TECH2

Testing the effects

To find out just how much output impedance impacts sound quality, I first measured the output impedance of various headphone-driving devices I had sitting around. I wanted something with high output impedance and something with low output impedance. I settled on a Musical Fidelity V-Can headphone amp (measured output impedance 5.1 ohms at 1 kHz) and my Dell Inspiron Mini 10 netbook computer (measured output impedance 75.0 ohms at 1 kHz).

I then measured the frequency response of four over-ear and four in-ear headphones, measuring each one driven first with the V-Can and then with the computer. Measurements were made with a Clio FW audio analyzer in swept sine mode and a GRAS Type 43AG ear/cheek simulator. Tested over-ear models were the Beyerdynamic DT-990 (32-ohm model), Grado RS2, HiFiMan HE-300, and Sennheiser HD598. Tested in-ear models were the Monster Turbine Pro Copper, Phonak Audéo PFE 232, Shure SE215, and TDK EB950.

Does it, or doesn’t it?

If your eye’s already been drawn to the frequency response charts included with this article, you can see that output impedance does have a substantial effect on headphone performance. Or it doesn’t. It depends on the headphones. Let’s consider the best- and worst-case results.

Look at the frequency response charts for the HiFiMan HE-300 (figure 1) and the Shure SE215 (figure 2) and you’ll probably wonder what all the fuss is about. Whether driven by the V-Can or the computer, both deliver nearly identical frequency responses. OK, when driven by the computer, the HiFiMan HE-300 is up +1.1 dB at 100 Hz and the Shure SE215 is up +0.6 dB at 20 Hz. Big deal. You could just barely hear the difference with the HE-300 and you’d never hear it with the SE215.

But when you glance at the charts for the Sennheiser HD598 (figure 3) and the Audéo PFE 232 (figure 4), it’s obvious something is up. Driven by the computer, the Sennheiser HD598 shows a little extra bump in the bass, up +3.3 dB at 100 Hz. Meanwhile, the Audéo PFE 232’s treble response is substantially reduced when driven by the computer; it’s down -5.4 dB at 10 kHz. You’d probably notice the extra bass in the HD598 and you’d certainly notice the treble roll-off in the PFE 232.

What I can’t say is which way you’d like the sound better. You might like the way the 75-ohm output impedance of the computer’s internal headphone amp interacts with these headphones. There’s no easy prescription here.