请问谁有《我如何设计Amity放大器》完整资料

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很早以前看过《我如何设计Amity放大器》，不过当时没有保留完整的实际线路图，不知哪位大侠还有？大家一起谈谈对这篇文章的看法吧!

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我如何设计Amity放大器
作者:lynn  | 文章ID号:69 | 点击:0次 | 发表日期:2001-12-09	对本文章感兴趣？到论坛讨论！
file:///G:/SIEMENS/PRO/E/PZH_BACKUP2004115/E/Music/文章/第1页%20-%20我如何设计Amity放大器%20-%20胆艺实作%20-%20胆艺轩网站.files/Amps.jpg 翻译：IMXP　　类似Ariel音箱，Amity放大器作为验证我设计思想的一个实验。 　　几年前我已经读了Crowhurst和其他的50年代作者关于放大器设计的一些文章，我打算设计一个尽量小内在失真的电路--特别是针对典型的具有反电动势的负载---例如扬声器。 甚至类似Ariel扬声器这样的阻抗仅仅在3.5和8欧姆之间变化不大的负载，对于SE设计的电路来说仍然是一个值得考虑的问题。 　　扬声器的阻抗程电抗性，我们在输出管的特性曲线上考察扬声器的负载线，是弯曲的。 　　三极管A类PP电路的带负载输出特性与理想状态比较接近，不受电抗的影响。作为对比的是，其他所有包括如三极管的SE、AB类的PP或五极真空管、晶体管或MOSFET的电路，输出特性统统受负载的影响。五极真空管（或者晶体管）AB类是最坏的情况：在零信号区域周围负载线是弯曲的而且弯曲的程度最厉害，...用另外的话来说就是“小功率状态下是最糟的”。 　　扬声器的阻抗程电抗性，其中有分频元件带来的，还有就是驱动单元本身能够储备能量并在几毫秒的时候反射到放大器的输出端。对于大部分的放大器来说，来自驱动单元的的反电动势也令放大器的失真大大增大。这是为什么放大器推不同的扬声器会发出不同的声音的另外一个理由。用SE电路，弯曲的负载线仍然存在，但是在零信号区域附近曲率不大；因此它的小功率输出特性比较好。 　　音乐信号大部分的时间是在较低的电平上，而突而其来的峰值却超出平均值10到20DB。经过统计，如果扬声器的效率不是特别低的话，很大一部分的时间里放大器工作在1瓦或者更低的情况下。 如果你想保持从低到高的输出功率放大器都工作在比较好的情况下，三极管A类PP电路是比较好的选择。 深度A类的状态使电路远离AB类区域；当负载在短时间变得很小的时候，SE放大器将会出现剪峰。 　　另一个目标是将电子管的染色和失真降到最低。这跟一些SE放大器采用的失真抵消的方法非常不同。（译者注，原文是：Another goal is getting driver coloration and distortion out of the picture ... quite different from the distortion-cancellation technique seen in some SE amps.这里的DRIVER应该是指电子管而扬声器单元） 　　一些设计主张使用失真抵消技术，我对于它们能否在宽广的电平和频率范围内能否起作用持怀疑态度。而且我希望放大器拥有用稳定失真特性（失真受频率和电平变化的影响小）。我怀疑耳朵可以轻易的补偿某类失真带来的影响--如果该失真是自然、稳定、单调的，这也是为什么SE电路有出色的主观评价的理由，但是对于量和类型经常改变的失真，耳朵是非常敏感的。不幸的是，传统的分相电路通常有不对称的输出阻抗、输出电容，或者他们对B+电路有不同的敏感性，因此，偶次谐波并没有达到很好的抵消而且随着信号的变化而变化。 不稳定的失真特性正是PP电路不如SE电路透明的根源。

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为了解答这问题，我们需要回到三十年代设计的古典的变压器耦合上面来。
　　为使三极管放大电路的失真最低，三极管的负载线必须水平——即采用非常高的负载阻抗，或许比一般情况下高10倍以上。完成这目标的唯一的方法是使用有源负载（但他需要额外的工作电压给有源负载的器件），或者扼流圈（电感）负载又或者变压器负载。既然我也想得到整个电平范围下的高度对称的裂相信号，我从用古典UTC和Sakuma放大器上得到灵感，采用级间变压器作为裂相元件。
　　我个人喜欢宽阔的带宽，所以选择了Lundahl的高质量的变压器。除Lundahl的产品外，Hiroshi和我在从Sowter、Peter Chappel的产品中也发现好的全平衡变压器。
　　在PP电路里，两个裂相信号的频率高端的平衡、对称度更难得；很多昂贵的变压器存在不对称的电容，这对于高频的失真来说影响特别大。如果变压器本身不对称，这样就算用精确对称的电子管也没太大用处。
本放大器和三十年代设计的“古典机”之间另一个差异是前者的频带要阔的多。在当时，根本没有宽带的信号源。都是窄带而且杂音大的信号源：AM收音机、78rpm记录的唱片和光学记录方式的电影的声轨道。但现在拥有65到100dB S/N和20-20 kHz带宽的信号源遍地都是。
　　使用古董零件和电路当然会发出同年代的--古董的声音。但那不是我的目标。我想使用三极管和变压器耦合—而且他们有无可匹敌的线性和超宽的通频带，例如15 Hz-70 kHz。这是为什么我要选择现代变压器，并且慎重地选低内阻三极管的原因。
　　当变压器的初级或次级的阻抗变低，它的带宽增加。级间变压器其实比输出变压器更难设计，原因就是它的初级或次级的阻抗都非常高，而输出变压器至少有一个低阻抗的次级。因为电子三极管要取得最低失真的特性，必须工作在无负载的情况下，因此级间变压器的次级没有接电阻负载。因此为了得到宽带，变压器初级的电子管的内阻必须足够小。
　　在这里合适的三极管是7119/7044/5687/E182CC系列，或者JJ Electronics新出的型号ECC99，或者Sovtek 的6H30，或者把6V6、6L6或EL34等接成三极管应用，又或者使用AVVT新出的直热系列管象AV5、AV8或AV20。（尽管常见的6DJ8/6922/E88CC系列有低的屏阻抗，但它的3次谐波失真比较大和有限的输出摆幅，因此不适宜在这里使用。）


电路的示意图如上（点击得到全图）


　　将灯丝与放大器的地线进行静电屏蔽似乎是次要的细节，但实际上也非常重要。这特点是朋友Tektronix向我建议的，原因是简单却又微妙的。
　　大多的DHT（直热三极管）放大器中，灯丝暴露在高电压的B+和从AC电源引进的噪音干扰下：这两种噪音干扰通过使电源变压器的分布电容等方面耦合进来。不单如此，典型的灯丝直流供电方法同样对付不了该类型的干扰；因为它们处在共模状态下，换句话说，它影响到灯丝的两端。因此就算加上直流稳压的方法也没有作用；它只仅仅影响到差模的噪音干扰。
　　直热三极管其实对差模的噪音干扰不太敏感；当进行屏蔽后，它至少使60 Hz嗡嗡声降低了40dB--降到原来的1%，为什么DHT要特别防止共模噪音？因为它的灯丝体积比较大因此对共模噪音很敏感。电子管不是单放大栅极的信号，而它真正放大的是栅极和阴极（对于直热三极管来说是灯丝的中心）之间的电压差别。如果你希望DHT寂静，虚拟阴极--灯丝的中心比较不受噪音的影响。如同前述：直热三极管的灯丝暴露在高电压的B+和AC电源的噪音干扰下。因此，听起来产生了高频杂音和嗡嗡声，并不是哼声。如果干扰比较小，虽然听不见，但可以想象同样影响着信号，类似给信号加了一层面纱。
　　可以通过为每个灯丝端增加精密匹配的LC过滤器去减少灯丝杂音，但是有更直接的解决方案：从根源上阻隔杂音。因此电源变压器内加了静电屏蔽层。静电屏蔽层是加在灯丝电源和其他绕组之间的薄铜片，另外在电源初级绕组也加上屏蔽层。屏蔽层减少绕组间电容量到原来1%-0.1%，10年前已经是仪器和医学装备里变压器的标准。屏蔽层同样也减少了因高压绕组故障引致的危险。
　　在电源变压器里增加一个屏蔽层其实并没有增加多少成本。而这两层屏蔽层往往比其他方法更有效改进声音。
　　再看看电源B+的供应，一开始我打算使用5R4-GY作为整流（通常应用在传统的300B电路中）。但由于其电压降性能不好和可靠性差而放弃了。或许它们太旧了，但我不允许这样的情况再次出现。作为对比的是，TV阻尼二极管特别是6C*3系列和新的Svetlana 6D22S电子管电压降比较稳定，它们还有电压降低（15 V），峰值电流大（2 A）和预热时间长（30秒）等不少的优点。
　　Matt Kamna做了一个示范，用示波器来观察各种整流器件的输出电压波形。在使用晶体二极管整流的情况下，在交流电经过零点时出现了明显的开关特性，从而产生了开关噪音。而传统的电子管整流则显得比较平滑，而使用TV阻尼管则得到最平滑的特性。因此甚至在低电流的前置放大器中使用，它也能提供最少的噪音。
　　我使用Tektronix公司的频谱分析仪也测量过，在滤波后的确排除了噪音。B+电源其实是从几百伏交流电压开关得到，这也是值得考虑的问题，因为这些开关噪音可以辐射到任何地方，包括B+电源、机器的底盘和交流电源进线处。因此为了进一步降低影响，我在其他级的电源供应上也增加了稳压装置--使用常见的VR管。但是真空稳压管的并没有太好的口碑，我觉得其实是使用手册指示不清楚的原因，其实真空稳压管并不需要并联电容，因此不要象使用齐纳二极管那样并联1UF左右的平滑电容。而且，真空稳压管仅存在1mV左右的平滑的宽带的噪音，而齐纳二极管的噪音却高达3-5mV，而且噪音类型是不规则的爆米花似的“BUMP”“POP”。真空稳压管同时也起到工作指示的作用。
　　我在真空稳压管的前面尝试了多种类型的滤波电路，发现它们的声音差别很大。最好的一种是什么滤波电路都不用，仅用一个电阻进行降压，但可以想象在大动态的时候，上一级电源波动会引起比较大的影响。当使用LC滤波时，声音变的非常慢，我不喜欢。简单的RC滤波电路可以说是一个折中的办法。
　　我把电路中所有的阴极旁路和滤波电路的时间常数通通设在3-4HZ，因此整个放大器的工作点的建立是同步的----我不希望在驱动级和功率输出级之间出现“打架”的现象。
　　我做的最后一个关于主观调节的地方是对灯丝分别使用AC和DC供电，结果说明DC供电的话，声音是非常乏味的。而AC灯丝供电的噪音电平仍低达1-3mV，因此我选择了AC点灯丝。有些人说使用恒流源对灯丝供电的声音是最好的，不过我没有尝试

音乐传真

考虑前置放大器
　　大部分的前置放大器推本功率放大器的声音都不大好---电子管的通常比较迟钝和闷而晶体管的通常又太冲、或者无力或者粗糙。不单如此，本功率放大器也不允许通过被动前级和激光唱机相连---因为输入变压器要求信号源的内阻远小于600欧姆。我尝试去掉音量控制而直接接到CD机的输出端，声音非常引人注目，十分类似AUDIONOTE ONGAKU、WAVAC和JACK STRAYER的SV572 PP放大器。
　　后来我借了一台JEFF ROWLAND的前置放大器来推动，声音非常好，接近直驳CD的效果。但它是如此的昂贵，因为我决心设计RAVEN(大乌鸦)前置放大器来推动它。这台RAVEN放大器的电路结构直接参考功率放大器，只增加了并馈的特征。通常在电子管的屏极的电感负载里并上电阻的原因是减少电感分布电容的影响。


前级电路示意如上（点击看全图）

　　使用SOWTER的线间变压器（变压比为3.2：1）,当使用7199/E182CC电子管后整个电路的增益大约为7,而使用5687、7044则增益稍小一点.可以根据自己的品味在3管之间进行选择：3个管用在本电路的失真都非常小。

　　以下是我的合作伙伴Hiroshi Ito，在完成了他的Raven/Amity/Ariel系统后的第一个反应：
　　听过本系统的人都很吃惊于本放大器和标准的PP放大器或者我的晶体管放大器之间的差别。细节非常多而且他们能很好地融合起来。另外一个问题是我的晶体管放大器现在看起来是非常的无活力、非常薄。Raven提供了从容、透明的声音和惊人的分析力，声音十分有生气、带情感。而用其他的晶体管前置放大器显得无活力和晦涩。欧洲的Ariel制作者也向我反映过听钢琴时出现中频凹陷的问题，我想原因就在这。当用此系统重播复杂的音乐例如管弦乐队和合唱时，达到了类似静电扬声器般的速度和透明感......


　　我本以为使用AMITY的爱好者通常会买或做一台中等水平的前置放大器,但实际上不是,AMITY对前置放大器的要求十分高,一般都很难满足要求.因此,我打算改进AMTIY放大器而成为AURORA放大器,增加一个内建的增益级而成为真正的合并放大器.眼利的读者可能已经注意到AURORA放大器的第一级其实是单端的放大级,不同于RAVEN。
　　我对SE放大器并没有偏见，特别是用在小信号的地方上，仅仅对于大信号输出的情况下，我觉得PP比SE要好。而SE电路的小信号输出失真也非常小。SE输入级的缺点，在于如何获得宽带全对称的SE-PP转换和更严格的噪音隔离。这电路其实在旧的WE电路里随处可见。但我加进了“并馈”、阴极旁路和SE-PP的耦合变压器在里面。Sowter 9100线间变压器在宽广的频带里能保持非常准确、对称的SE--PP转换。6SN7系列电子管是最线性、声音最佳的中小功率电子管之一。
　　在使用负反馈作为解决诸如失真、信噪比问题的普遍方法之前，电子管设计师不能不设计线性好的电子管，因此，一些古老的电子管线性非常好---这并不意外。AURORA的第一级电子管允许多种型号互换，如果你敢于尝试，你可以使用直热管如古董的26或现代的AVVT AV20。



AURORA合并放大器的第一级示意图（点击得到完整的电路）

　　尽管图上没有标示，你也可以增加输入选择开关或者JENSEN JT-10KB-D之类的隔离变压器用来连接到你的CD机上。1:1隔离变压器的好处是没有地的连接、两边也没有电连接，这样排除了地环路，而且它也滤去了数码信号里的RF干扰。如果你确定要增加1：1隔离变压器，要确保输入端到变压器端的连线要足够短，而且RCA的地端不要跟放大器的地连接（即浮地）。而静电屏蔽层必须接在变压器的次级：这样可以把数码音源含有的大量的100K以上干扰信号滤去，如果这些干扰能有效的滤去，声音会变得更清晰和自然而接近模拟音源。
　　最后，底盘上元件最好能以镜像的形式排列，而电位器放在中间，以缩短信号到两边的通路。 如下图：


点击得到完整的底盘设计图

　　底盘方面，虽然铜并非特别适合的材料---不够坚硬和难以加工而且非常重，但它的优点也不少，首先是导电性非常好，其次是能得到无侵蚀的接地点。这优点很重要，因为机器了一般都使用铜或者银作为导线，其中的部分导线必须接到底盘上。我们通常很难把铝和铜焊接起来，当两种不同的金属如铝和铜连接的时候，在连接处会它们会互相侵蚀。侵蚀产生了类似半导体的效应，因此该点就不再是理想中的零电位了。
要克服铜板上述的缺点，可以使用镀铜铝板，但我觉得镀银铜板或许是最好的，因为铜、铝的氧化物导电性能不好，银的氧化物依然是导电的而同的却是半导体。

音乐传真

设计哲学：　　在这里总结一下我的设计哲学，回想一下，严格来说我并不属于主流HI-END的支持者，也不属于热衷于小功率直热管推号角喇叭的爱好者。
　　我专注于一条自己的路，探测一些音响技术中不为大多数人注意的地方，而不大注重潮流，希望长时间保持自己的风格。或许是设计扬声器这工作影响了我，大多数的扬声器，它们并不带反馈，因此在选择的时候迫使我去选择最佳的单元。
　　具体几点如下：
1）电路结构、有源元件要有足够小的失真，尤其有低的高次谐波失真。
　　不采用五极真空管、束射管、晶体管、MOSFET、IGBT，和其他的非线性的电子管例如12AX7、12AU7、12AT7或者6DJ8。全部的有源元件要求小的高次谐波和适当的2次谐波。倾向于使用30年代中期直热三极管和60年代早期的高跨导三极管。
　　50年代中期通过大量使用负反馈来达到要求，设计出12AX7和12AU7等来替代了优秀的大八脚管如6SL7和6SN7，因此，设计这些管的时候并没有太注意他们的3次谐波失真。
　　12AT7等被设计在电视里的放大电路中，他的线性也不好，因此最好不要用在音频中。
　　而6DJ8/6922是普遍采用于彩色电视里RF放大，不是为了音频设计的。它声音里表现出来的“细节”其实是它本身高的3次谐波导致的，因此在PP电路中更不适合使用。
2）电路的每一部分都为了该级有源元件能达到最佳的线性度而设计。
　　对于三极管来说，意味着采用有源器件、电感或者线间变压器作为负载，而不是采用RC耦合。
　　RC耦合的原意是应用在HF的宽带上---同时必须加上大环负反馈，但其实它的失真比采用有源器件、电感或者线间变压器作负载的电路要大2-4倍。我个人更喜欢电感或者线间变压器因为他们的电路更简单可靠，而且没有有源器件带来额外的染色。同时我也不赞同采用晶体管作为恒流源负载，或许是害怕他们不够可靠。
　　采用电感、变压器耦合的另外一个优点是没有RC耦合里前驱动级管和输出级管的恢复时间问题：例如瞬间过载的话，RC耦合的输出级管通常需要几百毫秒去恢复到工作点上，而采用电感、变压器耦合则立即可以恢复到正常的工作点上，这对于HIFI放大器来说是必须的。
3）无局部或者全局的负反馈。
　　或者其他人会说所有的三极管都存在大量的局部反馈啊，但我觉得这种反馈跟我们平常理解的不同，一般我们平常说的反馈（无论是局部的还是全局的）对谐波失真里各次谐波的比例没有效果，减少的只是失真的量。 任何型的反馈都没影响对和声学的比率有，仅仅改变他们的大小。
　　直热三极管和类似直热三极管声音和其他管的唯一区别就是没有高次的谐波。
　　如果电量本身是线性的，反馈就没必要存在了，因为反馈不可能做的很好，特别是针对复杂、非线性的扬声器负载，会代来相位差和稳定性问题。
　　无负反馈可以将扬声器带来的反电动势和输入电路隔离开来。
4）足够的驱动裕量
　　一般商用的电子管放大器在驱动部分仅有1-2DB的裕量，因此大信号的剪峰会立刻出现。这导致了较长的恢复时间和夸大了剪峰的可辨别度。我宁可采用3-6db的裕量，因此驱动电路在输出级深度剪峰的时候仍保持其本身的线性。
　　或许驱动级有足够的电流和低的输出阻抗更重要。许多放大器的染色其实出现在驱动级，因为驱动级不能提供足够的电流来产生足够的栅压摆幅。假若有足够的电流，输出管会发出更透明的声音，并失去它们因此而著名的音色。这意味着输出管在一定的情况下会产生栅流。
5）屏弃整流器的噪音干扰
　　通常使用晶体管整流和大容量的滤波电容会产生最大的噪音干扰。
　　因为典型的晶体二极管的开关噪音频带在4-20K，它们往往在大容量电解电容的等效电感里和电源变压器次级分布电容里产生谐振。谐振的Q值是非常高的，通常在5-100之间。
　　 这是为什么在质量好的PP电容上并联大电解电容反而会令声音更差的原因。同时它也是为什么电源线可以改变声音的原因，因为电源线在此情况下担当了小线圈的角色。
　　这些噪音干扰向底盘、B+电源、音频电路和连线辐射。虽然这种噪音干扰能用过滤、屏蔽的办法减少，但最有效而简单的办法就是从根本上消除它。
　　我使用混合的扼流圈--电容的PI滤波器对B+电源进行滤波，可以把干扰降到最小。
　　从整流的波形来看，最粗糙的是一般晶体二极管的，其次是快速恢复二极管，然后是常规的电子管，最平滑的是TV阻尼管整流。
结论：　　这套电路结构和器件选择纯粹是个人爱好和理解，每一个设计者都有自己的爱好和选择，而这正是为什么音响器材听起来不同的原因。
　　我的选择并不表示声音一定是最好、完美的----因为从来就不存在哪样是最好的。
　　“完美的声音”只在我们的头脑里出现，或者在一些自封为“专家”的写手文章里出现。
　　不要迷信他们，要相信自己的耳朵。
　　每一个人对同样的声音有不同的感觉，而对它的理解是你自己所独有的。
　　我接触音响器材25年了，交了很多朋友，但大约只有2-3个人的看法跟我完全一样。
　　为什么你要向你不认识的陌生人去请教？请自己来听，不要在商店和HIFI展去听，尝试多接触一些喜欢冒险的DIY者，或许你会有惊奇的发现。

音乐传真

失真测量：　　下面的频谱图是Amity放大器在1.6瓦的的谐波失真，采用用Tektronix SG505信号发生仪和Hewlett-Packard 8535A频谱分析仪。

file:///G:/SIEMENS/PRO/E/PZH_BACKUP2004115/E/Music/文章/第5页%20-%20我如何设计Amity放大器%20-%20胆艺实作%20-%20胆艺轩网站.files/1W_Spectra.jpg

　　如果放大器没有失真，你看到的是单一的1 kHz信号一直延伸到顶，而旁边谐波、干扰信号降到最低的水平。
　　垂直轴是振幅（0-100dB ），水平轴是频率（0-10 kHz）。垂直轴内的刻度是每段10dB，水平轴内刻度为1 kHz。
　　2次谐波在主信号的右边，为2 kHz，从图中看出幅度是比较小的，与1K信号幅度对比，大约为-70DB，换成百分比是0.03 %左右。而3次谐波大约为-65DB（0.05 %）。在5K那里出现的一个极小的峰其实是噪音干扰，水平大约在-82dB（0.008 %）。
　　我们也可以看到本底噪音在-95DB的水平，这其实是测量系统本身的。
　　下面给出Harry F. Olson对6F6、2A3放大器的谐波失真测量结果：

file:///G:/SIEMENS/PRO/E/PZH_BACKUP2004115/E/Music/文章/第5页%20-%20我如何设计Amity放大器%20-%20胆艺实作%20-%20胆艺轩网站.files/6F6vs2A3.gif

　　注意垂直刻度是20DB，这两种电路是SE的，而非PP。
　　如果他测量的是完美对称的PP放大器，图中显示的偶次谐波会抵消掉（2、4、6倍等），而奇次谐波的比例则不变。但实际上不存在完美对称的PP放大器，因此还有一些偶次谐波残留在信号中，残留的程度跟电路不对称的程度成正比。如果电路里存在10%的不对称（相当大了），那样偶次谐波将会抵消掉20DB，而5%的不平衡（典型情况）则会抵消掉26DB的偶次谐波。1%的不平衡将抵消掉40DB。实际上，1%的增益匹配在动态条件下很难去实现。而采用线性好的三极管，典型情况（5%不平衡）下，经过对称电路的抵消后，2次谐波和3次谐波的量在同一水平上。而采用了线性不良的三极管在PP电路中，由于奇次谐波分量比较高，而产生类似晶体管的声音。这就是为什么在PP电路中要微妙地选择三极管的原因。
　　虽然我们测量的时间相差了近半个世纪，但我测量的AMITY放大器的谐波失真和频谱非常类似H.F.OLSEN测量2A3 SE放大器在1W时的情况。3次谐波几乎在同一水平-60DB上。但注意PP电路的2次谐波比SE电路的要低26DB，这样我们可以预言该PP放大器有5%左右的不对称。
　　就算没有局部和全局的负反馈，本AMITY放大器也能达到威廉类放大器的失真水平----要知道后者有高达20DB的负反馈啊。从这里也可以看出直热管的线性非常优良。
　　不单如此，谐波频谱更干净，没有高次谐波成分。
　　这是为什么直热三极管听起来率直、鲜活的重要原因。
　　耳朵并不受负反馈的欺骗。我们听的是放大器件的实际声音。这也是为什么我选用有源器件考虑最重要特性就是线性的原因。

结论：　　这里基本是技术性陈述；如果你觉得它比较难理解，请找一个有经验的DIY者，或者吉他放大器设计师、老龄的火腿爱好者来学习有关电子管放大器设计的手段和艺术。
一些布线等技术也需要熟练的DIY者来完成。
　　你可以选择上述电路的其中一个来尝试，一旦你取得成功，那种喜悦感远远超过你能从那些高价发烧线中能得到的！
　　祝你好运！

Julien

我也看过，不过也无保存了

资料很好

kenny

如题。

Julien

有源器件的选择问题上，首先是认同小量的三次谐波亦是不影响音质的，这点体现在平衡放大器上。

其次是不同的工作组态，产生的谐波失真是不同的。

当我们需要一个小输出电压，高放大倍数输出级时，五极管往往是一个好于三极管的选择。由于当需要输出信号电压小于最大输出电压能力的1/5时，优秀的音频五极管往往是趋向无失真的。即五极管的失真特性是：失真呈指数形式上升。而优秀的三极管的失真特性几乎是线性，偏对数的。
因此要根据需要权衡利弊的选择电压放大管。小信号放大级不应排斥低跨导的音频五极管。
因为此类五极管跨导较低，放大倍数较小，反馈量不大，较小的电流即拥有良好的线性，使用便捷，噪音亦在合理范围内。

大信号输出级则应当采用三极管，取其大信号失真小，输出阻抗低，动态范围宽等优势。

在输出级，选择工作点时，同样需亚考虑满功率输出，半功率输出时不同的失真特性。这给多极管输出级提供了使用的机会。还要注意到，多极管的栅地放大形式，能有效降低输出的三次谐波失真。

在三极管的选择上，还需要考虑多极管之间的失真互补，即同型号的电子管前后两级失真互补能力会较强，总体失真能降低。

Julien

The Amity, Raven, and Aurora
by Lynn Olson
--------------------------------------------------------------------------------

I've been designing loudspeaker systems since 1975, but I didn't move into electronic design until 1996, when I started the Amity project. I'd been reviewing various amplifiers for Positive Feedback magazine since 1991, and was intrigued by the whole SE-DHT phenomenon - writing the first US review of the Ongaku left an impression as well.
I auditioned many SE-DHT amplifiers after that, but in all honesty, none came up to the mark set by the Ongaku, although some came pretty close. I met the designer of the Ongaku a couple of years later at the CES, and Kondo confirmed my impression that the circuit of the Ongaku wasn't anything remarkable. It was the implementation - the all-silver signal path, especially the hand-made silver coupling cap and the all-silver output transformer, that gave the Ongaku its distinctive clarity and insight. Kondo-san said that building one on the cheap would just result in a quite ordinary SE amplifier - the Ongaku could be thought of as the ultimate parts-tweaker amplifier, a design that would sound completely different if all parts weren't exactly as specified.

Anyone that listens to SE amplifiers over any length of time is going to become aware just how different they sound from each other. Transistor amplifiers, at least if competently designed, sound more or less alike. Vacuum tube amps that replicate the design philosophy of the Fifties (Class AB PP pentode, quasi-Williamson, 20dB of loop feedback) also have a characteristic "group sound" - and of course different than transistor amps. These two classes of amps are what most audiophiles the world over had heard until the re-introduction of SE-DHT amps in the early Nineties, which created turmoil and dissension in the industry and trade magazines that lasts to this day.

Joe Robert's Sound Practices magazine promoted the combination of (very) low power, direct-heated triodes, single-ended circuits, and efficient speakers. By the mid-Nineties, SE-DHT almost became a religious cult, diverging rapidly away from the mainstream universe of hyper-expensive, super-power amps, low-efficiency speakers, with the whole industry driven by reviews in the Big Two magazines. Politics combined with a technological package, in other words.

As reviewer of the Ongaku (and designer of the 92dB efficient Ariel) I was drawn into the SE-DHT debate as well - but I never bought the religious angle. I agreed more speaker efficiency was a Good Thing, but didn't care for horn coloration (and still don't). The superiority of direct-heated triodes, by measurement and audition, is clear-cut, but there are design challenges related to filament power (and isolation) and serious demands on the driver stage - as well as coming to terms with the modest power, fragility, and considerable cost of direct-heated triodes.

Where I parted company with the SE-DHT crowd was single-ended circuits; for the output stage, at least, there are drawbacks to SE circuits compared to Class A PP (Class AB PP is another matter, and sonically is the worst of all). But a little research disclosed that PP amplifier design had essentially stagnated since the mid-Fifties; in fact, there was a period between the introduction of the Williamson in 1948 and the mid-Fifties when there were essentially no new designs at all!

More surprisingly, the mainstream high-end industry was just endlessly recycling trivial variations of Fifties designs over the last thirty years. Almost all products in the mainstream high-end market used the same stale tubes - 12AX7, 12AU7, 12AT7, 6DJ8, EL84, EL34, KT88 - and the same stale circuits - Williamson, Dynaco, Acrosound, or Marantz derivatives (which are all very similar). No wonder SE-DHT was making a stir - it was thirty years overdue!

By contrast with the US market, the Japanese had a wild variety of tubes and circuits, and had been playing with them since at least the early Seventies. SE, PP, OTL, hybrid, exotic ultra-rare tubes, specialized radio-transmitter tubes, circuits that ran in positive-grid bias, transformer coupling, anything you could imagine. In addition to the fun articles in MJ magazine, I'd been been reading Norman Crowhurst articles for many years, and I wanted to try a circuit that had the lowest possible intrinsic distortion, especially into typical speaker loads.

As a loudspeaker designer, I knew all too well that expecting speakers to be resistive was a vain hope - in fact, the best speakers are typically the most reactive and hardest to drive! I wasn't willing to cherry-pick speakers that were "SE-friendly" - to me, that's another way of saying "speaker-unfriendly." Speakers, by their nature as electromechanical transducers, are not just reactive, but store resonant energy for tens of milliseconds and transmit it back to the amplifier - and all speakers do that, electrostatic, planar, horn, single or multiple-driver. Speaker drivers are intrinsically resonant.

Thinking along these lines - an amplifier that remains linear when driving a complex, nonlinear, and poorly-defined load - leads to Class A PP, preferably with direct-heated triodes instead of triode-connected pentodes. (Tests in Vacuum Tube Valley magazine shows DHTs have about one-half to one-third the distortion of triode-connected pentodes, not a small difference.) The reason is clear when you look at the composite curves of Class A PP triodes: the grid-lines are essentially straight and parallel, so when the load-line opens into an ellipse (becomes reactive) there is no change in distortion.

BY contrast, all other other circuits and devices ... SE, or PP with Class AB, or pentodes, transistors, MOSFETs, etc. all have grid-lines that are curved. Class AB pentode (or transistor) is the worst, with high curvatures centered around the zero-signal region. In others words, the first watt is the worst watt ... but it only appears with reactive loads! Unfortunately, speakers are reactive: they can't help it, they're bandpass filters, and they also store energy for relatively long periods of time (milliseconds) and reflect it back to the amplifier. With most amplifiers, the reactive and delayed energy from the speaker driver greatly increases the amplifier distortion. This is why amplifiers can sound so different with different speakers.

With SE, the overall curvature of the grid-lines is still there, but is quite mild in the zero-signal region; the first watt really is the best watt. This is a critical point; music spends most of the time at low levels, with brief peaks that jump to 10 to 14 dB above the average. On a statistical basis, the greatest percentage of the time is in the 1-watt or lower region, at least if the speaker has any kind of reasonable efficiency.

But ... if you want linearity at low and high levels, near-total immunity from reactive loads, only PP triodes operating in very deep Class A can deliver straight and nearly parallel grid lines. The very deep Class A operating point keeps the circuit well away from the undesirable Class AB region; this is only entered when the load becomes a near-short, one that would current-limit and clip a SE amplifier.

Another goal is getting driver-stage coloration and distortion out of the picture ... quite different from the distortion-cancellation technique seen in some SE amps. I've never felt good about dissimilar devices maintaining cancellation over a wide range of signal levels and frequencies, and most of all I wanted an amplifier with a stable harmonic signature. Since I worked with cancellation techniques back when I patented the Shadow Vector Quadraphonic Decoder in 1975, I was all too aware that the "gotcha" of any subtractive techniques is the dirty nature of the residual, which calls attention to itself if the "dirt" changes character with different types of music.

I am beginning to suspect the ear compensates for amplifier distortion if it is simple in nature and stable (which is another reason for the excellent subjective quality of SE circuits), but does not like distortion that is constantly changing. Unfortunately, conventional phase-splitters have asymmetric output impedances, an asymmetric Miller capacitance, or sensitivity to B+ voltage, so the cancellation of the even-harmonic distortion terms (2nd, 4th, etc.) bounces up and down with the program material. The instability of the distortion characteristic is very likely the source of the usual PP "fog" and vagueness compared to the more direct and immediate SE sound.

To solve this problem, we need to go all the way back to classic Thirties-style interstage-transformer coupling. For the lowest distortion, the drivers need to have essentially horizontal load-lines, i.e. a very high impedance load relative to the plate impedance ... preferably more than ten times higher.



The Amity at night
The only way to accomplish this is some kind of dynamic load ... and the extra B+ headroom that requires ... or choke or transformer loading. Since I also wanted highly symmetric phase splitting that was invariant with signal level, I chose the interstage-transformer, and sought out the classic UTC and Sakuma amps for inspiration. My personal esthetic favors wide bandwidth, so the Lundahl transformers presented themselves as high-quality choices. In the PP world, high-frequency balance is more rare than you would expect; many expensive transformers have asymmetries in the capacitive balance, and this has a disastrous effect on HF distortion. It doesn't do any good to have precision-balanced tubes if the transformer itself is out of balance.

Another difference between this amplifier and the "classics" is of course much wider bandwidth than the "old days" in the Thirties when these circuits were last used. Back then, no wideband sources were available (with the exception of Major Armstrong's Yankee FM network). Everything else was both narrowband and noisy: intercity AM network radio (carried by Bell System land-lines), shellac 78rpm records, and optical sound-tracks for movies. Modern expectations for 65 to 100dB S/N ratios and 20Hz to 20kHz bandwidth simply didn't exist. Recycling antique parts and antique circuits will, of course, deliver vintage sound. That wasn't my goal. I wanted the unmatched linearity of triodes and transformer coupling, combined with modern bandwidths of 15Hz to 50kHz.

This is where modern transformers and carefully selecting low-plate-resistance tubes makes a difference. Transformer bandwidth is improved by having a low impedance on the primary, secondary, or both. One reason that interstage transformers are harder to design than a conventional output transformer is that impedance on both primary and secondary is high, while for the output transformer at least the secondary is low. Since the secondary is ideally unloaded in an interstage application (this gives the lowest distortion for a triode), the only good way to get satisfactory bandwidth from the interstage transformer is select a driver tube that has a low plate impedance.

Tubes that fit this application are the 5687/7044/7119 family or the Russian 6H30. (Although the familiar 6DJ8/6922/E88CC has low plate impedances, it also has moderately high third-harmonic distortion and limited output swing, which make it inappropriate for a PP DHT-triode amplifier.)

Moving on to the main B+ supply, I first tried a 5R4-GY rectifier (a traditional choice for 300B circuits), but was dismayed with the arc-overs and poor reliability in several examples. Maybe they were old and weak, but this sort of failure should never happen in the first place. By contrast, TV damper diodes, including the New-Old-Stock 6C*3 family and the new Svetlana 6D22S, have more-than-ample peak curves, and derating for continuous use gives more headroom in current and voltage than the traditional tube rectifiers seen in 2A3 or 300B amps. The low voltage drop (15V), huge peak currents (2A), and slow warm-ups (30 seconds) are just additional bonuses.

Matt Kamna also demonstrated a technique for zooming in on the waveform on the power-transformer secondary (about 10V/div on the scope screen). The rough appearance around the zero-crossing was very obvious with solid-state diodes. HEXFRED's gave a small improvement, but conventional tube rectifiers looked much smoother, and the TV damper diodes were by far the smoothest of all. So even in low-current preamp applications, TV damper diodes give the least noise. I know from experience in the Tektronix Spectrum Analyzer division that it's much easier to eliminate noise at the source than filter it afterward. If there was an even quieter device, I'd use that, but as far as I know, TV damper diodes are the quietest from the viewpoint of switching noise. Considering that the main B+ supply is switching five hundred volts, this is not a small consideration, since switch-noise is radiated in all directions, into the B+ supply, the interior of the chassis, and back into the power cord.



Another View, showing VV32B's and VR Tubes
A deluxe feature, which was easy to add, is the Voltage Regulator (VR) tube shunt-regulation for the driver. I wanted to drop 270 volts, so why not regulate at the same time? The noise of VR tubes is only 1mV of very smooth broadband noise, while the more common Zener diodes have 3 to 5mV of spectrally nonflat noise, occasional LF bumps and pops from "popcorn" noise, problems with temperature coefficients, and a huge amount of grossly nonlinear capacitance. Zener diodes need a lot of filtering and additional circuitry to isolate the problems. By contrast, VR tubes need no additional circuitry at all - just keep them away from capacitive loads.

Not only are old-timey VR tubes quiet, they're nice to look at. The purple glow of the OC3's is an subtle "ON" indicator. When the VR's light up after the lengthy 30 sec warmup from the damper diodes, you know the amp is well and truly running.

The original 1997 Amity used the simplest isolation between the VR tube and the main B+ possible; a pair of 2K 20-watt power resistors in series with a 100 uF filter cap between them. This simple RC filter worked well and was essentially unbreakable. The latest version, shown when you click the diagram, features Gary Pimm current sources for better isolation between driver and output stage. Either version works well; the choice is yours.



Click for full schematic diagram

Although the Amity is a beautifully simple design, it makes severe demands on the preamp. The preamp has to drive the combined capacitance of the interconnect cable, the input transformer, and the Miller capacitance of the input tubes. This can easily add up to 200 to 600pF of capacitance, depending on how long your cables are and if you choose to use a 1:2 step-up in the input transformer (which quadruples the 60pF Miller capacitance of the input tubes). In all honesty, this is difficult load for most tube preamps, with 12AX7 cathode-follower preamps falling down the worst. (1mA cannot drive 600pF!) Preamps with 12AU7 cathode-followers aren't much better, sounding very "tubey," rolled-off, and old-fashioned.

(Passive preamps cannot be used with the Amity since the input transformer must be driven with less than 600 ohms source impedance. A passive preamp using a 10K pot presents 2.5K source impedance to the power amp in the -6dB position, which will cause a significant decrease in transformer bandwidth. No harm to the Amity will result, just degraded sound quality.)

I borrowed many different preamps, bought some of them too, sold them at a loss, and just about the only one that sounded decent was a borrowed Jeff Rowland Consonance preamp. The A/B test was simple: I had a very high-quality DAC using the Burr-Brown PCM-63K converters, and I would compare a direct connection to the Amity versus a preamp. A preamp compared to a piece of wire - awkward on many CD's since the only volume setting was full up (no passive volume control allowed, remember). Still, it was obvious that most preamps added odd-sounding electronic colorations, as well as more serious deficits that flattened or removed qualities of air, space, musical textures, and an intangible quality that I call vitality, or immediacy. Most tube preamps, in fact, were downright dull and flat-sounding.

The Amity is transparent enough that with most preamps, you could never hear the Amity - just the preamp, mimicking the colorations of a lesser-quality power amp. When the preamp was removed, though, the truth was out - it was the preamp, not the power amp, that was adding the amp-like colorations.

A pretty disappointing state of affairs, and I didn't want to mindlessly copy the Jeff Rowland, although I appreciated the elegance (and boldness) of transformer-coupling a high-speed opamp in a preamp, thus keeping the solid-state electronics happy by filtering off ultrasonics from the CD source. So it was time to try my hand at designing a linestage preamp. I dreamed up the original Raven concept some time around 1998 - click the picture for the full schematic.



Click for full schematic diagram

(Historical footnote: The Raven is basically a Western Electric line-driver with modern components and shunt regulation. By the way, did you know the audio-engineering phrase "line-driver" dates back to Bell System telephone line repeater amplifiers which pushed the voice signal down another twenty miles of twisted-pair wire? Remember, no coax, RF modulation, or digital conversion back in the Twenties and early Thirties, just carefully-balanced voice circuits with repeaters and phase-correction networks. The NBC and CBS radio networks leased Bell System broadcast-grade intercity links for coast-to-coast broadcasts - dedicated sets of twisted-pair wires that were set aside for radio use only, with top-quality 50 Hz to 8 kHz bandwidth and special precautions to minimize noise over the many repeaters and hundreds, or thousands, of miles of wire carried on telephone poles.)

Interestingly enough, even though exotic audiophile preamps cost $7500 or more, when you look at the schematic, you'll almost always see a capacitor-coupled design, typically a cap-coupled cathode-follower with an active current-source pulldown. Transformer coupling is still quite rare in the vacuum-tube world, yet it has so many advantages in terms of breaking system ground loops and a powerful, high-current drive capability. You can parallel all the tubes you want, and you still won't have the current capability - and very low distortion - of a transformer-coupled design.

But I'm getting ahead of the story. Back in 2001, freshly arrived in Silverdale, and setting up Nutshell High Fidelity, the problem of the Amity preamp remained. There was a reasonable chance that folks would build or buy the Amity and use it with an average-quality off-the-shelf preamp. I knew from experience that this wouldn't work; the Amity doesn't appreciate most preamps, enough so that it is impossible to hear what the Amity actually sounds like. So I started work on the Aurora, which has its own built-in high-quality linestage.

Gary Dahl and I spent most of the summer of 2002 working on the Aurora. The Aurora is a direct extension of the Amity, with a simple SE preamp stage ahead of the PP driver and output section. Some of the best preamp/input tubes are the real oldies - the 6P5 (octal), the 76 (the same tube with a 5-pin base), and its predecessors - the 56, 37, or the grand-daddy of all indirect-heated triodes, the 27. All of them work fine with the 500-ohm cathode resistor and an operating current of 10mA, although the 27 and 56 require 2.5V AC for the heater instead of the more typical 6.3V AC for the 37, 76, or 6P5.

Since some of these types are also coveted by the vintage radio-restorer community (which is bigger than you might think), some models can be expensive - this corresponds to rarity, not sonics, so avoid paying big money for them. In the USA, at least, the 6P5 or 76 are not that rare. The prices you see in Antique Electronics usually corresponds to the rarity of a tube, so check their latest listing.

(Antique also carries octal, UX4, and five-pin sockets, as well as VR tubes and damper diodes. When you order the damper diodes, make sure you get the matching sockets for the models you order - they do not use conventional nine-pin miniature sockets, and the Russian and NOS tubes use different sockets.)

Don't be preoccupied with New-Old-Stock condition, either - moderately used tubes work just fine, especially in the light duty in this circuit. Avoid metal-can tubes - thanks to internal outgassing from the metal, these sound dreadful, while traditional globe, G, or GT styles sound fine (vintage radio collectors drive up the price of certain styles - remember, they're after appearance, not sonics).

You might think that really old tubes - designs dating from the early Thirties - sound "vintage" and "tubey." Actually, they don't. Tubes in this family (27, 37, 56, 76, and 6P5) actually sound fast, quick, and incisive, and have very low distortion, along with a favorable distribution of harmonics. If you want a traditional mellow "vintage" sound, you're thinking of a 12AU7, the tube used almost universally in late 1950's amplifiers and many guitar amps - with distortion many times higher than the early Thirties tubes.

The universal adoption of feedback in the late Forties allowed electron-tube designers to relax their concerns about distortion and focus on cost, heat emission, and package size instead. Audiophile vacuum-tube amps from the Seventies through today continued with the late-Fifties favorites of the 12AX7 and 12AU7, which are actually considerably worse than the octal predecessors, the 6SL7 and 6SN7.

The really superior tubes of the mid-to-late Fifties and early Sixties, the 5687, 7044, and 7119, were only used in commercial and aerospace applications, never in high-fidelity electronics sold to consumers. At the time, the American electronics industry was divided into two sectors, consumer and commercial/military, and the engineers didn't talk to each other. Putting it less kindly, the good engineers went into the commercial, instrumentation, and military sectors, while the not-so-good engineers went into the high-fidelity business. In the Thirties, the best electronic engineers in the world were designing radio and movie-sound systems, but World War II changed that, pulling the best engineers into the military/aerospace sector - which is still true today in North America and Europe.

It wasn't until the late Nineties that the space-age family (5687, 7044, 7119) and radio-age family (27, 37, 56, 76, 6P5) were re-discovered. It's not the looks, it's the performance. These triodes stand out for their low distortion and favorable distribution of harmonics, lower than any other type of analog device, pentode, bipolar transistor, JFET, or MOSFET. They sound good for a reason.

Another important part of the Aurora is the 2.2uF to 4uF 200V parallel-feed capacitor between the cathodes and the center-tap of the transformer primary. This is a critical part; the sonics of this single capacitor creates the sound of the entire amplifier. Do NOT use generic polypropylene caps in this location - you'll end up with a dull, murky-sounding amplifier. My personal favorites are Teflon caps, although these are astronomically expensive by the time you get to the required value. You can get some of the sonic benefits by using a smaller, higher-quality bypass capacitor, typically 1/10th to 1/20th of the value of the larger cap.

If you've gotten the impression that SE amplifiers are sensitive to parts tuning, you're right. Both the SE input tube and the choice of parallel-feed capacitor have a startling effect on the overall sound of the amplifier - make the wrong choice, and the amp can be either harsh and shrill or dull and muffled.

I never did find out what the Aurora sounds like - it just mimics the sound of the fairly large-value parafeed capacitor. Unfortunately, the best caps are the small ones, like air trimmers and silver-mica (only available with small values in the pF range), along with Teflon (shockingly expensive in larger values). Paper caps have a shortened lifetime when you apply 200V across them (measure the leakage current of old paper caps in vintage TV's and radios). Metallized polypropylenes (even boutique types) should be avoided in favor of high-quality metal-foil polypropylenes, which are probably the best real-world choice.

Capacitor sonics are less of a problem in less transparent (more distorted) amplifiers, but people who build the Amity and Aurora will discover the amplifiers are startlingly transparent and not forgiving of low-quality parts - coupling caps being the worst offenders. For some reason, transformers are better at retaining instrumental color and emotional expressiveness in the playing - while most caps, if not all, erase a lot of the expressiveness, kind of like MP3 digital compression flattening out in the inner detail and sparkle. With many caps in the signal path, this goes unnoticed, but when you get down to the last one, it's a lot more audible.

I don't know why this effect occurs, and many audiophiles don't hear it. I'm also sure most vacuum-tube amp designers in the high-end audio business would disagree with me - otherwise they wouldn't be designing RC-coupled PP-pentode amplifiers similar in style and sonic character to vintage amps of the 1950's.

Designing the Aurora wasn't a completely satisfactory experience. The Amity "dropped into place" in a matter of weeks, but the Aurora, in a sense, was never completed, since one critical part dominated the sound, and I never found a very good solution. Some audiophiles are thrilled to endlessly compare caps, cables, etc. on a few favorite pieces of music, but I'm not one of them. Parts-rolling is not something I enjoy; instead of listening for pleasure, it's listening for faults, and I can say caps have a lot of faults.

After reflecting on this for a while, I looked back at the Amity, and decided I'd gotten it right the first time around. Fully-balanced, no single-ended circuits anywhere, and most important of all, no coupling caps in the signal path. Time to start with a fresh sheet of paper.

Julien

The Karna Amplifier
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Many years ago when I was designing the Amity, Takuji Yamamoto of the Direct Heating Society of Japan urged me to try direct-heated drivers for the 300B tubes. Although the Amity ended up sounding different than the Sakuma-method amplifiers - more "American" in style - I very much appreciate Sakuma-san's poetic approach to amplifier building.
I never forgot Takuji's comment that getting the "tone" right on a 300B required a Direct-Heated-Triode driver. Putting on my Tektronix hat, DHT's have remarkably low distortion compared to nearly any other tube, and peak-current capability is far in excess of more conventional drivers. DHT's just sound better, more natural, more clear, and most of all, more emotionally engaging. "Presence" and "vividness" are the impressions that come to mind.

But the inevitable awkwardness of AC hum induction, microphonics, more complex power supply, etc. etc. kept me back from looking into it. I went through the Raven and Aurora before even considering it. At an intuitive level, the idea of a three-stage amplifier composed of an input 5687/7044/7119 driving another 5687/7044/7119 driver somehow just felt wrong.

Vacuum tubes are good at doing different things; a driver is not at all the same application as an input tube, which is really a preamp-style voltage amplifier and input buffer. Driver tubes have to confront the shock of power-tube grid-current whenever the amp clips, which happens more often than people think. They also have to deliver a massive voltage swing to the power-tube grids - more than two or three times the swing required by pentodes, combined with a heavy capacitive load. In the Karna, each VV32B (or equivalent) is biased at 100V; this means 282 rms volts of drive are required for the pair, and at low distortion - preferably less than the 0.3% distortion of the VV32B's themselves. This is a tall order for any driver. By the way, that's why high-end DHT amplifiers are sonically all over the place - very few have sufficient drive capability in the driver stage. Instead of hearing the transparency and directness of the DHT power tube, you hear an overloaded RC-coupled driver instead.

All of this stayed on the back burner until Kevin Carter of K&K Audio told me about building an amplifier that used a pair of PP 2A3 driver tubes instead of 5687/7044/7119's. His approach was a little different than what I was considering - he used direct coupling with center-tapped choke loading - but that it worked at all was interesting. Part of what kept hum reasonable was the low gain of the 2A3 driver. I was still reluctant to try DC heating after the disastrous experience with trying it in the 300B output section, so hearing from Kevin that hum levels were acceptable was reassuring. Kevin was the second person in the USA to build an Amity-style amplifier (and he already owned a pair of Ariel speakers), so I took his experience seriously.

I planned on using Gary Pimm's current sources combined with VR-tube shunt regulators to fully isolate the input and driver stages, along with the Western Electric bypass between the center-tap of the transformer and the common-cathode circuit of the tubes. This keeps the AC path from plate to cathode as short as possible, and minimizes the audio currents going through the shunt regulator.



Click for full schematic diagram

The sheer weight of all those transformers, combined with isolating the emission of the power transformers from the susceptance of the audio transformers, made a two-chassis design a good idea. It does have the drawback that grounding requirements become pretty subtle. Fortunately, Gary Pimm was the one to build the amplifiers, and he's pretty familiar with instrumentation-style grounding. So I could follow what he was doing, he recommended I buy a copy of Ralph Morrison's Grounding and Shielding Techniques in Instrumentation, which is pretty much the Bible in the subject.

Before you get too lost, a good place to start is simply put the word "grounding" out of your mind, and think instead of return paths for current. In this amplifier, there are two: the audio-signal return path, which flows from plate to cathode, and the DC return path, which powers the tubes, and ideally carries no audio at all. Thirdly, there's the connection to the chassis, which only occurs at one point in the circuit, and carries neither audio-frequency nor DC currents.

Note the power supply actually floats, with only the cases of the power transformers connected to the chassis. There are separate DC returns for each section of the circuit, as well as full isolation of the 400V and 520V supplies. The audio-signal returns are physically small in area (small loop area). Similarly, the loop area of each rectifier circuit is small, which reduces emission of hum and noise into the rest of the amplifier. Since the loop area acts like an antenna, keeping it as small as practical (limited by heat considerations of the tubes) reduces noise pick-up and emission.

By separating the audio-frequency return currents, the DC currents needed to run each stage of the amplifier, and the single connection to chassis ground, there's a substantial reduction in hum, noise, and background noise. Since the actual circuit has very low distortion and low intrinsic noise, it takes an instrumentation-style layout to realize the full benefit. In fact, even though all heaters and filaments are AC heated, the Karna is very quiet on 107dB/metre Avante-Garde Trios, with only a slight hum (audible no more than 1 foot away), no buzz at all, and just the faint hiss of the tubes themselves. Even on the Trios, the backgrounds are inky-dark and silent, not what you expect from a DHT amplifier. Instrumentation layout and grounding are probably even more important for DHT amplifiers than other types, since the intrinsic distortion is so low, and you can hear so far into the music.

At some point, I'd like to explore DC heating for the driver and output stage, but it isn't trivial to solve the problems of isolation from AC -> DC rectification noise, avoiding coloration from the DC heater supply regulators/capacitors, while also taking advantage of the DHT's intrinsic 40 to 50 dB of differential noise reduction.

Almost every DC-powering technique I've seen introduces subtle new colorations of their own, or fails to completely isolate the filament from the very high levels of switch-noise from the AC -> DC rectifiers. The DHT filament is carrying out three separate functions at once: it's an input node for the tube (the tube amplifies the voltage difference between grid and cathode), it carries the audio-frequency return currents that flow from plate to cathode, and last but not least, it heats the filament so it can emit electrons. The virtual cathode isn't a real connection that appears on a circuit pin; instead, it appears at the electrical center of the filament. If the audio-frequency return currents are connected to one side of the filament instead of the virtual center, then 40 to 50 dB of AC noise rejection of the filament is thrown away for no good reason.

The distortion from the DHT driver is far lower than any IDHT tube tried - in fact, the distortion from the 45 is so low it cannot be distinguished from the signal coming from the input stage - with measuring equipment with a noise floor of -130dB. In this amplifier almost all of the distortion from the 4th harmonic on up is coming from the input stage, not the driver, nor the output tubes.

Direct-heated triodes, especially the 45 and 300B, have extremely low distortion. The sonic result is vivid tone colors and remarkably transparent sound, limited only by the sonics of the transformers. I've tried the Lundahl's, which work well, and O-Netics are next on the menu. The most important requirement for the interstage transformers is a precise phase match between the secondaries - with no more than a few degrees of deviation - from 20 to 50 kHz. This can be confirmed by using a X-Y display on an oscilloscope, connecting one secondary to the X axis, the other secondary to the Y axis, and sweeping the transformer from 1 kHz to 100 kHz. The display should stay very close to a straight line, and only "open up" very slightly.

I was curious just how robust a DHT driver was - well, the driver can drive the 300B grids 30 volts (!) positive, with no crossover glitch as it passes from Class A1 to Class A2. In Class A1, it delivers 15 watts at 0.3% distortion, and A2, it delivers 30 watts at 3% distortion. The driver is so powerful it doesn't mind the onset of power-tube grid-current, and the 300B stays linear as it enters the positive-grid region.

The best-sounding drivers are old-stock 45's run at 28mA each, middle of the pack are NOS American triode-connected 6W6's, and the least impressive are new Sovtek 2A3's (at any current). You get what you pay for, I guess. Haven't tried the Chinese or Czech mesh-plates so far, although they have excellent reputations in the 2-watt SE world.

This gives the amplifier the subjective headroom of a 30 to 100-watt amplifier, not what you'd expect from 300B's. I can tell you, with the Azzolino or Avante-Garde horns, the dynamic range and sheer bass power is truly frightening. You do not want to get clumsy with interconnects with this amplifier. The input has a three-way rotary switch, which takes advantage of transformer coupling to offer normal phase, reverse-phase, and muting positions. This comes in handy for auditioning new equipment, quick interconnect changes, and checking to see if reversing the phase (of both channels) sounds better with a given recording (very recording and system-dependent).



Gary Pimm (looking attentive) and Gary Dahl (relaxing next to window) relaxing in my living room after delivering the just-completed Karna amplifiers. Mr. Pimm had been fine-tuning the amplifiers in Portland over the last three weeks, and was so impressed that he brought his complete system up here.

Gary P brought his Thorens/Grado phonograph, his superb phono stage, the Dave Slagle autoformer volume control, handmade cotton/litzwire cable, and Gary P's Klipsch/Heil speakers (modified Heresy woofer and crossover, AMT-1 box, modified passive radiator, and AMT-1 Heil tweeter). Although you might expect the worst from a Klipsch Heresy and a Heil tweeter, the 12" Klipsch woofer, modified Klipsch crossover, and Heil tweeter actually work very well together, giving dynamic and quick sound, and remarkably smooth response (+/- 2dB, not bad eh?).



Here's Gary Pimm (left) showing Gary Dahl (right) the layout of the Karna amplifiers, with Lundahl transformers underneath, O-Netics on top, Litzwire audio and power-supply wiring with cotton sleeving over high-voltage test probe insulation. The upstairs/downstairs dual-transformer layout was Gary's idea, so we could easily compare and switch between two types of input, interstage 1, interstage 2, and output transformers. In this picture, you can see the Stewart & Billington TX102 Mk III transformer volume control (TVC) set up for +6dB gain, followed by the Sowter 3575 for the input and phase-splitter, Lundahl interstages, and Bud Purvine's (O-Netics) Level 1 output transformer.

(O-Netics makes Level 1, 2, 3, 4, and 5 push-pull output transformers, with price doubling with each level. Mr. Purvine tells me the price is pretty much a direct reflection of labor content, with each level taking about twice as long to build as the level below it.)

Since their first arrival at the end of summer, the Karna's made a weekend visit to Gary Pimm's for a fine-tuning session. The picture below shows the latest 1.1 version of the Karna amplifier - a RCA input and ground-lift switch has been added, and the Lundahl interstage transformers are below decks. On the left side of the picture, directly behind the middle VR tube, you can see the DPDT toggle switch that sets the quiescent current for the driver stage, allowing direct comparison between 45 and 2A3 driver tubes. Near the front of the amplifier, a DACT 5-position input selector allows a choice of two input transformers, muting, and phase inversion. The ground-lift switch is behind the RCA input.



A few inches behind the input selector you can see the very small hum-nulling control, which is a screwdriver-settable 10-turn pot. On the 92dB/metre Ariels, you can just barely hear it with your ears 3" away from the woofer cones - any distance more than that, it goes away. It's quiet enough that adjusting the hum-nulling pot is a little awkward - the hum isn't audible when you're leaning over the amplifier making the adjustment.

The first experiment for the 1.1 version was neutralization of the 5687/7044/7119 input stage; this consists of cross-connecting a small (a few pF) capacitor from one plate to the grid of the opposing tube, and vice versa. This has the technical advantage of cancelling the Miller capacitance of the input stage, making the amplifier an easier load for the preamp, and speeding up the input stage as well. A "free lunch", as it were.

Nice in theory, a disaster in practice. Any value beyond 4.4pF resulted in full-power oscillation at some high frequency - hard to interpret on the digital scope, but Gary and I were guessing it was 20 to 50MHz or higher. 3pF would be a "safe" value for neutralization, but that's such a miniscule capacitance that you could get that just from sloppy layout. In fact, the onset of oscillation with 4.4pF of cross-coupling is a warning to keep the wiring point-to-point, going straight from the associated transformer to the relevant grid and plate. In this amplifier, as in the Karna, neutralization does not work. Don't bother.

I received a tip from John Atwood, the previous Technical Editor at Vacuum Tube Valley, the 6W6 is a "sleeper" tube that makes a great driver, very low distortion, really reasonable price, and readily available. How can I resist at $2.85 apiece from Antique Electronics?

Well, after testing for distortion, beware the Japanese variant of the 6W6, which has more gain and 15 to 20dB more upper-harmonic distortion than the genuine article. The old-stock American and Italian versions were OK in performance, maybe a little worse than a 2A3, but certainly usable. Since these are indirect-heated pentodes (triode-connected for this application), it saves worrying about with the 2.5VAC filament supply and using the hum-cancelling circuit of the Karna driver stage. Conveniently, the 6W6 uses an octal socket with the standard 6L6/EL34/KT88 pinout, so you can substitute any power pentode you like and listen for your favorite flavor of sound. The biasing is also the same as the 45 (in fact the overall specs are similar to a 45 in terms of dissipation and recommended plate current), so the audio circuit can be left at the 45 operating points. Replace the UX4 4-pin socket for the same-size octal socket, connect the screen directly to the plate, provide 2.4 amps of 6.3VAC heater current for the pair of tubes, and off you go.

Returning to direct-heated territory, Gary and I compared old-stock RCA 2A3's (might as well use the genuine article, right?) with old-stock 45's. Although 2nd and 3rd harmonic distortion levels were similar, the NOS 2A3 unfortunately had 5 to 10dB more of the 4th, 5th, 6th, 7th, 8th, and 9th harmonics than the 45 triode.

By comparison, the distortion from the 45 triode was unmeasurable, since overlaying the 5687/7044/7119 profile with the 5687/7044/7119 + 45 profile showed identical spectral curves, with no visible difference at all between the two. With every other driver - the 2A3, the different flavor 6W6's, etc. the 5687/7044/7119+driver curves showed more distortion than the 5687/7044/7119 by itself, which is what you would expect.

It should be mentioned that the raw THD figure is quite misleading, since it is usually dominated by the 2nd harmonic, even in Class A PP circuits. 2nd harmonic, by itself, is nearly inaudible, since it is a full octave higher than the original musical spectra, and is musically consonant. Similarly, the 4th harmonic is two octaves higher, and is also musically consonant. But 3rd, 5th, 6th, 7th, and 9th are very dissonant and ugly sounding, and are easily heard, especially with increasing order. In the 1950's, both Norm Crowhurst and D.E.L. Shorter (of the BBC) proposed weighting harmonics by the square or cube root of the order to better reflect their audibility. Small amounts of high-order harmonics - even when 60 to 100 dB down - are musically significant.

The high-order harmonics - 3rd on up - were where differences between drivers were easily seen on the FFT analyzer. The 45 was cleanest, in fact indistinguishable from the input tube driving it. The old-stock 2A3's came next, followed by the (genuine) 6W6's. Since the 5687/7044/7119 was delivering 20V rms per plate during these tests, using it as a driver (like the Amity and Aurora) would obviously have more distortion, since it would have to deliver between 60V and 70V rms per plate to drive the 300B or 320B-XLS.

As medium-gain, medium-current triodes go, the 5687/7044/7119 familiy are one of the lower-distortion ones out there - but not in the same league as DHT power tubes. Are there any tubes with gains of 20 or more that have lower distortion than the 5687/7044/7119 group, particularly in the upper harmonics? Good question, since the 5687/7044/7119 group have a more favorable distribution of harmonics than most other medium-gain tubes.

There's really old direct-heated exotica, like Western Electric 205D's and the RCA 26, but these have lowish gain, modest operating currents, and known problems with microphonics. As for newer tubes, 12AX7's, 12AU7's, 12AT7's, and 6DJ8/6922's are not as good the 5687/7044/7119 group for driver applications. As far as I know, the 5687/7044/7119 family has the most favorable distribution of harmonics, along with a moderate Rp of 1500 ohms per plate in the 16 to 20mA operating region. (Adjust the cathode resistor to give each section anywhere from 16 to 20mA of quiescent current, depending on personal taste, and the sonics of your favorite tube and your favorite operating point.)

This leaves the tube lineup of input=6H30 or 5687/7044/7119, driver=45, output=300B or 320B-XLS as the best for the Karna amplifier. I've yet to evaluate the 45 and 2A3 mesh-plate exotics from China and the Czech Republic, so I'm curious how these turn out.

Julien

Design Philosophy

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1) Select active devices for minimum upper harmonic distortion. Although some devices have reasonably low second-harmonic distortion, the ear is not very sensitive to 2nd harmonic. It's the 3rd and higher-order harmonics that create unpleasant "electronic" colorations. D.E.L. Shorter of the BBC Research Labs and Norman Crowhurst both proposed weighting harmonics by the square or cube of the order in to reflect audibility and annoyance-factor, and it's a shame their suggestions were never carried out. To this day, it's the 2nd harmonics that dominates THD device measurements, but it's the ones that are higher than that (even though they may be 20dB lower) that we hear. That's why a THD spec, without reference to the complete spectrogram, is essentially useless, and not only that, potentially quite misleading.

In the absence of a full spectrogram, try and find a 2nd and 3rd-harmonic spec for the device you're interested in. It should be at least 20 to 30dB lower than the dominant 2nd-order distortion term. Failing that, there are some devices to avoid if you want to minimize the proportions of high-order harmonics - pentodes, beam tetrodes, transistors, MOSFETs, and IGBTs. That leaves triodes, which are not all the same.

The triodes that were the most popular in the Fifties (12AX7, 12AU7, 12AT7, and 6DJ8) were not in fact the most linear available. At the time, commercial amplifiers used at least 20dB of feedback, giving no incentive to retain big, old-fashioned octal tubes like the 6SN7 (size mattered back then). The 6SN7 came out of a prewar family of medium-mu radio tubes, starting with the single triodes 27, 37, 56, 76, 6P5, 6C5, 6J5, and then, the dual-triode 6SN7, introduced in 1940.

The 6SN7, used in millions of radios and early black-and-white TV sets, was replaced by the 12AU7 miniature tube in the early Fifties. Although the electrical characteristics appear identical in a databook (Rp=7700 ohms, mu=20), the 12AU7 has quite a bit more distortion than it's octal predecessor. In fact, the 6FQ7 miniature was introduced as a direct-replacement for the 6SN7, carrying forward the low-distortion characteristics of the 6SN7, but saw very little use in Fifties hifi equipment as a result of higher cost.

Tubes in widespread use in the Fifties have acquired a "cult" status that has nothing to do with performance, or even sonics. Believe it or not, there were better tubes both earlier and later than the "Golden Age" favorites. The 6SN7 and it's predecessors have very low distortion, almost certainly because they were designed before widespread use of feedback.

The Space Age high-transconductance tubes of the late Fifties and early Sixties never saw use in commercial hifi equipment, mostly due to cost considerations. But commercial and aerospace-grade tubes were some of the best ever made for low distortion - and favorable distribution of harmonics. The 5687, 6900, 7044, and 7119 are at the head of the line for low output impedance, wide voltage swing capability, high current, and low distortion. As for modern tubes in the same family, I've heard good things about the Sovtek 6N6 and 6H30.

Speaking of exotic, for brave souls who aren't afraid of microphonics or 50 MHz oscillations, there's the exotic (and hard-to-use) WE417A/Raytheon 5842, and the truly over-the-top WE437 or 3A/167M. (Available in modern form as the Russian 6C45pi.) Beware of large sample variations (20% or more) in this family - it's very hard to get matched pairs unless you burn them in for 100 hours and then re-test for gain. It isn't that quality is bad - the same problem is seen in genuine Western Electric 437's - it's that ultrahigh transconductance specifications requires extremely close-tolerance manufacturing. By comparison, the more moderately specified 5687, 6900, 7044, 7119, and ECC99 family is typically quite well matched, with variations of less than 5% from sample to sample.



2) Design circuits that optimize the linearity of the active device. For triodes, this means active, choke, or transformer loads. RC-coupling, although essential for the extended HF bandwidth required by feedback circuits (like the Williamson, Dynaco, or Marantz), degrades the distortion of triodes anywhere from 2 to 4 times relative to active, choke, or transformer loads.

An additional advantage of no RC coupling between the driver and output tubes is instant recovery from overload. RC coupling usually requires hundreds of milliseconds to recover the correct bias point for the output tubes. This is great for guitar amps, not so good for hifi applications, where immediate recovery is much more desirable.



3) Zero feedback in either local or global circuits. Some would say all triodes have large amounts of local feedback, but I would say, take a close look at the ratio of upper to lower harmonics. For the lower-order harmonics (2nd and 3rd), you'll see a reduction in direct proportion to the amount of feedback. But feedback generates harmonics of its own, in small magnitudes perhaps, but in greatly increased order. A classic article by Crowhurst mathematically demonstrates that a non-feedback amplifier with 9th-order harmonics acquires 81st-order harmonics when feedback is added. So feedback, while substantially reducing lower-order harmonics (the most sonically benign), also creates entirely new high-order harmonics (albeit at very low magnitudes).

So if the theory of triodes having large amounts of local feedback is correct, they'd have less distortion at low orders of harmonic distortion than pentodes, but more distortion at higher orders. This is exactly the opposite of what measurements disclose. Measurements going back as far Harry F. Olson (see below) show triodes not only have less distortion overall, but most significantly, have far less upper-harmonic distortion. In fact, vanishing amounts of upper-harmonic distortion is the distinctive hallmark of triodes, compared to all other amplifying devices.

The exceptional linearity of Class A triodes removes the need for feedback in the first place, sidestepping problems with phase margin and stability with complex and nonlinear loads ... which pretty much describes all loudspeakers. In particular, complex back-EMF currents from the drivers stop at the plates of the output tubes, instead of being mixed at the summing node with the incoming signal. This avoids load-dependent distortion terms being generated in the feedback circuit.



4) Generous driver design. Most commercial tube amps only have 1 or 2dB of headroom in the driver circuit, so the entire amp clips at once. This leads to longer recovery times and exaggerating the audibility of clipping. I prefer 3 to 6dB of headroom, so the driver can retain its (voltage) linearity even when the output stage is deep into clipping.

Perhaps more important is adequate current and low output impedance in the driver. Much of the amplifier coloration is actually in the driver, and is a result of not enough current to properly charge the grids of the output tubes. I give Arthur Loesch credit for pointing out that the "sound" of different DHT output tubes is greatly exaggerated by not having enough current in the driver. With enough current, DHT's become more transparent sounding and begin to lose the characteristic colorations they are known for. This implies that grid current is present during much of the duty cycle and is quite nonlinear. The more current available and the lower the source impedance, the less important this grid-current nonlinearity will be.

The capacitive load the output tube presents to the driver changes the load-line into an ellipse, which pushes the driver into it's nonlinear low-current region - once per cycle for a SE amp, twice per cycle for a PP amp. Pentodes have both less capacitance and half - or less - the voltage requirements of DHT's, so the driver requirements are relaxed (EL84's are particularly easy to drive, needing only 10 volts). DHT's, on the other hand, are much more severe loads, more capacitive, needing twice the voltage swing, and worst of all, exposing the driver distortion since DHT's have such low intrinsic distortion of their own. The sad fact is that many commercial DHT amplfiers, certainly all the RC-coupled designs, have more distortion in the driver than the output tube! What's the point in spending hundreds of dollars on a handmade 45, 2A3, 300B, or 845 if all you hear is the driver distortion? But that's why DHT sound quality is all over the place - the driver section isn't up to the job.



5) Absence of rectifier switching noise. The noisiest circuits of all are solid-state bridges driving large values of electrolytic capacitors. (As found in almost all transistor gear and the DC supplies for heaters and filaments.)

The commutation noise of the diodes shock-excites the RLC of the stray L in the cap bank and the stray C in the power trans secondary. The resonance of this tank circuit is typically anywhere from 4 to 20KHz and the Q's are large, anywhere from 5 to 100, depending on the DCR of the caps. This is why paralleling large values of electrolytics with "better", faster polypropylenes can frequently result in worse sound. It is also the reason power cables are audible ... they act as antennas for the small Tesla coil that most power supplies resemble. The supply radiates noise into the chassis, the power supply B+ lines, the audio circuit, and the power cable. This broadband noise can be filtered and shielded (at considerable trouble), but it is much easier to eliminate the commutation switch-noise right at the source.

Choke-fed supplies are much quieter due to the choke slowing down the rate-of-charge of the main cap bank. I use a hybrid choke-fed/pi-filter to minimize the shock-excitation of the main PS choke (this tip from the Radiotron Designers Handbook, Fourth Edition).

In terms of ragged waveforms, solid-state diodes are the worst, followed by Schottky diodes and HEXFRED's, followed by conventional tube rectifiers, followed by TV damping diodes, which are the smoothest of all in terms of the AC waveform on the power trans secondary. This, along with 2 amp peak current, is why I use them. The 30 second warmup is just a bonus.



Conclusion: This is a set of personal preferences, not the Holy Writ to Good Sound. Every designer has a set of different priorities, and this is why electronics sound different. There Is No Best. Get the idea out of your head. This is a pernicious and ugly myth propagated by greedy marketers and lazy magazine reviewers. The entire notion of an "Absolute Sound" is false. Absolute to who? To some self-appointed "expert" who knows how to write and publish a magazine? Don't be fooled.

Trust what you hear; people really do hear different things, and your perceptions are unique to you. I've been in high-end audio for thirty years now, and I've only found a handful of people who hear things the same way I do. Audio perceptions are every bit as individual as food preferences or your idea of the best lover or life companion; why should you take the guidance of a stranger you've never met? Listen for yourself, get out of the stores and hi-fi shows, and hear what's going on with the more adventurous builders. You'll be surprised.


Distortion Measurements
--------------------------------------------------------------------------------



This spectrogram shows the harmonic distortion of the 1997 Amity amplifier at 1.6 watts, and was made by Matt Kamna with a Tektronix SG505 signal generator and a Hewlett-Packard 8535A spectrum analyzer. If the amplifier were distortionless, all you would see would be a single 1kHz tone extending to the top of the display, and the "grass" or noise floor at the bottom of the display.

The vertical axis is amplitude (0 to -100dB), and the horizontal axis is frequency (0 to 10kHz). The display is calibrated with horizontal lines that show 10dB intervals, and vertical lines that show 1kHz intervals. The 2nd harmonic is one interval to the right of the big signal, being at 2kHz. The smaller it is, the less the distortion. Since the top of the little bump is 7 intervals below the top of the display, that means it is 70dB below the 1kHz fundamental. -70dB corresponds to 0.03%, so the 2nd harmonic is around 0.03%.

The next little bump to the right of that is about 6.5 intervals down from the top of the display, which is 65dB below the fundamental, or about 0.05%. That's the 3rd harmonic. A little bit of 5th is peeking out from the noise, and it's around 8.2 intervals down from the top, which is -82dB down, or about 0.008%. The rest appears to be noise at the -92 to -95dB level ... this is the limit of the measuring system, not necessarily amplifier noise.



The next set of spectrograms were measured by Harry F. Olson (no relation as far as I know) and show the harmonic distortion spectra of a single-ended 6F6 pentode and a single-ended 2A3 triode. (Vertical intervals at 20 dB instead of 10 dB.)

Note that H.F. Olson's measurements are for single-ended circuits, not push-pull. If he had measured a perfectly balanced PP circuit, the even harmonics (2nd, 4th, 6th, etc.) would be completely cancelled, leaving no change in the magnitudes and proportions of the odd harmonics (3rd, 5th, 7th, etc.). Since perfectly balanced PP circuits don't exist in the real world, there is always some residue of even harmonic, which is in direct proportion to the degree of imbalance. A circuit in 10% imbalance (quite large) would have a 20dB cancellation of even harmonics, 5% imbalance (more typical) would have 26dB of even-harmonic cancellation, and 1% imbalance would have 40dB of even-harmonic cancellation.

In practice, 1% gain-matching is not going to realized under dynamic conditions. Morever, what good is accomplished by almost entirely removing even harmonics while leaving odd harmonics alone? With well-behaved triodes, the typical 5% imbalance results in even and odd harmonics conveniently having the same levels. With nonlinear triodes, though, the proportion of odd harmonics are higher, so they will dominate in a PP circuit, resulting in a more "transistor-like" sound. This is a subtle reason to seek out triodes with low proportions of 3rd-harmonic distortion when building a PP circuit (no 6DJ8's, in other words). The same qualities that make a triode desirable in a SE circuit (moderate 2nd harmonic, very low 3rd) make it even more desirable in a PP circuit.

Despite the passage of a half-century and totally different instrumentation, H.F. Olson's 1 watt SE 2A3 spectrograms are remarkably similar to the Amity PP amplifier at 1.6 watts; the 3rd harmonic is almost exactly at the same level below the fundamental, or -60dB down. Note the PP circuit gives a 26dB reduction of the 2nd harmonic compared to single-ended; this is almost exactly what would be predicted from a PP circuit with a residual 5% imbalance.

With zero feedback (local or global), a Class A PP direct-heated triode amplifier has the same (or less) distortion as a Williamson-style pentode amplifier with 20dB of feedback, which is a comment on the impressive linearity of direct-heated triodes. Not only that, the distortion spectrum is much cleaner, with almost no high-order harmonic components. This is an important reason that triodes have that hard-to-describe "direct" and "fresh" sound, while other devices sound more "canned" and "electronic" in character. The ear is not fooled by feedback; what we hear are the actual characteristics of the amplifying devices themselves.

This is why I feel that linearity right at the device level is the most important quality of an amplifier; this preference comes from a background in speaker design, where driver build quality sets the upper limit on the sonic potential of the entire system. In a similar way, the amplifying elements themselves set the upper limit on the sonics of the system.

KLOVE

  再见 A B C  

kenny

等我回家仔细阅读。现在在外地，东西很贵的地方。

Julien

等我翻一下电脑，再找点资料给老兄看

Adams_Leo

完整的线路图，包括方法和前级部分，请大家好好看看把



同时我这里有完整的原文，不过是英文的，但是比较大250K的PDF的文件，论坛文件大小限制了，我没办法把原文章上传上来，需要的请发短消息吧，我邮件给你

音乐传真

原帖由 Adams_Leo 于 2007-8-1 23:52 发表
完整的线路图，包括方法和前级部分，请大家好好看看把
214914
214915

同时我这里有完整的原文，不过是英文的，但是比较大250K的PDF的文件，论坛文件大小限制了，我没办法把原文章上传上来，需要的请发短消 ...

还是有有心人保留的资料！ 多谢了！ ：）

另外，原文是由 IMXP 翻译的，是我们现在的一位版主吗？

Adams_Leo

传真兄是否打算做这个线路，我打算搞哪个前级，有什么好的想法么，不知道J斑竹有什么建议和看法，谢谢

音乐传真

回楼上的兄弟，我还暂时没有实做的想法，因为那两对变压器太不容易做好了，
我曾经咨询过牛魔王，他说好的银线的变压器就要3000元/对，
所以还是先研究电路，再做下一步的打算。

Julien

不是很赞成这样的做法。那个电路依赖变压器性能，器件配对性能过重。
若12AX7等工作在小电平幅度下，三次谐波失真也基本不见了，不存在问题。