INTRO: The output Impedance of headphone sources is one of the most common reasons the same headphones can sound different depending on what they’re plugged into. This important parameter is rarely specified by manufactures but can make a huge difference in sound quality and headphone compatibility.
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THE SHORT VERSION: All you really need to know is most headphones work best when the output impedance is less than 1/8th the headphone impedance. So, for example, with 32 ohm Grados the output impedance can be, at most, 32/8 = 4 ohms. The Etymotic HF5s are 16 ohms so the max output impedance is 16/8 = 2 ohms. If you want to be assured a source will work well with just about any headphone, simply make sure the output impedance is under 2 ohms.
WHY DOES OUTPUT IMPEDANCE MATTER? It matters for at least three reasons:
- The greater the output impedance the greater the voltage drop with lower impedance loads. This drop can be large to enough to prevent driving low impedance headphones to sufficiently loud levels. A real world example is the Behringer UCA202 with a 50 ohm output impedance. It struggles with some 16 - 32 ohm headphones.
- Headphone impedance changes with frequency. If the output impedance is much above zero this means the voltage delivered to the headphones will also change with frequency. The greater the output impedance, the greater the frequency response deviations. Different headphones will interact in different, and typically unpredictable, ways with the source. Sometimes these variations can be large and plainly audible.
- As output impedance increases electrical damping is reduced. The bass performance of the headphones, as designed by the manufacture, may be audibly compromised if there’s insufficient damping. The bass might become more “boomy” and less controlled. The transient response becomes worse and the deep bass performance is compromised (the headphones will roll off sooner at low frequencies). A few, such as those who like a very warm “tube like” sound, might enjoy this sort of under damped bass. But it’s almost always less accurate compared to using a low impedance source.
THE 1/8th RULE: To minimize all three of the above problems, it’s only necessary to keep the output impedance less than 1/8th the headphone impedance. Or, put another way, just divide the headphone impedance by 8 to get the maximum output impedance without potential audible degradation.
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THE BEST HEADPHONES ARE DESIGNED FOR THE BEST SOURCES: If you do a quick survey of the most well reviewed high-end headphone amps and DACs, they nearly all have very low output impedances. Examples are products from Grace Designs, Benchmark Media, HeadAmp, HeadRoom, Violectric, etc. It only stands to reason that most high-end headphones are designed to be at their best with similar products. Some of the most highly regarded headphones have relatively low impedances including several models from Denon, AKG, Etymotic, Ultimate Ears, Westone, HiFiMAN and Audeze. All of these, as far as I know, were designed to be used with low (ideally near zero) impedance sources. I’ve also had a Sennheiser representative tell me they design their audiophile and portable headphones for zero ohm sources.
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WHAT OUTPUT IMPEDANCE DOES MY SOURCE HAVE? Some manufactures make it clear they strive for a low output impedance (such as Benchmark), while others specify the actual output impedance of their products (such as Behringer does with the UCA202 at 50 ohms). And most, sadly, keep it a total mystery. Some product reviews, such as the ones on this blog, include measurements of the output impedance as it’s critical to the sound of the device with various different headphones.
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EXCEPTIONS TO THE RULE: There are a few headphones supposedly designed for significantly higher output impedances. I do wonder if this might be more myth than reality these days in terms of audiophile and consumer headphones as I’m not aware for any specific examples. But it’s certainly possible. If so, using these headphones on a low impedance source might cause under-damped bass performance and a different frequency response than the manufacture intended. This might explain some of the “synergy” claims when certain headphones are mated with a certain source. But those “synergies” are entirely subjective—one man’s “bright and detailed” is another man’s “harsh”. The only way to get
consistent performance is to use a low impedance source and follow the 1/8th Rule.
A CHEAP TEST: If you’re wondering if your current source is compromising the sound quality because of an unknown output impedance, consider buying the $19
FiiO E5 amp. It has a near zero ohm output impedance and has enough output for most many headphones under 100 ohms. If it obviously improves the sound, it’s likely your source has an output impedance that’s too high.
BOTTOM LINE: Unless you know your particular headphones sound better with a specific higher output impedance, it’s best to always use a source with an output impedance no higher than 1/8th the impedance of your headphones. Or, to make it even simpler, an output impedance of 2 ohms or less.
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FINAL WORDS: Hopefully I’ve made it clear the only way to get consistent performance between headphones and their source is to follow the 1/8th Rule. While some may prefer the sound using a higher output impedance, that’s very specific to each particular headphone, the particular output impedance, and the person’s own subjective tastes. Ideally a new standard should be developed and manufactures should be encouraged to design headphone sources with an output impedance below 2 ohms.