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I'm trying to wrap my head around input- and output impedances. From this question I learned that the output impedance of a headphone amplifier should be lower than the input impedance of the headphones connected to it, because this improves the "damping factor".

However, when looking at actual headphone amplifiers, I noticed that the output impedances are often crazy high, for instance 200 ohm on this Sound Devices HX-3 model:

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This seems to be a pro-grade piece of equipment. Does its output impedance of 200 ohm mean that it is unable to drive the majority of headphones on the market? Or am I missing something?

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First a short historical notice.

In the old days, professional headphones often where 600 Ohm, sometimes 250 Ohm. The IEC standard from 1996 recommended an output impedance of 120 Ohms for the headphone amp. This worked perfectly as the supply voltage in the headphone amplifier often was 20V or more. The value of 200 Ohms output impedance on the HX3 worked very well with that era of headphones.

Then came the consumer stuff, such as the iPod. Here the headphone amplifier only had a few volts to work with. This meant that old type headphones only could be driven to a low sound volume. Of course, as the consumers where many and had money, we started to see a lot of low impedance headphones. So much that low impedance now is more or less the standard. The low impedance headphone could be driven by the lower voltage to a high sound volume and the headphone amplifiers where modified to have a lower output impedance.

Now, what happens when you use a headphone amp where the output impedance of the headphone amplifier is larger than the impedance of the headphone? The first effect you will hear is that the sound volume will be higher -- often easily adjusted by turning down the volume. The second effect is more variable, depending on a lot of factors. As the headphone will be less dampened by the amplifier, it may create various types of sound "distortion". It could be variations in frequency response or harmonic distortions of various types. Highly dependent on the exact headphone and I do not believe there is any general answer for all situations.

The reason for the "distortion" is that the headphone amplifier is designed and optimized to work together with a type of headphone. This will include optimizingg the volume of the various cavities in the headphone, the dimensions and material used in the sound membrane, electrical components and so on. How these design decisions will interact in a different setting cannot really be generalized. Also remember that some headphones are designed to intentionally distort the sound, example may be "bass boost" on some headphones. How this bass boost will react to a different output impedance of the headphone amp is not really possible to predict.

(There are a lot of "crap" headphones out there, made only to sell cheaply. These will sound bad regardless but that is something different).

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  • Does this mean that oldschool, pro-grade equipment will distort the sound of modern, consumer-oriented headphones?
    – Jaap Joris Vens
    Apr 18, 2021 at 15:09
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    @JaapJorisVens Perhaps, perhaps not. You need to try the exact headphones. My guess is -- not much.
    – ghellquist
    Apr 18, 2021 at 16:25
  • And thank you for the edit. My computer keeps on spell-correcting to Swedish, very annoying.
    – ghellquist
    Apr 18, 2021 at 16:27
  • Weirdly enough, the 200 ohm model in the original question runs of AA batteries. It is not able to supply more power than an iPhone, but presumably it transforms the power to a higher voltage before delivering it.
    – Jaap Joris Vens
    Apr 19, 2021 at 15:28
  • @JaapJorisVens It contains an internal DC to DC converter. According to the manual it uses +/- 15 V internally. The DC converter adds a bit of cost to the product, something the early walkmans and iPods did not budget for. Instead the consumer market moved to low impedance headphones. Incidentally, the 600 Ohm headphone impedance specification comes from the equipment used in analog telephone equipment. But the world moves on.
    – ghellquist
    Apr 20, 2021 at 6:59

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