iPhone 6S Plus Audio Quality
iPhone 6S. I'd get the iPhone 6S Plus at Amazon or at this link directly to them at eBay (see How to Win at eBay).
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January 2017 audio reviews Apple headphones tube amps all reviews
Introduction
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These are measurements of the 3.5mm analog audio output of my iPhone 6S Plus playing signals in its Music app.
Few people realize that Apple iOS devices have some of the best audio digital-to-analog converters and low-impedance headphone amplifiers available at any price.
Here are some quick measurements of my iPhone 6S Plus made in my laboratory with a state-of-the-art Rohde & Schwarz UPL audio analyzer.
Maximum Output Levels
At 0 dBFS at maximum volume setting with various resistive loads:
200kΩ: 0.989 V RMS @ 0.0012% THD.
600Ω: 0.979 V RMS @ 0.0012% THD.
300Ω: 0.969 V RMS @ 0.0012% THD.
37.5Ω: 0.834 V RMS @ 0.0012% THD.
This is exceptional; almost nothing else has this little THD, and very few portable devices have this much clean output in to low-impedance loads.
Output Source Impedance
Output source impedance measures 5.9Ω from 31 to 1,000 Hz, and rose to 6.8Ω at 20,000 Hz, all very low.
Higher output source impedances lead to frequency response imbalances caused by the varying real impedance of actual headphones with frequency, and also leads to low-frequency distortion when driving real headphones.
The ultra-low output source impedance of the iPhone 6S Plus is so low as to stop any of these potential problems
Frequency Response
iPhone 6S Plus driving a resistive load.
Frequency response is super-flat, ±0.14 dB from infrasonics to at least 19,350 Hz.
I measured only -0.50 dB @ 20 kc, -1.35 dB @ 20.59 kc and a sharp cutoff of -93 @ 21.82 kc with steady sine waves.
It's easy to make an amplifier which tests well driving resistive loads. Driving actual headphones is where it gets difficult, and where the iPhone 6S Plus excels.
Let's really push it and measure frequency response while driving some real headphones, the 32Ω beyerdynamic T51i:
iPhone 6S Plus driving the 32Ω beyerdynamic T51i headphones.
YES! The low output source impedance leads to ruler-flat response driving real loads, much flatter than most gear with higher output source impedances.
Let's increase the vertical scale just to look more closely:
iPhone 6S Plus driving the 32Ω beyerdynamic T51i headphones.
Marvelous, also better than ±0.2 dB driving a real set of low-impedance headphones. Bravo!
Harmonic Distortion Driving 200kΩ
Driving 200kΩ is what we see when using the iPhone 6S Plus' analog 3.5mm output to drive the line input of a HiFi system.
0.0012% THD and harmonic distortion components at 1 kHz at 0 dBFS into 200KΩ at maximum output level.
The harmonic distortion is mostly second-order, just like a tube amplifier. Bravo!
0.003% THD and harmonic distortion components at 4 kHz at 0 dBFS into 200KΩ at maximum output level.
0.0065% THD and harmonic distortion components at 31 Hz at 0 dBFS into 200KΩ at maximum output level.
Harmonic Distortion Driving 300 Ω
Driving 300 Ω is what we see typically when using the iPhone 6S Plus' analog 3.5mm output to drive high impedance headphones. The 300 Ω resistive load I'm using is a much easier thing to drive than real headphones.
0.0012% THD and harmonic distortion components at 1 kHz at 0 dBFS into 300Ω at maximum output level.
The THD driving 300 Ω is just as low as into 200kΩ and harmonic distortion again is mostly second-order, just like a tube amplifier. Bravo!
0.0033% THD and harmonic distortion components at 4 kHz at 0 dBFS into 300Ω at maximum output level.
Again, 300Ω measures the same as 200kΩ.
0.0066% THD and harmonic distortion components at 31 Hz at 0 dBFS into 300Ω at maximum output level.
300Ω measures the same as 200kΩ. Brilliant!
Harmonic Distortion Driving Real 32Ω Headphones
It's easy to make an amplifier which tests well driving resistive loads. Driving actual headphones is where it gets difficult, and where the iPhone 6S Plus excels.
Let's really push it and measure distortion while driving some real headphones, the 32Ω beyerdynamic T51i:
0.5% THD and harmonic distortion components at 31 Hz at 0 dBFS driving the 32Ω beyerdynamic T51i headphones at maximum volume setting.
AHA!!! While 0.5% seems high, very few headphone amplifiers can come anywhere near this close at 31 Hz driving a real set of headphones. This is extremely good performance for this full-scale torture test. THD is of course much lower at higher frequencies, driving headphones at low frequencies is where this becomes difficult for headphone amplifiers.
0.08% THD and harmonic distortion components at 31 Hz at 0 dBFS driving the 32Ω beyerdynamic T51i headphones at -20dB volume setting.
This is a much more reasonable level; there it no music at full scale at 31 Hz as tested above.
This is excellent real-world performance — which few portable audiophile products can match at any price!
Analysis & Recommendations
As expected, the analog audio output of the iPhone 6S Plus is extraordinarily good. It has only half the distortion of the already extraordinary iPhone 6 Plus.
Apple has more smart people and more resources than any other audio company on the planet, so as we see when it comes to audio engineering, the iPhone easily outdoes many so-called "audiophile" products.
For enjoying music, you will probably get poorer performance if you waste your time and money with outboard DACs or headphone amplifiers; the iPhone already has the best there is.
Why do commercial audio magazines tout external DACs and amplifiers? Because that what their advertisers are trying to sell!
The only reason to get an outboard headphone amplifier for use with your iPhone 6S Plus is if you have high-impedance (100Ω or greater) headphones like the 600 Ω beyerdynamic DT880 which often require more voltage than the 1V RMS maximum from iOS devices. In this case, you still don't need a DAC; the analog output from the iPhone's 3.5mm jack will probably be better than what you'd get fromo an expensive outboard DAC!
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