The Apple USB-C to 3.5 mm Audio Cable Might be The World’s Smallest bi-directional Audio Interface.

In order to use my AirPods Max with things like in-flight entertainment, or any 3.5mm audio jack device, the only supported way seems to be the Apple USB-C to 3.5 mm audio cable. This cable coming out was the reason I finally bought the USB-C AirPods Max in the first place, as unlike the lightning model, the USB-C ones don’t work with a simple lightning-to-3.5mm adapter.

So. What’s this cable? $39 is pricey, but it isn’t far off what apple usually charges for cables in general. This simple cable with a 3.5mm TRS connector and a USB-C connector on the other end actually packs a bunch of tech and might be the world’s smallest bi-directional interface.

A standard headphone jack usually just carries an analog audio signal. The common 3.5mm TRS connector carries the signals for left and right ear, as well as providing a common ground. This signal is really just the electrical current, waves of the audio signal. For unpowered earbuds, that current is enough to drive the earbuds. In contrast, USB is inherently digital and USB devices can be all sorts of things. Without diving too much into the different USB classes and handshakes, one of the things a USB device can be is an audio interface.

The wild thing about the Apple 3.5mm to USB-C cable is that it works both ways. It can be used to, say, plug in an iPhone into an older audio receiver with only an analog input. In this case, the cable acts as a digital-to-analog converter, providing the iPhone with an analog input.

It can also be used to plug an analog device (say, an older iPod) into the AirPods Max headphones’ USB port. In this case, the cable acts as an analog-to-digital converter.

The 3.5mm connector is very thin and the cable feels light. It’s impressive that, when buying this cable, you essentially get a little audio interface.

How does it know which way to operate? Without analyzing the handshake, I can’t tell for sure, but It seems to initialize into different modes depending on which USB device it’s plugged into. So, say, on a MacBook it will act as an output. on the AirPods Max it acts as an input, because that’s the only thing that makes sense. This works every time and automatically, but the downside to this is that you can’t choose what you want it to be. If you want to use it, say, as an input (for example to record audio from a analog cassette player to a Mac), it won’t work, because plugging it into the Mac will turn it into an output by default. This would be useful to, say, record from an analog tape deck, but I have my Rødecaster Duo for things like that, so this is not a big deal.

What is a bigger deal is the way it initializes and the resulting problems with volume.

An input device could have any level of volume on the 3.5mm Jack. So that the AirPods Max don’t blow your eardrums when plugging the cable into certain devices, they seem to adjust the maximum output volume based on… some arcane and obscure initialization process. The issue is that sometimes, when plugging it in it assumes some high volume level and sticks with it (perhaps because it registers the noise from inserting the cable into the jack). When trying it with the SteamDeck a few times, for example, the volume was way too low and even when turning the AirPods Max as well as the SteamDeck all the way to the max, I could barely hear anything. Other times, it was very loud and I could comfortably reduce the volume of the headphones as well as the SteamDeck.

After talking to Apple Support about this, in typical Apple fashion, they said that the SteamDeck isn’t a supported device and it’s supposed to only work with Apple devices or computers, not the SteamDeck.

This is hilarious, because the whole point of this cable is to open up compatibility with other devices!

I since learned that the volume is more predictable when plugging the 3.5mm end into the SteamDeck first and then plugging the USB-C connector into the AirPods Max. This is counterintuitive, because one could argue that the headphones with the cable are the “thing” to plug in.

Overall, this is a nifty little “cable”, though. I put that in quotes, because it’s really more of a tiny USB-powered audio interface with a built in 3.5mm TRS connector.