原文出自head-fi的討論串
impedance matching for maximum power transfer is important . . . in long distance power lines! But not really important with headphones. Typically you want the source to have a maximum of 1/10th the impedance of the driven device. With speakers you would want a maximum output impedance of 0.8ohms for a nominal 8 ohm speaker. Less is more in this case, and the lower the output impedance the better. This is because the output of the amp looks like
a short circuit to energy reflecting back from the headphones. Reflecting is really the wrong word here, back EMF (electro-motive force) would be a more accurate term. At any rate, this Juice comming back from the headphone, if not shorted out by the output impedance of the amp would develope a voltage at the output of the amp that would become part of the feedback signal, which will cause distortions in hte amp. HeadRoom amps have less than 0.1 ohm output impedance.
前後端阻抗匹配以取得最大效益的功率傳輸很重要！
──才怪，只有在長距離傳輸時才比較有影響，對耳機而言其實不是那麼重要
大部分情況下，只會要求前端的輸出阻抗不多於後端輸入阻抗的十分之一，
以8歐姆阻抗的喇叭系統為例，會要求擴大姬的輸出阻抗不大於0.8ohm
然而以耳機的情況而言，壓低輸出阻抗還是比較好的
因為把耳擴的輸出阻抗壓低，形同一個對從耳機方向反射回來能量的短路，反射這詞實際上是誤用了，反電動勢是比較精確的詞，而如果從耳機回流的能量沒有低輸出阻抗短路掉會造成輸出端的電壓，變成一部分整體線路回授的信號，造成失真，我家的耳擴可是有著低於0.1歐姆的輸出阻抗
And that would be the end of the story but for the fact that headphone
manufacturers build into the headphone acoustic damping. Part of the acoustic damping formula is the electrical network around it. And part of the electrical network is the output impedance of the amp. So when headphone manufacturers develop the acoustic damping for headphones, they usually assume a crappy headphone amp; with a crappy, relatively high, output impedance. Then they design the damping. THe European "standard" is 120 ohms,and this is why Jan puts a 120 Ohm output impedance plug on his amps.
問題本來應該如此單純，但是實際上耳機廠商們把這視為耳機聲學阻尼考量
的一部分
耳機一部分的聲學阻尼途徑正是這個以耳機為中心的電能網路，而耳擴的輸出阻抗
正是這個電能網路的一部分，當耳機廠商們在調整耳機的聲學阻尼要素時
他們通常會以一個很爛的，有著相對較高輸出阻抗的耳機擴大線路為前提去考量
然後再去安排阻尼
歐洲佬們的"標準"是120歐姆，這就是為什麼Jan Meier要在他的耳擴上安個
120歐姆輸出的原因
Unfortunately, it's not really a "standard"; even in the standard it's not really a standard but more an observation. Headphone manufacturers know that they are going to be plugged into things with widely varying output impedance; no manufacturer (that I'm aware of) has ever setr the output impedance at 120 Ohms, in fact, I'm sure they usually do the ncommon sense thing and make the output impedance low as they can and still meet the power and parts price targets they have for the headphone jack----which is usually next to nothing so they just slap in an op amp and call it good.
This state of affair mainly makes headphone manufacturers to have to design for as wide a range of output impedance as they can. Lovely, let's optimize by making the headphones insensitive to the quality of electronics.
不幸的是，120歐姆並不真的是個普世通行的標準，就算去翻原始的規定內文
也只是個"建議"多於"規定"
耳機廠明白他們的耳機會被接上各式各樣輸出阻抗的前端
而就我所知，沒有任何一間訊源製造商會把耳機孔的輸出阻抗設在120歐姆
實際上，我確定他們通常按照著常識去設計：盡可能的讓耳機孔的輸出阻抗降低
──然而這些訊源的耳機輸出通常只是勉強堪用，用個OP應付了事
這樣的情況逼得耳機廠商不得不盡可能的考量各種輸出阻抗的情況：
很"可愛"的，盡量讓這些耳機對這些前端的品質不是那麼敏感*
*可以跟我上一篇那個談耳機阻抗的文章對照
To sum up, two things:
1) Output impedance---lower is still better on average, but some headphones
may benefit by tweeking output impedance.
2) Headphone could sound MUCH better if headphone makers could rely on good amps. As it is now they mainly assume the amp is crap and if they have to do damping they have to do it acousticly rather than the RIGHT way wich would be to rely on a good amp for it's low output impedance and damping factor.
最後總結，兩個要點
1.輸出阻抗：壓低還是比較好的，但是某些耳機可以透過調整輸出阻抗得益
2.如果廠商能夠以優秀的耳擴為基礎去調整，這些耳機實際上能夠做得更好
然而他們寧可用爛耳擴去調整而阻尼過頭，而不願意用低輸出阻抗、高阻尼係數的"好"耳擴.
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這篇文章相當的極端，或者說這麼說吧，雖然譯者也明白這問題，但是不會下這麼絕對的"好"、"不好"、"對"、"錯"判斷
Tyll Hertsens講的這些現象基本上沒有問題
然而他後續的意見或判斷我個人覺得不是那麼站得住腳，或多或少是出於一種
"耳擴"設計者立場的方便
我想可能有人還不是很清楚什麼是阻尼，簡單的講，訊號電流流過音圈，
導致振膜變位，推動空氣產生疏密波，也就是聲波之後
振膜並不是"動了就馬上停"的，而是需要一些機制與設計幫助振膜能盡快的停下來,這樣的機制就叫阻尼
而阻尼大體上可分為機械性的與電氣性的：機械性的例子像是喇叭的懸邊、
箱體內的空氣等等，電氣性的例子就是前述的反電動勢了
要類比的話，那就像是個剎車調校的問題
Tyll的立場很明確，壓低輸出阻抗，抬高整個系統的阻尼係數等等，簡單的講
在擴大機與耳機的交互作用範圍，偏向由耳擴"單方面"施力
然而，歐陸三廠可能並不這麼想，甚至發展方向和Tyll希望的是背道而馳
揚聲器系統中，通常用Q值、品質因數來衡量阻尼的強度，數值越低，系統的阻尼越強(我這邊省略/簡化了很多，有興趣的版友可以再自行研究)
一般調校喇叭系統時，一個通行的標準值是0.5
而我所知的，從HD650到HD800的變化是這樣 (Sonove那邊估的)
HD650：0.38 > HD800：0.19
明顯有越來越強的趨勢
而認為輸出阻抗越低、阻尼系數越強的耳擴才是"好"的的意見恐怕也一竿子打死了大部分的真空管耳擴與恆流源輸出的耳擴......
不過從Tyll的文字中，可以看出一種設計上的取向，或流派吧
順帶一提，要提高阻尼係數(壓低內阻)最簡單的手段就是拉高回授量.....而這或多或少又牽扯到耳擴設計者們對這回事的意見與規劃
我想音響的確很少存在什麼金科玉律，大部分都是在兩難之間的取捨與各自堅持的"正義"
