10 OHM OUTPUT IMPEDANCE: Some might look at the above example and think it’s extreme with a 43 ohm source. But plenty of sources have around a 10 ohm output impedance. Here’s the same headphones with a 10 ohm source—there’s still a very audible 6 dB of variation. This sort of curve creates weaker bass, a “glaring” midrange emphasis, muted high frequencies, and odd phase characteristics due to the sharp “notch” at 10 khz that can alter spatial perceptions:
FULL SIZE SENNHEISERS: Here are the full size, higher impedance, Sennheiser HD590 cans with the same 10 ohm output impedance. Now the variation is only a bit over 1 dB above 20 hz. While 1 dB isn’t that much, it’s right in the most “boomy” bass region which is the last place most want any sort of emphasis:
DAMPING EXPLAINED: Any dynamic driver, in a speaker or headphone, moves back and forth with the music. That’s how it creates sound and they all have moving mass. The laws of physics say an object in motion tends to stay in motion. Damping is used to help avoid unwanted motion. Without going into too many details, if a speaker is under-damped, it keeps moving after it should have stopped. And if it’s over-damped (rare) its ability to accurately follow the signal is compromised—imagine a speaker trying to operate submersed in maple syrup. There are only two ways to damp a driver—mechanically and electrically.
BOUNCING CARS: Mechanical damping is much like the shock absorbers on a car. They add resistance so when you hit a bump the car doesn’t keep bouncing up and down long after the bump. But they also add harshness because they reduce the suspension’s ability to accurately follow the road. They’re a compromise—soft shocks give a softer but more bouncy ride and stiff shocks control the bouncing better but make the ride harsher. Mechanical damping is always a compromise.
ELECTRICAL IS BETTER: There’s a better option to control unwanted motion of headphone drivers and it’s called electrical damping. The voice coil and magnet of the driver work with the amplifier to control the motion of the driver. This kind of damping has fewer negative side effects and allows headphone designers to create headphones with less distortion and better sound. Just like a car suspension that can better follow the road, an optimally damped headphone driver can better follow the audio signal. But, and this is the critical part, electrical damping is only effective when the output impedance of the amplifier is much lower than the impedance of the headphones. If you plug 16 ohm headphones into an amp with a 50 ohm output impedance, there will be almost no electrical damping. That means when the driver is supposed to stop moving it might not. The headphone is more like a car with worn shock absorbers. If the 1/8th Rule is followed, however, there will be sufficient electrical damping.
A SPEAKER ANALOGY: Back in the day, before my time, speakers were mostly driven by amplifiers that used tubes instead of transistors. Tubes are high impedance devices that operate at high voltages so nearly all tube amps use output transformers. Without going into all the details, tube amps had widely varying output impedances that were often significant and violated the 1/8th Rule. Speaker manufactures couldn’t rely on amplifiers having a low enough impedance to provide much electrical damping. This compromised speaker design much like headphone design is compromised today if a headphone designer can’t rely on a low impedance source for proper electrical damping.
ACOUSTIC SUSPENSION: In the 1970’s the situation changed as solid state amplifiers became popular. Almost all solid state amps easily pass the 1/8th Rule. In fact, most pass a 1/50th Rule—their output impedance is generally below about 0.16 ohms—known as a damping factor of 50. Suddenly speaker manufactures were free to design better speakers that could take advantage of these much lower output impedances. And the first really good acoustic suspension sealed box speakers like the original AR's, Large Advents, etc. were developed. They had deeper and better bass than any of their tube-powered predecessors could manage from a similar box size. It was a big milestone in "hi-fi" to rely on lots of electrical damping from the amplifier. It’s too bad many headphone sources are 40+ years behind.
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.
WHY DO SO MANY SOURCES HAVE A HIGHER OUTPUT IMPEDANCE? The most common reasons are:
Headphone Protection - More powerful sources with a low output impedance might be capable of delivering too much power into low impedance headphones. To help protect such headphones, some designers raise the output impedance. This is a compromise to try and have the amp adapt to the load used. But it comes at a big price with many headphones. A better solution is offering two gain options The low gain setting can lower the maximum output voltage when using low impedance headphones. And, in addition, active current limiting can be used so the source will automatically restrict the maximum output into lower impedance headphones even if the wrong gain setting is used.
To Be Different - Some manufactures raise the output impedance on purpose claiming it makes their source sound better. Sometimes “different sells” as it’s a way to differentiate the sound of their product from their competitors. But, in this case, the particular “different sound” you get is entirely dependent on which headphones are used. With some it might be an improvement and with others it’s more likely a big step backwards. The odds greatly favor degrading the sound.
It’s Cheap – A higher output impedance is a band-aid for many inexpensive headphone sources. It’s a cheap way to achieve stability, a crude form of short circuit protection, and it can allow using an otherwise substandard op amp or output device that would be unable to drive 16 or even 32 ohm headphones directly. By adding some series resistance to the output all these things get “fixed” with a $0.01 part. But the cheap “fix” comes at a substantial price in the sound quality with many headphones.
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.
