Quote:
Not to go off topic, but if you think the $129 uDac2 with it's 6 ohm output impedance is bad for sound, then what do you think about the Dacport($400) and Dacmini($800) having 10 ohm of output impedance? Both are regarded as very good products and are used to drive IEMs enjoyably for many people.
You can probably guess what my answer is. It's a fact that an output impedance over 2 ohms will create audible frequency response variations that widely differ from headphone to headphone. That's not my opinion, or some wild theory, it's just fact.
So if a headphone amp manufacture has an output impedance above 2 ohms, you have to ask why would they want the sound of their product to change in audible and
unpredictable ways with different headphones? I can't answer that, you would have to ask the manufacture. We do know how at least NuForce justified the 6 ohm output impedance of the uDAC-2:
- They used lots of words to try and say a low output impedance creates a high damping factor and hence gives headphones "anemic bass" response. And it's true when the output impedance starts to approach the driver's impedance it is likely to change the bass response. But the problem is that "change" is very dependent on the impedance and inherent "Q" (internal damping) of the headphones. It's likely to change the bass in ways the headphone manufacture did not intend. To put it another way, it may help with a few headphones, but it's much more likely to make other headphones sound worse. It could easily give you more boomy uncontrolled "flabby" bass. See below for why.
- They argued a short circuit might damage the headphone amplifier so they added a series resistor (raising the output impedance) to help protect it. This is a really poor way to protect an amplifier because it has readily audible negative consequences. It's also entirely unnecessary as there are much better ways (many of which are built into the IC's used as headphone amps).
The damping of a driver is a delicate balance. Here are the 3 basic choices:
- Critically Damped (Q = 0.7) - This is widely considered ideal as it provides the deepest bass extension without any frequency response variations or excessive "ringing" (uncontrolled driver motion).
- Over Damped (Q < 0.7) - This keeps even tighter control over the driver but at the expense of less deep bass (the response rolls off sooner). So manufactures rarely over damp their products.
- Under Damped (Q > 0.7) - This trades off some low bass extension for a peak at higher bass frequencies. The driver is also no longer well controlled and exhibits excessive "ringing" (i.e. it doesn't stop soon enough when the audio signal stops). Under damping typically creates frequency response variations, loss of deep bass, poor transient response. Under damping is a cheap way to provide the illusion of more bass at the expense of the quality of the bass. It's frequently used in the cheapest headphones and speakers to provide "fake bass". Under damped headphones/speakers are frequently described as "boomy", "flabby", "sloppy", etc. This is the sound NuForce is giving you more of.
Headphone manufactures have to decide how much damping they want, and they have to factor in the expected output impedance of the source. The "defacto" standard appears to be close to zero ohms. This makes sense as this is the only standard that assures broad compatibility with virtually all headphones. So if headphone manufactures are designing to "near zero ohms" their headphones will perform in ways they didn't intend, and have less damping, with higher output impedances.
So if a headphone is designed for ideal damping with a near zero ohm source. When it's connected to a higher impedance source, it will be under damped.
By raising the output impedance of their amp, NuForce is generally making expensive headphones behave more like cheap ones with under-damped boomy "fake" bass.
Because there are no advantages to over damping, few headphones are over damped by design. Most are either already under damped, or have an ideal Q of around 0.7.
So the odds are against NuForce improving bass performance.
And most of the above is just talking about bass performance. It's ignoring the sometimes much greater frequency response changes caused by the headphone impedance varying with frequency. Here's the frequency response of the uDAC-2 in blue with the Ultimate Ears SF5's. Some might like the sort of "glaring" broad mid range boost and sharp high frequency "notch" in the sound, but I suspect most would prefer something closer to the yellow line--which is exactly what you'd get plugging those same headphones into the Benchmark DAC, a FiiO amp, or even a $29 Sansa Clip+:
My personal opinion is a lot of amps have an output impedance above 2 ohms because it makes the design easier, and cheaper. It's often likely a matter of taking the easy way out.
For the reasons above, it's much harder to argue higher output impedances generally result in better sound.
Most of the better, reviewed, higher-end, well respected headphone amps I'm aware of have an output impedance below 2 ohms and often well below 1 ohm. The Benchmark products are just one example. It's also interesting Apple has lowered the output impedance on their products resulting in subjective reviews of better sound quality and wider headphone compatibility.
edit: fixed typo
