Why We Need To Test Low-Impedance Headphones Soon
So far, all of our tests employed Sennheiser's HD 800 headphones. As a reminder, they're relatively high-impedance (300 Ω) cans.
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 Ω||0.1 Ω||6000|
|HD 800 / JDS Labs O2+ODAC||600 Ω||0.1 Ω||6000|
|HD 800 / Asus Xonar Essence STX||600 Ω||10 Ω||60|
|HD 800 / Realtek ALC899||600 Ω||77 Ω||7.8|
|K 550 / Benchmark DAC2 HGC||35 Ω||0.1 Ω||350|
|K 550 / JDS Labs O2+ODAC||35 Ω||0.1 Ω||350|
|K 550 / Asus Xonar Essence STX||35 Ω||10 Ω||3.5|
|K 550 / Realtek ALC899||35 Ω||77 Ω||0.4|
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.