Review and measurements of MacBook Air (2024) headphone output

[Roland301]

I wish they didn't have the impedance sensing. There needs to be an option to control it manually.

I couldn't use the headphone jack on my old LG phone because it wouldn't put out enough voltage for my 30 ohm planars, despite being perfectly capable of driving them with the extension cable + dummy load workaround. But at that point it was easier to just use a dongle...

 

[staticV3]

I wish they didn't have the impedance sensing. There needs to be an option to control it manually.

That would make it possible for the headphone output to clip, which Apple would never allow.

The best solution IMO would be a more granular impedance sense which keeps the output at 90+% of clipping across the entire impedance range.

 

[fws]

It only does this check when you physically plug in the jack, which caught me out a couple of times in testing!

Do you mean there is a way to force it? Like switching headphones when powered off or connecting one with adapter and then changing to another? Apple certainly dropped the ball here, not taking into consideration 2kOhm headphones.

 

[mcdn]

Do you mean there is a way to force it? Like switching headphones when powered off or connecting one with adapter and then changing to another? Apple certainly dropped the ball here, not taking into consideration 2kOhm headphones.

Say you plug some 300ohm headphones into an extension cord, plug the cord into the MacBook, remove the headphones and replace them with 30ohm headphones, then the MacBook will run at 3V into your 30ohm headphones. It gives more power but I didn’t put it in the review because it’s not how the system is designed to work.

 

[Roland301]

That would make it possible for the headphone output to clip, which Apple would never allow.

The best solution IMO would be a more granular impedance sense which keeps the output at 90+% of clipping across the entire impedance range.

To be fair, the thresholds Apple set are reasonable, aside from limiting the 'line out" mode (>1k ohm) to 1V.

With LG, plugging in a 30 ohm headphone meant you were limited to something like 150mV which was only usable with IEMs.

 

[fws]

Ye, reasonably we are looking mostly at some planars possibly being an issue, and realistically that is a niche within a niche.
When it comes to popular dynamic headphones, if we would take 110dB SPL as target, then the main suspect would be HD599 50Ohm 106dB/V = ~1.6V, while something like DT770/990 80Ohm 96dB/mW = ~1.4V would likely be still fine. Some of my older Sennheisers go like HD330 100Ohm 94dB/mW = 2V, but I'm quite sure they are much more sensitive than that, and likely Sennheiser was just printing 94dB/mW in the spec of all their headphones for some years. But no going hip with HD414 on Mac I guess.

A software control where you can set the output to Line Out mode and chose between some preset line level "standards" like 0.775, 1, 2V would be good for professionals while still user friendly solution.

 

[xxie]

This is a review and measurements of the MacBook Air 2024 headphone output. It was prompted by a request from @DrSpan, as the little Mac has a very strong reputation as a headphone source. Here it is, measuring itself:

View attachment 454294

Since 2021 Apple has supported impedance detection on the headphone outputs of its laptops, meaning low impedance headphones get more current, while high impedance headphones get more voltage. When you plug in the headphones the MacBook checks the impedance, and sets the output voltage to 1.25V if it's less than 150ohms or 3V if it's up to 1kOhms/. Above that it assumes you're using it as line interface and sets the voltage to 1V. It only does this check when you physically plug in the jack, which caught me out a couple of times in testing!

So the first question is does the impedance sensing work? It absolutely does, and the voltages are superbly accurate at 1.00V (4.7kOhms) and 1.25V (30ohms), dropping a tiny bit from spec to 2.97V at 300ohms.

The second question people have is how much power it can actually deliver? I won't make you wait, it delivers exactly to spec into 300ohms at 29mW, so essentially no voltage drop. Into 30ohms I got 48mW against an ideal of 52mW, and into 15ohms the gap widened a bit more with 85mW against an ideal of 104mW.

OK, let's do some graphs! The measurements were taken with a Cosmos ADCiso and a resistive dummy load. The ADC was running at 96kHz/24bits, with the laptop outputting 44.1kHz/float32, which is the default. The laptop was connected to external power via a thunderbolt hub. The headphone output itself is a 3.5mm TRS jack, i.e. unbalanced stereo.

Here's the best SINAD I could get, which was at 0dBFS output into 30ohms, delivering a SINAD of 97.9dB:

View attachment 454454
Frequency response is ruler flat to 16KHz, then drops about 6dB by 20KHz. If you change the output sample rate in "Audio MIDI setup" then you get the same result, e.g. the drop starts around 44kHz when using a 96kHz sample rate. The good news is the filter is steep and clean with no rise in noise after. It does look quite tailored, but I didn't get into impulse tests:

View attachment 454290

Here's THD+N vas power at 15, 30 and 300ohms:
View attachment 454286

And here's CCIF IMD (19 and 20kHz tones) at 30 and 300ohms:
View attachment 454287
The multitone distortion is very clean - I've overlayed all three impedances but they are pretty much the same:
View attachment 454288
Crosstalk was about -50dB. This was measured by driving one output at 0dB while measuring the other, so they share about 1m of cable before being split out for measurement:
View attachment 454289

At 50mV into 30ohms (with the Cosmos Scaler boosting the signal to about 1V into the ADC) I was able to get a SINAD of 89.1dB, which is less than the 95dB L7 Audio Lab measured but I think their measurement may be A weighted, and they have an APx555:
View attachment 454453

In conclusion definitely 5 quokkas from me! There's simply no reason to use an external headphone amp with any MacBook made since 2021 unless you have demanding headphones. It's not nearly as good for power as a good desktop headphone amp, but it is better than popular interfaces such as the Scarlett 2i2.

I actually wrote the driver for this headphone amplifier at Apple which was my last job there around 2020. It was done during the Covid time and I thoroughly enjoyed it just because I have some old AKG k240/K501 at 600ohm/120ohm lying around somewhere. I remembered I specifically set the max output to 4Vrms, but it became 3.2Vrms when released which was after I left, I am still curious why.

One issue was planar magnetic headphone has low impedance but requires high output voltage, I raised this issue but the consensus seems like very few customers has planar and they probably have dedicated amplifier already.

 

[mcdn]

I actually wrote the driver for this headphone amplifier at Apple which was my last job there around 2020. It was done during the Covid time and I thoroughly enjoyed it just because I have some old AKG k240/K501 at 600ohm/120ohm lying around somewhere. I remembered I specifically set the max output to 4Vrms, but it became 3.2Vrms when released which was after I left, I am still curious why.

One issue was planar magnetic headphone has low impedance but requires high output voltage, I raised this issue but the consensus seems like very few customers has planar and they probably have dedicated amplifier already.

The circuit seems perfectly capable of driving low impedance headphones at 3v, any idea why 1.25v was chosen for them?

 

[staticV3]

The circuit seems perfectly capable of driving low impedance headphones at 3v

I'd be surprised honestly, and have seen no indication thereof.

3V@32Ω=280mW per channel
3V@16Ω=560mW per channel

 

[xxie]

The circuit seems perfectly capable of driving low impedance headphones at 3v, any idea why 1.25v was chosen for them?

Mainly because it might damage some headphone (or your hearing) considering the customer base is so big.

I think you might already noticed it in your test, the impedance detection is done at the time 3.5mm plug is inserted. In theory, you can fake it by plug a 300ohm fake load and swap it to 32ohms real headphone.

 

[mcdn]

I'd be surprised honestly, and have seen no indication thereof.

3V@32Ω=280mW per channel
3V@16Ω=560mW per channel

Well no, not to every impedance! I meant it could be current limited to say 160mA (which L7 shows still performs fine) while leaving the voltage constant. But @xxie has given the answer, which makes sense

 

[PeakDodger]

Same for me with a M1 MacBook and 650s.

Mmh, Apple doesn't list M1 MacBook Air as able to drive high impedance headphones. Neither my M1 iMac. But I have an M2 Air and have Senns HD660S2 incoming today, will make some comparisons with both

 

[Steve356]

Mmh, Apple doesn't list M1 MacBook Air as able to drive high impedance headphones. Neither my M1 iMac. But I have an M2 Air and have Senns HD660S2 incoming today, will make some comparisons with both

Mine isn't an Air if that matters, it's a M1 MacBook Pro. All I know is that it has zero problems driving my 650s from the headphone output even with EQ applied.

 

[PeakDodger]

Mine isn't an Air if that matters, it's a M1 MacBook Pro. All I know is that it has zero problems driving my 650s from the headphone output even with EQ applied.

Then yeah, M1 Pro is 2021 and it has the updated amp for high impedance headphones, you're good. M1 MacBook Air and M1 iMac sadly do not.

 

[stefanovesa]

Then yeah, M1 Pro is 2021 and it has the updated amp for high impedance headphones, you're good. M1 MacBook Air and M1 iMac sadly do not.

for MacBook Air only from M2 ongoing. Surely M3 has it because I own it and it drives HD650 also very loud

 

[PeakDodger]

for MacBook Air only from M2 ongoing. Surely M3 has it because I own it and it drives HD650 also very loud

Yeah M2 too (I have it and will try the 660S2 on them). All those listed here https://support.apple.com/en-us/108351

3V of voltage swing is not bad at all. I'll still get a Qudelix for my iMac and for the phone, which reach 4V on balanced, but I don't think they will sound perceivably better than on board M2, especially at low volumes

 

[stefanovesa]

Going a bit OOT, but never understood why the internal DAC is screwed to 96 kHz bitrate. One of the my initial reasons for purchasing a USB DAC is for going to 192 kHz at least: so many audio files are at that sampling rate.

 

[CedarX]

Going a bit OOT, but never understood why the internal DAC is screwed to 96 kHz bitrate. One of the my initial reasons for purchasing a USB DAC is for going to 192 kHz at least: so many audio files are at that sampling rate.

Which DAC are you referring to?

 

[stefanovesa]

Which DAC are you referring to?

the DAC inside the MacBook

 

[caocaolatre199x]

Are you aware of the trick to plug a headphone extension cable in (no headphone connected) and the LG phone will detect high impedance? Then plug your headphones in. Works great on my beloved LG G7, which I now use a dedicated DAP.

Thank you. I will try to do that.