When I compared impedance measurements of the headphones with the frequency response measurements, it was easy to see why some headphones were affected by the differing output impedances of our sources while others weren’t.
The headphones that were essentially unaffected by the amplifiers’ output impedances were the ones whose own impedances measured fairly consistently at all frequencies. The ones whose impedance varied a lot were more affected by the computer’s high output impedance. Look at figure 5, which compares the impedance of the Sennheiser HD598 with that of the HiFiMan HE-300. The HD598 has a huge impedance spike at 100 Hz, right where the +3.3 dB bass boost occurred. The HE-300’s impedance is relatively flat in comparison.
Now examine figure 6, which compares the impedance of the Audéo PFE 232 with that of the Shure SE215. The PFE 232’s impedance drops dramatically above 3 kHz, the exact same region where it showed a change in frequency response. Meanwhile, the SE215’s impedance is almost perfectly flat.
Clearly, the impedance of a headphone amp can have a major effect on what you hear in the ’phones. However, given the sometimes wide variance in listeners’ preferences in headphone response, I can’t say whether you’ll like the sound of a high-impedance or low-impedance amplifier better. All I can say is that you’ll get more consistent results with a headphone amp that has low output impedance.
What appears to be just as important, though, is the impedance of the headphones. Based on my admittedly limited survey here, headphones with a reasonably flat impedance curve will react with different headphone amplifiers in a more predictable way than headphones that exhibit major impedance swings.
It might be safest to recommend seeking out a decent headphone amp or amp/DAC with a relatively low impedance, as well as a set of headphones with fairly consistent impedance at all frequencies. Now that S+V is adding measurements to our reviews of headphones and headphone amps, we hope sorting all this out will soon be much easier.
Or to put it more simply: Headphone amps aren’t useless creations intended merely to capitalize on commercial trends. They’re the real deal.
Brent Butterworth and Geoff Morrison combine their years of gear testing and knowledge in one überblog of irreverence and techiness.
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