In my experience, and based on comments from professionals, headphones are resistance controlled, and not mass controlled, and therefore are not as susceptible to the effects of damping factor (Loudspeaker and Headphone Handbook 1998)
Headphone transducers are resistance-controlled, not mass-controlled like loudspeaker drivers above the main resonance. In any case 'damping factor' is largely nonsense - most of the resistance in the circuit is the voice-coil resistance and reducing the amplifier source impedance to infinitesimal proportions has an exactly corresponding effect on damping - infinitesimal.
However, the source impedance affects the *frequency response* of a loudspeaker because the motional impedance varies with frequency, and thus so does the voltage drop across the source impedance. This means that the source impedance (including the cable) should be less than about one-twentieth (not one two-hundredth or less!) of the rated impedance of the loudspeaker, to give a *worst-possible change* in frequency response from true voltage-drive of 0.5 dB.
Personally, most professionals don't use the term "damping factor" as it is a non-quantifiable term. From all the measurements I've seen, damping factor is a colloquial term used as a rule of thumb when contrasting the Zout of an amp, to the impedance curve of a loudspeaker or headphone. The Zout acts as a voltage divider, and for loudspeakers which have a huge impedance curve, high Zouts can lead to huge changes in FQ response. Headphones have very flat impedance curves, and it seems that only Zouts that are higher than the load impedance (damping factor <1), can have audible changes in FQ response. Likewise, impulse response doesn't really change until the Zout equals or exceed the load impedance. So for all intents and purposes, damping factors of even 1 for headphones just "start" to potentially create audible differences.
I've calculated the FQ response with several of my headphones (which range from 32ohms to 250ohm) with my Presonus HP4 which has a Zout of 51ohms. We're looking at damping factors that are worse than 1, and as high as 5. At worst (less than 1 damping factor) there is a 1.15db boost @100hz which is audible, but everything with a damping factor of 1.5 to 5 has a change of less than 0.5db at the resonant frequency, which is essentially inaudible unless you're REALLY trying to spot the difference. Of course, each headphone has it's unique impedance curve, and some headphones might be less immune to damping factors than others. With a zout of 120ohm, a 250ohm DT880 would have less than a 0.5db increase at the resonant frequency of 100hz, which is just bordering on the threshold of audibility (PM me if you want the calculations). Indeed, this is a damping factor of 2, and yet there is virtually no change to how it will measure (unlike a loudspeaker which would severely suffer)
I would imagine some tube amps with a high Zout (greater than 200ohm) might produce very slight changes in FQ response compared to the 600ohm version but the differences would likely be subtle enough that they couldn't possible account for the "amazing improvement in the 600ohm version" that so many head-fiers claim to hear (while others don't find any difference)
Edited by Catharsis - 8/26/10 at 6:48am
Originally Posted by maverickronin
Do those calculations include damping factor, or is it just based on power transfer? I was thinking that the change in damping factor would account for more of the audible differences. I don't know how important damping factor is to the much lighter diaphragms of headphones as compared to loudspeakers though. I had this idea because most of the people who recommend the 600s over the 250s pair them OTL tube amps. If those differences in the distortion graphs are too low to hear, then this is the last physical reason I can think of for them to be different.
The load/Z out ratio does hold true for all other headphones, but no other manufacturer AFIK makes such near identical models that only seem to vary in impedance which creates the problem we are trying to solve. These choices would seem to give you a chance to better match your amp.
My hypothesis is based on the 600s having a higher damping factor than the 250s when used with OTL tube amps. I'm going to guess that most OTL tube amps have a Z out in the 120 ohm range. That's what the Z out on my Crak that I'm going to build this weekend is, so I'm over generalizing a lot here. Tell me if I'm wrong. Back to damping factor. I've read that for speakers a damping factor of about 20 is all you need, since anything higher won't be audible. I'm guessing that the reduced mass and excursion of a headphone diaphragm would require a smaller damping factor than a loudspeaker.
If the damping factor of just over 2 (for Z out = 120 and 250 ohm drivers) is already past the threshold of audibility the a damping factor of 5 for the 600s won't make a difference. I don't thing the power transfer differences account for differences reported by some, but I don't know enough about it to say with much certainty. Since many SS amps have very low output impedance and damping factor improvement is inaudible after a point it makes sense that the 250s and 600s should measure very closely on such gear. Assuming a 1 ohm Z out, a damping factor of 600 shouldn't be audibly different than one of 250.