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发表于 2011-3-8 01:47
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Nelson Pass 细论阻尼及全频
全文在此下载：
www.passdiy.com/pdf/cs-amps-speakers.pdf
09年搜到这个档，图文并茂，娓娓道来，Nelson是神人一个。先翻部分，慢慢再续：
Current Source Amplifiers and Sensitive /Full-Range Drivers
by Nelson Pass
Introduction
Conventional wisdom holds that a pure voltage source amplifier is ideal for audio applications, andgenerally designers of loudspeakers work to that assumption. This belief hasparticularly been dominant since the development of high power solid-stateamplifiers that began in the 1960’s.A small minority of audiophiles thinks otherwise, and these are often peopleusing low wattage tube amplifiers with unusual looking speakers. Well, of courseentertainment is full of fringe elements.
前言：
传统智慧认为，纯电压源的功放是理想的音响功放，而且通常喇叭系统设计师据此从事设计。这个信念雄霸世界，尤其在60年代始的晶体大功率功放潮流。小数烧友不同意，通常就是那些小功率胆机配置古怪喇叭之人。边沿人何其多。
A couple of years ago, Kent English and I were playingaround with various ribbon tweeters, noting how the ribbons themselves seemedperfectly happy being driven by a current source and without their usual matching transformer. We builtan audio current source that delivered 1 amp of AC current per input volt to amaximum of 10 amps. This kind of amplifier is known as a power transconductanceamplifier. Such circuitsare fairly common in small chips which don’t generate the high currentnecessary to drive a loudspeaker. Our amplifier had a high output impedance and thus no damping factor,but we found that the ribbon in the tweeter didn't seem to need much dampingfactor, measuring and sounding a bit better without the matching transformer.We decided that a current source was likely delivering more accurate force/accelerationto the ribbon than a voltage source and after playing with it for a couple of days,we put it away and went on to other projects.
几年前，我和Kent English把弄几个铝带高音，发觉无需耦合变压器，用纯电流源功放就很好声。我们搞了个功放，输入1伏时输出1安培，最大输出10安。这是个跨导功放。这种线路在小芯片很多见，输出电流极低，不能推喇叭。我们的功放输阻抗极高，所以几乎没有阻尼因素，但铝带高音无耦合牛都似乎无需多少阻尼，比有牛更靓声。所以我们总结：电流源貌似比电压源功放推高品铝带更准更快速。跟着就忘记整个事件了。（老余按：铝带高音结构是一片铝片代替音圈，夹在前后两块磁极，铝带导电率蛮高，通常铝带内阻在1欧以下，需要用匹配牛或叫偶合牛变换阻抗，老Pass不用牛，直接功放直入，不怕烧鸡，因为他用的是电流式功放，内阻高，可以不怕过量电流烧鸡烧铝带高频——小朋友如我，没有这个“神Zen”功放，一定不要学.......）
About this same time, Kent and I began playing with full-rangedrivers, those loudspeakers which deliver bass, midrange and treble from onesingle cone. In a number of ways, they don’t measure as well as multiplespecialized drivers particularly at the bottom and the top, but there issomething aesthetically appealing about the simplicity of the idea and on many occasions,they manage to sound very good especially driven by tube amplifiers.
大概同一时段，我和Kent正在玩全频，这些把低频、中频、高频混在一个纸盆出来的东东，测试数据并非特别好，尤其是高低两端，但声音常常有种美态，其靓非凡，尤其是用胆机推。
Most interesting are the full-range high-efficiencydrivers that deliver the goods with only a watt or so. It’s a big designchallenge to produce a good sounding full-range acoustic transducer with100dB/watt efficiency. When it is properly achieved, you get a wealth of detail,exceptional dynamic range and a sense of musical “liveness” that you don’toften hear elsewhere.
最过瘾的是，全频高效率，用一瓦半瓦就可以推的瓜瓜叫。所以设计个100dB高效又靓声的全频，非常够挑战。搞得掂，细节一大把一大把，动态超极限，乐声之灵动，秒杀其他。
Tube amplifiers seem to bring out the best in suchdrivers. They have more bottom end, a warmer, mellower mid and upper mid-range,and often more top octave. By comparison, the “best” solid-state stateamplifiers make them sound more like transistor radios – less bottom and a,occasionally strident upper midrange. If you are a solid-state kind of guy(like me) you start wondering how that could be, and if you are a tubeaficionado, you smirk and say, “I told you so.” The solid-state guy probablystarts fixing the response with a parametric equalizer, and the tube guy enjoyshis music with a nice glass of wine.
胆机貌似是全频绝配。低端丰满，中频又温又纯，更经常在极高频段更出彩。要比较，用最靓的晶体推，变成收音机——低频一点点，中高段吱吱叫，（高频他就不提）。你如果是像我一样的晶体佬，必定嘀咕嘀咕：搞神马？ 但是胆机佬就一定嘎嘎大笑：咋才知道啊？晶体佬折腾如何搞个等化，胆佬早就叹着红酒加靓乐。
2
Critical damping -- that resistive combination ofelectrical source impedance, suspension friction and acoustic load -- occurs whenyou apply a step pulse to the voice coil and the cone’s motion doesn’tovershoot. Under-damping results in bass notes that hang around a little longerthan the amplifier intended. Over-damping has good transient bass control but alsosuffers a significant loss of bottom end response. Generally, we want somethinginbetween, something closer to critical damping. Whether we over-damp orunder-damp seems to be a matter of taste.
临界点阻尼（阻尼刚刚好）——当你加个脉冲波到音圈，音源的内阻、悬边的阻力和纸膜声学负载等等，齐齐出现。功放阻尼不足，低频就来了不大愿走；阻尼过大，低频瞬变快速但最低端却若有所失。我们通常想两边讨好，更接近临界点阻尼。是否太多或不够阻尼，大概是口味问题。
The need for electrical damping is different for eachtype of loudspeaker and acoustic environment. High-efficiency full-rangedrivers are more easily damped than other types due to their powerful efficientmotors and light cones. Looking at their bass response curves, we conclude thatthey are easily over-damped, resulting in excessive loss of bottom end. This partiallyexplains the preference for tube amps with such loudspeakers. Anyway, this assortmentof observations arrived at a confluence when I hooked up a Son of Zen amplifier (Audio Electronics 1997 #2)to a pair of Fostex 208Es in sealed enclosures. The Son of Zen operates withoutfeedback and has an output impedance of about 16 ohms. This nets a dampingfactor of 0.5, miniscule compared to the 100 to 1000 you can achieve withregular solid-state amplifiers.
电器阻尼，不同类型的喇叭系统有不同的需要，又和房间声学有关系。全频较为容易阻尼，因为磁力强大、振盆轻盈。看看频响图，我们归类为“容易过分阻尼”的单元，低频容易消失。大概可以解释何以配胆机更妙（因为胆机的阻尼系数较晶体低）。咋说，这些调调儿，就是在密封箱Fostex209E配Son of Zen功放搞出来的。Zen不用负回输，输出阻抗大概16欧——就是阻尼系数大概0.5，是正常晶体机器的零头。
With the low damping factor, the Fostex became a totallydifferent speaker. It suddenly had bottom end response and a better top end. Itstill had the same annoying upper midrange that had Dick Olsher (www.blackdahlia.com) devise his passive equalizationnetwork. The low damping factor didn’t cure the upper midrange faults but itseemed to work improvements everywhere else. A year later, Kent hasacquired about 20 different full-range mid to high efficiency drivers for us toplay with (that’s part of his job description) and we’ve spent as much timeexploring them as we reasonably could, trying different things to coax the bestsound out of them with a current-source amplifier and various passive parallelnetworks.
阻尼如此低，Fostex好像变性。他突然屁股大大，低频丰厚，连高频也更好。虽然中高部分还是有时刮刮不绝，所以Dick Olsher搞了个等化器给他。低阻尼无助于中高。一年后，Kent搞来20个全频，重效率到高效都有，玩个不亦乐乎（他们付工资给我玩，哈），逐一折腾个够，当然也是使尽浑身解数，用电流源功放加个等化网络，压榨他们的美妙声线。
What is a High-Efficiency / Full RangeLoudspeaker?
To arrive at high efficiency and wide bandwidth, wefundamentally need two things – a great motor and a great radiating surface. Agreat motor means getting the most force/acceleration for the least amount ofelectricity. It means a big magnet with lots of magnetic density in a preciselymachined gap where a very light voice coil sits perfectly aligned. This voicecoil is wound in a cylindrical assembly that maximizes the current exposed tothe magnetic field and generates the highest amount of force (acceleration) fora given amount of electrical current.
到底高效/全频的为何物？
要高效和宽频带，基本上两个元素——更厉害的磁头结构和更好的振膜。磁头大，小量电力就力量大速度快。就是说大磁头，磁束密配合准确的磁隙，用上很轻的音圈调准位置。音圈绕在滚筒校正在电能集中、磁力集中的区域，发出最大的力量（加速度）。
In other words, we need an expensive motor.
换句话说，要有个又贵又靓的驱动磁头。
\ The other half of the equation is the cone assemblyattached to the voice coil. In contrast to the fine motor engineering, here westart seeing more of the black art involved in designing such a drive unit. Theradiating surface for such a loudspeaker must be very light to maximize theacceleration from the voice coil. At the same time, the cone needs to be stiffand inflexible so that this acceleration can be accurately transmitted to theentire radiating surface at once.
这个方程的另一边，就是与音圈连在一起的椎盆组件。对应这个动能工程，我们遇到的是驱动组件的“黑色艺术”。振盆必须极轻令音圈移动更快速。同时，振膜要够硬无弹性，令全个振膜一起震动。
When this fails at high frequencies (which it will), thecone needs to decouple the force gracefully to a smaller and smaller surface sothat it effectively shrinks at the highest frequencies. In practice, it’s a lotof art coupled to even more trial and error. Thomas Edison could have made afantastic version of such a speaker and for his time, I believe he did.
当这个硬度，在高频时注定无能推动整体振盆（一定不能），椎盆就必须逐渐把震动范围缩小，令越高的频率有效发送。实际制造时，更多的是不断的尝试多于艺术处理。爱迪生肯定做过这个屡败屡战的喇叭发明过程。
3 Besides obviously requiring low power and only one driverper channel, the advantages to a full-range efficient loudspeaker are found inthree areas. First, the speaker requires no crossover network to apportionfrequency bands to different drivers and thus does not suffer the phase shiftsthat come with such filters. Second, the sound radiates from one point source - diffraction effectsbetween multiple drivers are removed. Third, the electrical and mechanicalqualities required by such drivers give rise to subjectively good dynamic rangeand detail, assuming we deal with an ideal example of a full-range efficientdriver.
在低功率的同时，每声道单一单元带来的好处有三个方面：一、无分音器则无相位漂移。二、
单点声源——衍射效应消弥与无形。三、够水平的单元，电器和机械质量令单元有更佳动态及细节。
Naturally, the reality can fall well short of thepromise. Real drivers tend to be too limited to satisfactorily deliver both thetreble and bass octaves of audio. The decoupling of high frequencies on thecone is often not smooth, and with that come response peaks and dips in theupper midrange often followed by a high frequency roll-off. On the bottom end,overdamping of the very light cone contributes to a roll-off starting an octaveor more above resonance so that many examples ofsuch drivers fall short below 100Hz. Driven from a voltage source in a closedbox, the response of such light-weight cones can be down by as much as 15dB at resonance. Some of these drivers are betterthan others, most are delicate and some of them are very expensive.
现实世界不应该完美。真的单元发送高频和低频段都属有限。高频的发送又难以平顺，中高频段谷山交叠，跟随着高频滚降。低端频率时，轻纸盆的过度阻尼，把100周以下减低。用电压功放、密封箱体，这种轻振盆在谐振频率会减少很多，曾经见过减低了15dB。折腾的单元，素质参差，但大部分都是非常精致，更有些是钱包出血价。
Power Transconductance Amplifiers
This is a fancy name for a power current source. An inputvoltage causes the amplifier to deliver a proportional output current. Ofcourse this same sort of thing would occur with an ordinary amplifier driving apure resistance but a loudspeaker circuit is not purely resistive. It possesses numerousreactive elements, some due to inductance and capacitance in the electricalcircuit, some from the reaction to motion of the voice coil in a complexmechanical system. Fed by a voltage amplifier, the current through the driver’svoice coil is not directly or instantlyproportional to the input to the amplifier. Ordinarily, loudspeakers aredesigned around this assumption but the “piston model” of loudspeaker designassumes that the acoustic output mirrors the acceleration of the voicecoil/cone assembly over a specific range and this is reflected by the currentthrough the voice coil.
跨度功放
名字过瘾。电压的输入令功放按比例放送电流。当然，普通功放对应电阻式负载也有这个功能，但单元并非纯电阻式阻抗，当中有电感和电容，动能会从音圈回灌电能到功放。用电压功放，电流通过音圈时，并非直接和线性。普通喇叭，设计基于假设的“活塞” 运动形式——“音圈-椎盆”整体跟着信号电流运动的。
The most precise way to develop that specific current is witha current-source amplifier. Such an amplifier ignores the impedances in serieswith the circuit, the resistance and inductance of the wire and voice coil andthe back electromotive force (EMF) produced by the cone motion. As I said, mostspeakers are designed around voltage sources but there are few instances wherea current source can be used to advantage. One of the best ones is the categoryof full-range high-efficiency drivers.
而最准确的对付这些电器特性的就是电流源功放——它们可以忽视串联的负载，即如音圈的电感和电阻，甚至无视振膜（在谐振频率的）发电的回馈电压。上面说过，绝大部分喇叭都是给电压推动，但有些喇叭用电流源功放，效果更佳——例如高效率的全频。
[ 本帖最后由 henry余 于 2011-3-8 01:50 编辑 ] |
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