why below.
An iPhone, plus the right cable. The EP-133 has a USB-C port.
A recording app. Reel is the one I make and use for this, but any class-compliant iOS audio app works.

USB-C iPhone: one cable

If your iPhone is USB-C (iPhone 15 or later) you need a single USB-C to USB-C cable. Plug one end into the EP-133 and the other into the phone. That is the whole connection.

Lightning iPhone: camera adapter

If your iPhone is Lightning (iPhone 14 or earlier) a plain cable will not do it. You need an Apple USB camera adapter in between and the Lightning to USB 3 Camera Adapter is the one to get because its extra power port keeps the phone charged. Run a USB cable from the adapter to the EP-133 and plug a charger into the adapter's power port.

Power it from batteries

The EP-133 can take power over USB-C, but an iPhone is a weak power host and asking it to feed the EP-133 tends to be unreliable. The steady setup is to run the EP-133 on its own AAA batteries and let the USB cable carry audio only.

Batteries have a second benefit. Some people hit a faint hum or USB noise when recording over USB and it is almost always a power or ground issue. Running the EP-133 on batteries and trying a different cable or phone port usually clears it.

Record it, no setting to flip

Once it is updated and connected, there is no menu mode to switch on to send audio out. The EP-133 simply appears as an input. Open your app, pick the K.O. II as the input and record. In Reel it shows up automatically and each take lands as a clean capture and there is no sample rate to set because Reel takes the K.O. II's rate for you.

You capture the stereo master mix, so set your levels and balance on the EP-133 before you record rather than expecting separate tracks. One thing not to touch: the SMP to USB setting with its MON option is for sending audio the other way, into the EP-133, so leave it alone when you are recording out to the phone.

See it in action

Here is the OS 2.5 USB audio going straight from a K.O. II into Reel, no interface in the chain.

Frequently Asked Questions

Does the EP-133 K.O. II work with an iPhone?

Yes, as long as it is on firmware 2.5 or later. Teenage Engineering added class-compliant USB audio in the OS 2.5 update in June 2026, so the K.O. II records into an iPhone with no drivers and no interface. On firmware 2.4 or earlier there is no USB audio at all.

Why is my EP-133 not sending audio to my iPhone?

The most common reason is firmware. USB audio only exists on OS 2.5 and later, so update the K.O. II from Teenage Engineering's downloads page first. Also make sure you are not in the SMP to USB MON setting, which routes audio into the EP-133 rather than out to the phone.

What cable do I need to connect an EP-133 to an iPhone?

The EP-133 has a USB-C port. For a USB-C iPhone use a USB-C to USB-C cable. For a Lightning iPhone use an Apple Lightning to USB 3 Camera Adapter with a USB cable to the EP-133. Power the EP-133 from its own AAA batteries for a stable connection.

Can I record the EP-133's tracks or pads separately into my phone?

No. Over USB the EP-133 K.O. II streams a stereo master mix, which is what an iPhone records. Set your mix and levels on the EP-133 before recording. For separate stems you would need to record passes one at a time.