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Why We Need To Test Low-Impedance Headphones Soon

What Does It Take To Turn The PC Into A Hi-Fi Audio Platform?
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So far, all of our tests employed Sennheiser's HD 800 headphones. As a reminder, they're relatively high-impedance (300 Ω) cans.

HD 800s and K 550s in their respective housingsHD 800s and K 550s in their respective housings

As we talked about testing the O2+ODAC, JDS Labs asked that we also try using low-impedance headphones, and the company sent a set of of AKG K 550s, rated at 32 Ω. Its point is that one of the O2+ODAC's main advantages is a vanishingly small output impedance (close to 0 Ω), which is supposed to be great with low-impedance headphones.

Now, the Benchmark DAC2 also has vanishingly small output impedance; its HPA2 headphone amp is rated close to 0 Ω, too. Naturally, then, testing against the DAC2 again was fairly redundant. But what about Asus's Xonar Essence STX, which implements the TI TPA6120A2 datasheet-recommended 10 Ω output impedance level, or Realtek's codec, rated at 2 Ω but subject to a suggested 75 Ω resistor in series on the output path, yielding a typical total of 77 Ω?

Output and Load Impedance

In order to understand why output and load impedance might matter, we need to introduce a concept known as Damping Factor.

As speaker (or headphone) drivers oscillate, they generate a voltage difference of their own that affects all directly-connected electrical components. Without going into too much detail, if an amplifier's output impedance is high compared to the load's impedance, speaker motion and control are impeded. This is particularly true at low (<500 Hz) frequencies, and extremely so at the driver's resonance frequency (resonance, as you can imagine, is very bad for hi-fi audio). The ratio between an amplifier's output impedance and a load (headphones, in this case) impedance is called Damping Factor.

Impedance is a concept that applies exclusively to alternating-current circuits. Furthermore, impedance is not a set figure. It varies based on the frequency of the electrical signal. The 300 Ω-rated HD 800s, for example, typically measure in the 600 Ω impedance range below 1 kHz frequencies.

Headphone / Amplifier
Headphone Actual Load Impedance at <500 Hz
Amplifier Output Impedance
Damping Factor
HD 800 / Benchmark DAC2 HGC
600 Ω0.1 Ω6000
HD 800 / JDS Labs O2+ODAC
600 Ω0.1 Ω6000
HD 800 / Asus Xonar Essence STX
600 Ω10 Ω60
HD 800 / Realtek ALC899
600 Ω77 Ω7.8
K 550 / Benchmark DAC2 HGC35 Ω0.1 Ω350
K 550 / JDS Labs O2+ODAC35 Ω0.1 Ω350
K 550 / Asus Xonar Essence STX35 Ω10 Ω3.5
K 550 / Realtek ALC899
35 Ω77 Ω0.4

A DF of 50 or more is typically considered excellent. That means amplifiers rated for up to 12 Ω output impedance should encounter little trouble driving the HD 800s (600/12 = 50 DF at <500 Hz), even through deep bass. The benefits of even higher damping factors, which you see in the chart above can approach 6000, are debatable.

But using a 32 Ω headphone as a load yields quite different numbers. According to InnerFidelity, the K 550 measures between 34-37 Ω impedance below 500 Hz. With that load on the 77 Ω Realtek ALC899 codec, the Damping Factor is a fairly poor 0.4, and on the 10 Ω Xonar Essence STX, it's a not-so-stellar 3.5.

Technically, you also need to add cable impedance to the amplifier's impedance. But given that the 10-foot cables and connectors we're using have an impedance of <0.1 Ω, I felt that could be excluded. If you're using long or thin cables, the same might not be true.

Coming Soon: Low-Impedance Headphone Testing

We would have loved to test AKG's K 550 as part of this article. But at over 12,000 words, this piece was already a behemoth before going down that road. Still, we know that testing the AKG K 550 (or another low-impedance headphone, for the matter) is important since high-impedance headphones tend to be exotic, expensive stuff. A vast majority of PC users own headphones rated at or around 32 Ω.

Because we haven't yet covered 32 Ω headphones, the results discussed throughout this article only apply to 300 Ω headphones.

We plan to explore whether the same conclusions can be drawn about lower-impedance headphones in a future article.

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Top Comments
  • 12 Hide
    SuckRaven , February 25, 2014 12:29 AM
    Bravo ! Awesome, and a very thorough review. Even though as you mention, audio gear is not usually the forté/emphasis of the reviews here, it's refreshing to have someone at least try to cut through the (more often-than-not) overpriced arena of bullshit that is the field of "high-end" audio. I applaud the review, and the effort. Keep up the good work. More please.
  • 11 Hide
    shahrooz , February 25, 2014 1:05 AM
    this article just won Tom's Hardware Readers Elite award
  • 11 Hide
    kitsunestarwind , February 25, 2014 12:52 AM
    The biggest thing I have found for the PC is no matter how good your DAC is , if your speakers and AMP are crap, then it will never sound better.People spend big money on DAC's and forget that you need a high Quality amp with very very low THD (total harmonic distortions) and a very good set of Full Range speakers with high sensitivity if you want good sound, instead of crappy (albeit expensive) computer speakers especially sets with a sub.
Other Comments
  • 12 Hide
    SuckRaven , February 25, 2014 12:29 AM
    Bravo ! Awesome, and a very thorough review. Even though as you mention, audio gear is not usually the forté/emphasis of the reviews here, it's refreshing to have someone at least try to cut through the (more often-than-not) overpriced arena of bullshit that is the field of "high-end" audio. I applaud the review, and the effort. Keep up the good work. More please.
  • 8 Hide
    PudgyChicken , February 25, 2014 12:44 AM
    Just wondering, why not test a Creative X-Fi Titanium HD or something like that alongside the ASUS Xonar? It would be interesting to see some of the differences between different PCIe sound cards in this matchup. However I understand that what you were really going for was showing the difference between price point and form factor at the same time, so perhaps not testing two PCIe cards makes sense.
  • 11 Hide
    kitsunestarwind , February 25, 2014 12:52 AM
    The biggest thing I have found for the PC is no matter how good your DAC is , if your speakers and AMP are crap, then it will never sound better.People spend big money on DAC's and forget that you need a high Quality amp with very very low THD (total harmonic distortions) and a very good set of Full Range speakers with high sensitivity if you want good sound, instead of crappy (albeit expensive) computer speakers especially sets with a sub.
  • 11 Hide
    shahrooz , February 25, 2014 1:05 AM
    this article just won Tom's Hardware Readers Elite award
  • 1 Hide
    maestro0428 , February 25, 2014 1:27 AM
    Wonderful article! I love listening to music and do so mostly at my PCs. I try to set up systems where audio is important in component selection. Although we all love drooling over expensive equipment, many times it is not all that necessary for an amazing experience. I'd love to see more! Including smaller, studio speakers as I believe that speakers/headphones are the most important part of the equation. Keep up the great work!
  • 5 Hide
    blackmagnum , February 25, 2014 1:28 AM
    Don't forget that for PCs: the hardware is as good as its software (drivers).
  • 0 Hide
    Someone Somewhere , February 25, 2014 1:43 AM
    Agree totally with this. It always annoys me when people say they're spending over $100 on a sound card, especially when it turns out that they're using Optical out, and the whole thing is basically moot.I now have a nice source to link to.
  • -1 Hide
    1zacster , February 25, 2014 2:01 AM
    The thing is you can't just pick up two sets of good headphones, try them on different DACs/AMPs and expect to hear major differences, it takes longer than 5 minutes for your ears to adjust to newer headphones and for the differences to actually show. This is like taking food from Left Bank and then bringing in a bunch of hobos and asking them tel tell the differences between the foods.
  • 1 Hide
    dogman-x , February 25, 2014 2:34 AM
    I use an optical cable from my PC to a home theatre receiver. With this setup, stereo CD audio content is sent as raw PCM to the receiver, not compressed into DD or DTS. These days you can buy a very good quality home theatre receiver for less than $200. Audio quality is outstanding.
  • 1 Hide
    Memnarchon , February 25, 2014 2:50 AM
    I would love to see ALC1150 in these tests too, since its widely used at most Z87 mobos.
  • 0 Hide
    outlw6669 , February 25, 2014 2:56 AM
    Excellent in depth review Filippo! It is good to see a bit of Tom's roots shining through after all this time :) 
  • 3 Hide
    loosescrews , February 25, 2014 3:01 AM
    I would have liked to see some hard to drive planar magnetic headphones in the mix (maybe some of the Audeze LCD-X or LCD-XC headphones or HiFiMAN something) and also a cheaper DAC/Amp solution like Maybe the Schiit Audio Modi + Magni or Vali. Another nice addition would be the Creative Sound Blaster Z Series ZXR with its TI Burr-Brown DAC.
  • 4 Hide
    BrightCandle , February 25, 2014 3:17 AM
    Can we get game surround sound audio tested as well? A lot of the reviews recently are focussing on sound quality differences in music but as you have determined there really isn't any difference there. But there is a clear difference I can hear in the comparative videos of battlefield with cmss, sbx pro, razor and realtek on youtube videos and the different surround sound effects really do seem to change positioning quality. This remains the only reason I think a sound card is worth it over realtek but it would be good to get to the bottom of whether its just EQ or its genuine quality differences related to the HRTF or something else.
  • 0 Hide
    bstaletic , February 25, 2014 3:27 AM
    Great article. I also came to similar conclusions. I had bought High Resolution Technolies Musicstreamer II 2013 edition for ~$140and an Asus (I don't remember which one) for ~70$. I have technics SU-V8 amplifier and Wharfedale E50 speakers (cool stuff, look it up). Muscistreamer made bass a bit better (though not everyone could hear the difference) and now I say it was a waste of money. Asus on the other hand could make a difference if you set it up correctly, but you have to do it for every album so forget about shuffle. Only DAC I'm willing to hear is DACmagic for ~$400 and I doubt I'm going to be impressed.Conclusion: Buy any PC (the cheaper the better), and spend the rest of money you have on speakers and amplifier. Also make yourown cables.
  • 4 Hide
    ilovetea , February 25, 2014 3:34 AM
    What's the purpose to invest into some special pc hardware, if major reciever brands have digital inputs and also usually unify inflows of audio through digital filters? This makes the reciever to serve as DAC both supporting and limiting the final quality.
  • 3 Hide
    Someone Somewhere , February 25, 2014 3:53 AM
    Quote:
    What's the purpose to invest into some special pc hardware, if major reciever brands have digital inputs and also usually unify inflows of audio through digital filters? This makes the reciever to serve as DAC both supporting and limiting the final quality.


    What I have been saying for quite a while.
  • 5 Hide
    vmnej , February 25, 2014 3:57 AM
    Electronics are negilable. The hard part ist turn ing the electrical signal into a mechanical signal (sound waves). That' why most of the money should go into the speakers and then maybe room acoustics. I highly recommend a pair of Nubert nuPro speakers.
  • 2 Hide
    gaymer1984 , February 25, 2014 4:01 AM
    I have a challenge to lay down for the writers of this article as an audiophile.Nothing you have particularly referred to can be contested; you do get more features with more expensive hardware, but price isn't necessarily an indicator of quality and it is high quality audio you are looking for, not necessarily the price point. That assumption doesn't work with sound cards as the first point in the signal path to the speakers.My challenge is this: compare your ALC 889 to an E-MU 1616m PCI-E. The quality of the DACs is higher on this £250 board than other PC sources I've heard myself, and you aren't spending £2,000 to get there. I challenge you to NOT find a difference. Don't change anything else in the signal path - keep the cable that feeds to your amp, and the speaker cables the same. Then listen to audio you know very well, and you know has been recorded well. This is harder to find with current music.You aren't looking for things to sound "better" or "louder", you are looking for greater detail. A better stereo "image" as it is called, where you can place instruments being reproduced by the speakers in a notional 3-D space. That is the mark of "good" audio.I ask you to accept this challenge because without following up this statement of $2 is as good as $2,000 you will potentially mislead budding enthusiasts down a misguided path.
  • 0 Hide
    martel80 , February 25, 2014 4:10 AM
    Why not include readings from the RightMark Audio Analyzer? They don't tell you anything about how it sounds but still...
  • 6 Hide
    Someone Somewhere , February 25, 2014 4:11 AM
    Oh, great.

    Do you believe that the E-MU 1616m is significantly better than their $2k amp? If not, then they're still not going to find a difference.
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