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Ari, how would using a 300 ohm adapter make the sound of a low impedence can better? It was always my understanding that the amp's output impedance should be lower than the headphone's impedance. With the 300 ohm adapter wouldn't the damping factor be 0.083? I'm not really sure what damping factor is, but I'm told it's important.
Damping factor is the ratio of the amplifier's output impedance to the driver's impedance. It is important (or important not to do it
) because of the way this interacts with the actual impedance VS frequency of the driver.
The first thing to look at is that very few drivers have a truly flat impedance VS frequency curve. Some do, most dont. There are peaks and dips in the impedance VS frequency.
If you have a very high damping factor (typically given as 10 or more per rule of thumb, IE amps output impedance<1/10 headphone impedance) dips and peaks in the impedance VS frequency have basically no effect on the frequency response of the headphone. As you start reducing damping factor, the dips and peaks in the headphone's impedance VS frequency causes the frequency response to change at those places. If everything lines up this can be used to your advantage. What we are looking for here is "the right kind of wrong."
Take a hypothetical headphone whose impedance increases pretty steadily below 100hz but has obviously rolled off bass when driven from an amp that has a high damping factor. When driven from an amp with a low damping factor, the interaction of the output impedance and load impedance there causes a bass boost, but without the funkyness of more junk in the signal path to force that to happen. So by getting the damping factor set to " the right kind of wrong" you have picked up some bass. The headphones that had sucky bass extension now have better extension for the sonic penalty of a resistor.
With headphones we have an easy life because we dont typically have crossovers. Single driver speakers without passive notch filters can also see VERY strong benefits from getting damping factor "the right kind of wrong."
In speakers things get MUCH harder if passive crossovers or notch filters are used. The problem with crossovers & damping factors is that the crossovers must be designed/tuned around a particular source impedance from the amplifier. If you start mucking about with the output impedance from the amp with a speaker with passive crossovers you are now shooting at many many targets at once. Inevitably the crossovers stop lining up with each other correctly and your speakers which sounded great driven by your amps with a 0.0001ohm output impedance turn into little heaps of day old poop.
This above bit, with speakers with passive crossovers is the reason that damping factor is soooooo strongly ingrained in the hi-fi culture. Its not that high damping factor is necessary for good sound, in fact its easy to show that there are cases where its better that you have a low damping factor, its just that its easy to give 1 rule and stick to it. As long as everyone follows the same rule things work out OK. The problem is that the people making the rules are the people building high power amps & HUGE multi-way speakers, which only work if you get the damping factor right, by their definition anyways.