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研究这个一波多折，尽量以简明的文字描述俺的观点
背景： 部分商品化合DIY的耳放输出设计有输出电阻，而部分没有，有加输出电阻的耳放，其阻值也各不相同，这导致俺的困扰
加电阻和不加电阻的目的：
- 加电阻使得放大器输出阻抗增加，耳放驱动接近电流驱动，而不加电阻则为电压驱动。
- 加电阻的原始目的是为了保护耳机和放置短路损坏耳放电路
国际标准对加耳放输出电阻的说明：
- 推荐所有的耳放串联120欧电阻，这个推荐对于输出阻抗为16~600欧的耳机都适用
- 这个标准是IEC61938，页码16， 表格8
-标准上对加电阻的说明：
1）对大多数耳机而言，加这个电阻对性能影响有限
2）如果耳机阻抗在8~2000欧，这个120欧输出电阻也是适用的
问题来了：
- 这个标准对耳放厂家来说不是法律，只是指引，即guidance。。。
- 有一些耳机的频响-阻抗曲线是平直的，但很多并不平直，差异很大。120欧的输出阻抗实际还是严重影响了耳机的频响曲线，如 HD600，重放频率内，阻抗的变化超过100%， 即标称300欧的耳机在100hz左右阻抗高达600欧。
- 这个规则要耳机生产厂和耳放共同遵守，如果耳机频响曲线是在120欧的耳放驱动下进行调教的，则我们使用120欧阻抗的耳房将可期望获得设计时的性能 （频响曲线）
一般网上专业论坛的理解和猜测如下：
-耳机大厂，如歌德，拜亚动力，森海塞尓，AKG肯定是遵守 IEC61938，则耳机的调教和评价都是在120欧耳放的输出下进行的
- 低阻耳机适用于电压驱动的耳放，高阻耳机适用于电流输出的耳放
但事实是否如此？有老外向耳机原厂发起询问，问耳机大厂是否遵循 IEC61938，即耳机是按照120欧源电驱动下进行调教？ 耳机厂对耳放的输出阻抗有何建议，以下是收集到的答复：
- 森海塞尓的答复：
耳放阻抗 0~10欧之内即可，否则低音会过量。森海塞尓的耳机是基于0内阻放大器设计的。
- Hi,
I call Sennheiser (Germany) about in IEC61938 recommand 120 Ohm.
Sennheiser tells me that a good headphone amp for Sennheiser headphones should be have an output resister from 0 ... 10 Ohm - otherwise the bass will rise.
另一个答复
I remember Jan Meier has visited the Sennheiser factory quite a few times and they told him that the Sennheisers are designed to sound as intended with an amp as close to zero ohm output as is possible.
- 来自AKG的答复
AKG的耳机是为低阻抗放大器打造的，如K701，即不遵循120 ohm规则，但是，如果一定要用120欧的电流驱动，问题也不大，因为K701的频响-阻抗曲线比较平缓。
Karl Peschel is in semi-retirement - anyhow he answer
K701 are constructed for low impedance output amplifiers.
But the rise of impedance at the resonance frequency is unincisive, so the effect to the sound is low.
The K701 works good (without diversification in sound) with all the current headphone-jack output-impedance
- GRADO 的观点：
RA-1已经说明了GRADO 的耳机是基于 0内阻耳放设计的。
- 拜亚动力对120欧耳放阻抗的观点
拜亚 DT880在耳放输出阻抗为250欧时频响最平坦，建议耳放输出阻抗在200~250欧， 120ohm勉强可用，但是零输出很不合适。
Whoah! Been on the phone to Beyer for 30 minutes, phoned up to ask them "what impedance?" and the guy went into some seriously mind boggling stuff which went totally over my head! He was talking so fast and so passionately I just couldn't digest it!
Basically Beyer say the optimal output impedance of an amp using their DT-880 is 250 ohm. This will give you the flattest response, plugging them into the 16 ohm output of an ipod will not do them any favours but will result in a more unnatural (yet pleasing sound) He described it as the classic smiley face response with bass up mids down and treble up.
He went on to say that some of the competitors design their headphones to sound like this... for flattest response with DT880, however, 250ohm is the desired output impedance.
He explained that impedance is not everything we are also taking the materials, the loading, the enclosure into equation... closed, open, semi open. Impedance should only be considered a rough starting point and as the 'phones are to be used in domestic situations trade offs must be expected but for optimum performance, if you use Beyer DT-880 / DT-770, DT-990 then your amp output impedance should be 200 - 250 ohm.
I really wish the guy had been talking slower as he was giving out some VERY interesting stuff! I didn't have a pen handy so couldn't take notes. He did say that no way is 0 ohm suitable for beyer 'phones.... 120ohm is marginal but 200 - 250 ohm optimal for the most honest reproduction.
拜亚技术支持的另一个人的回复，认为120ohm可用
Further to the Beyer tech guys 250ohm recommendation for the DT-880 / 770 / 990 somebody else has sent me the following:
"Dear Mike,
Many thanks for your email and apologies for the delay in replying. In most applications we would recommend an amp with an output driving impedance of 100 ?120 ohms to drive most of our headphones.
We currently offer our new A1 headphone amplifier - http://www.beyerdynamic.de/cms/Headphone....uctdatabase_pi1[showUid][showUID]=1876&tx_sbproductdatabase_pi1[showUid][backPID]=108&cHash=b11daed01b and this offers a headphone output impedance of 100 ohms.
结论来了！！！
虽然IEC61938 推荐了耳机放大器需要120欧 输出阻抗，部分耳放采纳了这个指引，但绝大多数的耳机厂都无视这个指引，所以，我们选择耳放的输出阻抗还不能跟着这个IEC61938跑，我的建议是，耳放0内阻输出可以适用于大多数耳机，且比120欧的源阻更贴近原厂耳机的设计期望，当然，耳放也可以选择三档输出 0欧，适合大多数耳机， 120欧，符合 IEC61938，但目前没有适用的耳机来匹配， 250欧，来对付拜亚动力 |
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发表于 2010-5-1 11:51
楼主
