Apple has released a support document explaining in detail the technology behind the high impedance headphone output on the new MacBook Pro using the new Apple Silicon powered M1 Pro and M1 Max chips.

In the document published October 27th 2021 and titled Use high-impedance headphones with new MacBook Pro models Apple explains;

The 3.5 mm headphone jack on the MacBook Pro (14-inch, 2021) or MacBook Pro (16-inch, 2021) features DC load detection and adaptive voltage output. Your Mac can detect the impedance of the connected device and will adapt its output for low and high-impedance headphones, as well as for line-level audio devices.

When you connect headphones with an impedance of less than 150 ohms, the headphone jack provides up to 1.25 volts RMS. For headphones with an impedance of 150 to 1k ohms, the headphone jack delivers 3 volts RMS. This may remove the need for an external headphone amplifier.

With impedance detection, adaptive voltage output, and a built-in digital-to-analog converter that supports sample rates of up to 96 kHz, you can enjoy high-fidelity, full-resolution audio directly from the headphone jack on your MacBook Pro.

More information can be found on the Apple support website.

What Does Headphone Impedance Mean?

The impedance of headphones is often overlooked but it shouldn’t be. So what does this mean and why is it significant?

Lower impedance headphones are louder - right? So get low impedance headphones so you’ll be able to run them nice and loud from your phone or laptop? This is true but it’s only part of the story. Impedance isn’t well understood. If you don’t know what it is then thinking of it as resistance for alternating current (audio signals are alternating current) is a good place to start. Headphone impedance affects the efficiency of headphones so low impedance headphones can draw more current from a given voltage. That makes them better at turning electricity into noise i.e. more efficient.

Low Impedance Is Louder, And Louder Is Better - Right?

So is that it? If so it looks like you should get low impedance headphones. Well things like this are always about compromise and the compromise here is that to achieve a low impedance the headphone driver’s voice coil has to be constructed is a particular way and if you are prioritising low impedance to get the most efficient driver possible then this is probably at the expense of something and in this case its at the expense of distortion.

It’s interesting to note that while loudspeakers occupy a narrow range of impedances, typically 4-8Ω headphones are available from 8-600Ω. That’s quite a difference. Anything over 25Ω is now considered high impedance and the impedance is telling you something about the construction of the drivers in the headphones, specifically the wire used in the voice coils. Very fine voice coil wire is desirable when designing a low distortion headphone driver because it has a lower mass and is more tightly packed than the equivalent number of turns in a thicker wire. The lighter mass improves performance, particularly in the high end and a more tightly packed coil can generate a stronger magnetic field, further improving performance. All of this results in lower distortion and an improved sound.

Of course there is a payoff and in this case the downsides are increased manufacturing costs, voice coils made with very fine wire are more difficult to make, and if you use finer wire it affects the electrical performance. It raises the impedance.

So we can see that high impedance isn’t desirable in itself, it’s a consequence of optimising the driver design towards audio performance at the expense of efficiency and cost.