Apple's Lightning to Headphone Jack adapter, aka The Dongle.
It's $9. So how bad could it possibly be?
2nd column: Apple Lightning to Headphone Jack adapter, attached to iPhone 6S
4th column: Apple Lightning to Headphone Jack adapter, attached to iPad Pro 9.7"
All measurements at maximum volume, no load, and with iOS 10.
More accurate distortion measurements appear below.
The frequency response from the same set of measurements. They remain flat for all devices with a 16-ohm load on each channel, measured when the volume is lowered.
Apple Lightning to Headphone Jack adapter playing 1 kHz sine wave from iPhone 6S, no load.
Apple Lightning to Headphone Jack adapter playing 1 kHz sine wave from iPad Pro 9.7", no load.
Compare that to the iPhone 6S headphone out on the left and the iPad Pro 9.7" headphone out on the right.
Apple Lightning to Headphone Jack adapter playing 1 kHz sine wave from iPhone 6S, maximum volume, 16 ohm load on both channels
Volume one click down: Apple Lightning to Headphone Jack adapter playing 1 kHz sine wave from iPhone 6S, one click down from maximum volume, 16 ohm load on both channels
Apple Lightning to Headphone Jack adapter playing 1 kHz sine wave from iPhone 6S, 16 ohm load on both channels, onset of clipping. I used the volume control slider to get the distortion just under 1% on the left channel. It's not easy to be really precise, but it's at a level between maximum volume and one click down.
SMPTE IMD, no load
CCIF IMD, no load
Playback of 16-bit, 44.1 kHz J-test file
Playback of 24-bit, 48 kHz J-test file
Impulse response, 1 kHz square wave, rising edge of 60 Hz square wave "fake step response"
So far, the performance seems to be in the same ballpark as the iPhone and the iPad. Here's some good news for those of you who like those crazy multi-driver balanced armature IEMs:
1.
Left: Apple Lighting to Headphone Jack adapter, 1 kHz sine, 4 clicks down from maximum volume, no load.
Right: Apple Lighting to Headphone Jack adapter, 1 kHz sine, 4 clicks down from maximum volume, measured 16.6-ohm load on each channels.
Therefore,
http://www.wolframalpha.com/input/?i=200+*+16.6+%2F+(R%2B16.6)+%3D+196
Which is lower than that of any other device running any version of iOS. It's almost an order of magnitude better than on the iPhone 6S.
2.
Subjective take on the noise level: I used the Ultimate Ears UE600 IEM to evaluate the noise level because it is very sensitive to hiss.
The iPhone 6S had a good bit of audible electronic noise when it first came out. It was very noticeable on the task switcher screen and worse still when Siri was listening. This was a problem with IEMs like the UE600, Shure SE535, etc., but not for headphones, dynamic-driver IEMs, and less sensitive balanced armature IEMs like the Etymotic ER4P. With each update to IOS, the situation seemed to get better. Using IOS 10, I can still hear the noise on the iPhone 6S, but it's not nearly as bad as it was before. It's still there, but it's hard to notice when the phone isn't doing anything but playing music. The iPad Pro doesn't have this problem. It has more of a steady background hiss and it's quieter too.
The Lightning to Headphone Jack adapter is more like the iPad. If you listen closely on a sensitive IEM, you'll hear it. It's also louder when the phone is on the task switcher or when Siri is listening. But it's much softer than the noise on the iPhone 6S headphone jack; it's also a soft, steady background hiss like on the iPad, not the irregular-sounding electronic noise on the iPhone 6S.
This adapter can come in handy if you want to use a very sensitive IEM that has impedance that changes much with frequency. On the iPad it's not as useful because the Lightning connector is in a more inconvenient spot.
EDIT 9/19: Added IMD graphs, cleaned up THD pics, corrected typos
EDIT 9/24: Changed CCIF IMD graphs to show up to 30 kHz, added J-test results, recalculated output impedance with measurements on new setup.