Hi ASRs!
Long time reader first time poster here.
I'm really interested on this topic of output impedance. I have a V2 which I enjoy using it as DAC for my active speaker and also as headphone amp for my headphones. The output impedance never bothered me before as most of the time I'm using planar headphones. Since the recent discussion on how output impedance alter the sound I want to explore more on this rabbit hole.
I fortunately also have Tin T2 to test. I use it to compare between DX3 Pro and NX4DSD, which have very low output impedance. I don't have equipment to do volume matching so it was manually adjusted as best as my ear allow me to. After listening to numerous songs, especially on bass heavy genre, try as I might but still I could not hear any difference between the 2 devices. Dissatisfied, I try my dynamic driver headphone which is 35 ohm impedance, still can't hear any difference. Why is it so?
I been reading more on output impedance and that lead to damping factor. Some of the most famous article on this topic I believe will be from
Stereophile,
NwAvGuy and
Inner Fidelity. They showed increase of 6 dB or even 12 dB in some frequency response, but those are when the output impedance are much higher than the headphone impedance, and I think at maximum volume level? How much difference if it were only at normal listening level? Usually I'm listening at around -30dB, low gain.
I think NwAvGuy is the one that popularized the idea of 1/8th of impedance, or 8 damping factor. Basically it means headphone impedance must be higher by 8 times from amp's output impedance, if headphone impedance is 35 ohm then amp output impedance can't be higher than 4.375 ohm. But why in his article Sennheiser HD590 which is 120 ohm impedance paired to 10 ohm output amp still increase 1 dB above 20 hz? Is he meaning 12 damping factor is still not enough?
Inner Fidelity article is interesting as Dr. Jan Meier show AKG K420 which is 32 ohm give zero oscillations when the output impedance is 88 ohm while Tyll show HD800 getting bass bumped when output impedance change from 0 to 100 and 200 ohm. When output impedance is 100 ohm, that is only 3 damping factor, we're seeing bump up about 1.25 dB. For 200 ohm which is 1.5 damping factor, we're seeing bump up about 3 dB. Again, such increase in dB is only on max volume level? If in lower level then the bump in dB will be smaller? And why Dr. Jan Meier result and Tyll are the opposite of one another?
I found
this post from head-hi when searching about this topic. The headphone is 250 ohm while the amp output impedance is 10.3 ohm (which give 24 damping factor), and he also use serial resistors to bump up the output impedance to 100 ohm (which give only 2.5 damping factor). In both test the headphone still produce almost the same frequency response, there's only very small bump in some part of the frequency because of low damping factor. Are damping factor does not need to be as high as 8 if measured from headphone response?