Hi, actually I don't know what is impedance and how it causes hiss/noise? How do I know the impedance of each DAC? Is it the Signal to Noise ratio?
Higher impedance or lower impedance is better?
I'm going to give you a somewhat detailed explanation which you don't want or need, because I feel like typing something.
Impedance is resistance. You have two impedances to consider:
Headphone impedance
This determines the relationship between the voltage, current, and power the headphone receives from the amplifier. The power a headphone receives from a given voltage is defined as P = V^2/R, where P is power in Watts and R is impedance is ohms. As you can probably see, higher impedance means less power, so if two headphones have the same sensitivity (usually decibels per milliwatt) the one with the higher impedance will require more voltage to reach the same volume. This is why most portables, and IEMs in particular are designed with low impedance: portable players often have very limited voltage output.
However, because Power = Voltage * Current, and low impedance headphones draw more power from the same voltage, they must also draw more current. This makes them a more "difficult" load from the amp's perspective, increasing noise, distortion, and other things. That's why a lot of high-end headphones have a high impedance, as it improves performance of the system. These days it's pretty easy to make an amp which has inaudible noise and distortion even into low impedance loads, so no big deal.
Output impedance
This is an aspect of the amplifier, not the headphones. When an amp has an output impedance, it acts as a voltage divider when driving headphones. This means it will absorb some of the voltage the amp is producing. The lower the impedance of the headphone the amp is driving, the more voltage will be divided and the less will reach the headphones, leading to lower volumes and more heat generated by the amp. This is problematic for a few reasons:
Many headphones don't just have one impedance across all frequencies, their impedance changes across the spectrum. Because output impedance has a larger affect on voltage at low impedances, frequencies with a high impedance will receive relatively more voltage and sound louder. This is usually inaudible unless the headphone has very wild changes in impedance (like in balanced armature IEMs with crossovers) or if the ratio of the output impedance to load impedance is very high (even some 300 ohm headphones can be affected by 120 ohm output impedance).
High output impedance can reduce electrical damping. You can determine the damping factor by dividing the headphone impedance by the output impedance. If the damping factor is low, the amp will have trouble controlling the headphone driver, causing it to bounce back and forth instead of stop ("ringing"). This will be most noticeable in bass distortion and roll-off of sub-bass.
In both cases it's advisable to have a damping factor of 8 or more (that is, an output impedance that is 1/8 the headphone impedance). The higher the better; you can't have too low output impedance.
USB DACs are usually not designed to drive headphones, even if they have a plug for them. DACs normally have high output impedance because they're designed simply to feed a low current signal into a high impedance load like an amp. If they don't have a dedicated headphone amp, they may just be routing the headphone plug through the same high impedance circuitry. And unfortunately the output impedance is rarely listed in the specs.
You'll have low output impedance and high damping factor for sure with the E10K, it has a dedicated amp. With the others you listed you're gambling. Both on what their impedance is, and if you'll even hear the difference it makes.
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Probably none of this has to do with the hissing, as that's a noise and sensitivity issue.
