Option #1 (USB output required)
NOTE: USB audio streaming with iOS devices is not compatible with all turntables with USB output
USB-B to Lightning
By far the simplest setup you can have, but only if your turntable has a USB port! This cable also comes in a USB-B to USB-C style.
Option #2 (USB-C required)
The Sonos USB-C adapters are meant to connect your iOS device directly to a Sonos speaker, but they also work great as line-in adapters for AirChord.
Sonos Line-In Adapter
Accepts your 3.5mm male (headphone) source.
Sonos Ace Aux Cable
Go directly from your 3.5mm source to your USB-C iOS device.
Option #3 (phono preamp required)
iRig Stream
No need for additional adapters. This is an all-in-one solution that sounds great. (Thanks to AirChord user Mike G. for the great recommendation!)
Option #4
The following options could be better/cheaper if, say, you already have USB output from your turntable or a Lightning to USB adapter. The Behringer adapter converts your RCA turntable output into a digital signal over USB-A. The Lightning to USB adapter will then convert USB-A to Lightning for input to your iOS device. (If either of these are out of stock on Amazon and elsewhere, there are plenty of alternative options available.)
Behringer U-Phono UFO202 Audiophile USB/Audio Interface with Built-in Phono Preamp
A solid entry-level preamp (and what I personally use in my setup)
Apple Lightning to USB Camera Adapter
A versatile adapter that can be used to connect just about any USB peripheral to your iPhone/iPad. NOTE: only the genuine Apple version has been tested and is known to work.
Option #5
I get a lot of emails asking why going straight into the headphone port doesn’t work, since that is apparently what is done in the YouTube demonstration video. If you look closely, there is actually this little in-line adapter before the headphone jack.
Headset Buddy Line-level Adapter
If you just want to use your headphone jack (or Lightning-to-headphone-jack adapter), here’s your adapter. Note: this adapter will convert your stereo audio to mono.