تقویت کننده Power Amplifier

Audio Innovations

جمعه 6 ژوئن 2014
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http://dagogo.com/audio-innovations-first-audio

As audiophiles many of us have products we have a very particular appreciation of. It could be a beaten up old valve amp found in a parent’s attic that utterly transforms music or a long desired component that can only be obtained by mortgaging the family home. Although some take this interest a step too far and become distinctly fanatical, there are occasions nonetheless when such abundant enthusiasm is partly or fully justified. I believe the Audio Innovations First Audio power amplifier is an excellent example of the latter. Its importance is not acknowledged today, which in turn prompted me to write this advocacy about its significance historically and its continued worth as an amplifier in its own right.

Although the First Audio, and its monoblock relation the Second Audio, seem to be largely forgotten they represent a genuine milestone in audio technology. They are the culmination of a design aesthetic that emerged in the 1970’s where there was a gradual realisation that feedback can be problematic and valve technology began to be reappraised. Not only has approach to amplification radically altered how amps are designed today but the First and Second Audio have also been influential.

Introduced in 1986, the First Audio was extremely radical for that era: a high-end amplifier producing a grand total of 7.5 watts per channel! Other than a few relatively exotic Class A transistor and lower cost valve amps, virtually all amplifiers producing 20 to 30 watts occupied the budget market. Any outputting less than two figures seemed to cross some sort of psychological barrier. Somehow it wasn’t right. The fact that it was a relatively costly amp made the notion more bizarre, perhaps even shocking!

The First Audio uses output triodes. This was almost as novel as the very low output. While small signal triodes were and still are the most common valve type found in amplification, amps using output triodes hadn’t been widely available for some thirty years! Tetrodes and pentodes became the norm as they produced greater power, and in the 80’s manufacturers sought higher transistor-like output ratings.

The rebirth of triode amplification can be understood by looking to the past. Output triodes with direct-heated cathodes were the mainstay of high quality amplification in the 1920s and 30s. Output pentodes and beam tetrodes became popular in the 30’s. They are more power efficient devices as they generate higher wattage in a given circuit. Unfortunately, this comes with a much higher output impedance and significantly higher levels of distortion, including a disproportionate increase in levels of odd order distortion which is more audible to the human ear than the even order distortion that triodes primarily generate. They were used in applications where power and cost took precedence over sound quality.

After World War II negative feedback was used in amplification to reduce distortion levels. By the turn of the 1950’s with the rise of the hi-fi industry and its associated commercial pressures, output tetrodes and pentodes were increasingly used for better output power, combined with more negative feedback to reduce output impedance and distortion. Increasingly Class AB circuits replaced Class A to further increase output power but as with the switch from triodes it yielded greater levels of distortion.

Clearly high output power is not a virtue of triodes. The key reason for their use rather than tetrodes, pentodes and transistors is sound quality. This is mainly a product of their lower and more benign largely even order distortion characteristics, allowing little or no global or local feedback to be used.

Tests in the 1930’s indicated that second order distortion is imperceptible at levels up to 5% although some perceive it at lower levels. Shorter in 1950 and Wigan in 1961 published articles discussing the greater aural impact of odd order distortion. In 1957 Crowhurst referred to the increasing complexity of distortion when feedback is applied. Lower order distortion is reduced while higher order forms increase. These orders are lower in level but potentially more audible especially when modulating with the signal. These observations were not necessarily a rejection of feedback but raised issues about use. Such concerns were not widely accepted so the affair with feedback continued without complications.

By the late 1960’s, while the world was embracing the transistor and high feedback, a small movement developed in Japan comprising of enthusiasts who advocated the use of low power triode amps coupled with very efficient speakers. Jean Hiraga was influenced by the Japanese enthusiasts. In 1970’s articles of his were published which contended that the harmonics of distortion rather than total distortion levels should be evaluated. In 1986 he received an award from a well-established magazine for his contribution to the understanding of audio reproduction. That year the “First Audio”, the first commercial triode amplifier in decades, utilising neither global nor local feedback, appeared in a largely unsuspecting audiophile market.

Audio Note built a small number of quite powerful amplifiers based on the large 211 triode from 1980. The Ongaku, issued circa 1989, was sold internationally more so than previous models with the help of Peter Qvortrup, then owner of Audio Innovations. It caused a quite stir partly due to its £30,000 price!

The First and Second Audio pre-date the Ongaku by a number of years but failed to build up anything like the same level of mystique. They were far more utilitarian and cost a mere fraction of the price. In the early 1990’s the First Audio retailed for a relatively modest £1300. Its bigger brother the Second Audio (a monoblock double valve 15 watt equivalent) retailed at £2600. Both sold surprisingly well.

One gets the feeling hardly a cent was wasted in the construction of these amplifiers. They have a fairly lightweight construction for models in their price range, utilising the standard Audio Innovations art deco cabinet design common to cheaper models like the 500. Being more substance than surface, these amplifiers have their share of internal goodies. The power-supply uses a hefty mains transformer with six secondary windings. Components changed with time but handmade paper-in-oil capacitors (rare at the time), bulk-foil non-magnetic resistors, and fully hardwired circuitry are common. The output stage uses cathode (auto) biasing. Four and eight ohm binding posts are fitted to suit different speakers; early models also feature a 16 ohm binding post. A switch on the rear alters the earthing to minimise hum.

There were three versions of the First and Second Audio. The initial version (c. 1986-88) uses ECC88 (6DJ8) and PCC88 small signal valves along with 6B4G direct heated output triodes. The 6B4G is an 8 pin 6.3 volt (cathode heater) version of the 2A3 valve (2.5 volt heater, 4 pin) which itself dates to 1932. It was a popular variant used in amplification into the 1950’s but is rarely used today. The ECC88 is a double triode popular since the classic era of hi-fi. PCC88’s are similar but use a higher heater voltage.

After a short time 6B4G NOS supplies began to run low so new versions were launched using 2A3 tubes instead (c. 88-92). The company took the unusual step of having modified 2A3’s produced by Shuguang in China. The most peculiar feature of these amplifiers is the use of 2A3’s with octal (8 pin) bases rather than standard 4 pin bases. This was required since 4 pin bases were rare at the time. These octal bi-plate valves sound reasonable but don’t compare with NOS or modern monoplate 4 pin 2A3’s. Changing the bases in the amplifiers to 4 pin or buying good 2A3’s with octal bases specially fitted is an important upgrade. The PCC88’s were jettisoned for uniform ECC88 usage in the preceding stages.

The last version of the amplifiers (c. 92-96) were designed and manufactured when Audio Components ran Audio Innovations. The redesigned circuit features lower gain 12AU7 (ECC82) valves to drive the 2A3’s. These changes are a significant departure from the initial design and an improvement sonically.

The First and Second Audio operate in push-pull class A1, a highly linear mode of operation. However push-pull topologies have become controversial partly due to losses caused by splitting the signal into opposing cycles. Paraphase phase splitters are used for this task but have a less symmetrical output than other splitter types although they perform well. Phase symmetry would become a critical issue with the popularisation of single-ended amplifiers like the Ongaku in the 90’s that completely avoid the process.

Despite the low continuous RMS output, these amps have very generous dynamic power, perhaps more so than non-triode valve amps. This yields an output power similar to transistor amps of around three times their rated output! Although more tolerant of speaker loads than equivalent single-ended amps, care should be taken with matching. Difficult or insensitive speaker loads compromise performance.

Two amplifier gain stages drive the 6B4G’s/2A3’s resulting in very high input sensitivity since there is no gain loss from using feedback. The ECC88 versions of the amplifiers have a sensitivity in the region of 100 mV! The later ECC82 version has a sensitivity of over 200 mV – high but far more usable. With such sensitivities most of the gain produced by active pre-amps is redundant. For this reason the amps, with a suitably high input impedance of 220 Kohms, were designed to suit standard passive pre-amps.

Whilst aged triode amps like the Western Electric 300B cinema amps were highly influential, The First and Second still represent the first experience of triodes for many designers, reviewers and consumers alike. Reviews at the time testify to a very different sonic experience quite unlike anything to be found in competing amps. This was the case until the early 90’s when a number of new triode amps appeared.

While current amplifier technologies offer different pros and cons, it does seem that triode amplifiers offer the kind of performance other amplifiers rarely hint at. In a way these amplifiers transcend issues like treble, bass, and detail. Although these qualities are important, the difference between an excellent triode amplifier and more conventional designs is the difference between experiencing an almost metaphysical “thereness” and the audible distance experienced with conventional reproduction. This was quite a revelation for some reviewers. There is an uninhibited intensity that the amplifiers either bring to music or do not impede, often in contrast with other amplifiers that slightly restrict the sound.

Since the 1990’s the triode has become extremely popular for high-end amplification. The higher powered 300B has become the valve of choice. More recent trends in design vary with that of the First Audio. Class A push-pull triode configurations have largely given way to single-ended triodes (SET). Although some still prefer push-pull, the majority have a preference for single-ended topologies today.

While other amplifiers of the era like the Ongaku and Pioneer A400 are better remembered, and it can be argued that the common design of triode amplification has moved on to a certain extent, it is still the case that the First and Second Audio represented a very prescient insight of some radical paradigmatic changes to come. They also stand up when evaluated as products in their own right. They are extremely fine quality amplifiers that compare favourably with many of today’s high-end equivalents.

Audio Innovations: The Story

Audio Innovations was founded by Peter Qvortrup and Erik Andersson in 1984. Their first products were the 800 Series power amp and matching pre.

The valve amplifiers of the 1980’s occupied the high-end almost exclusively. Most of the valve amplifiers of the 1950’s and 60’s were not nearly as expensive as their newer equivalents. In contrast to many other companies, Audio Innovations continued the noble tradition of making the valve amplifier an affordable item – the aim was to have one in every home!

In 1986 Audio Innovations were seemingly the first to reintroduce integrated valve amplifiers! The Series 500, a mid-priced 25 watt integrated, become probably the biggest selling valve amp of the late 1980’s while the 300 integrated was the least expensive valve amplifier then available. In keeping with the firm’s cost conscious approach early models like the Series 800, 500 and 300 cost the equivalent figure in Sterling when first introduced!

Most frequently Audio Innovations used EL-34 power pentodes. Their smaller amplifiers use EL-84 output pentodes and the Series 300 uses ECL-86 triode-driver/output-pentodes. These designs typically use paraphase phase-splitters, and Class A push-pull output stages with ultra linear feedback configurations. Relatively little negative feedback is used. Their amps were typically designed to suit conventional efficiency loudspeakers. A few rarer zero feedback pentode models like the Series 200, 400, and 800 Mk2 are extremely intolerant of speaker loads which limit their application unduly.

Erik Andersson returned to Sweden in 1986 but designers like Guy Adams, and others like Guy Sergeant, maintained the design ethos of the company. A firm called Audio Components bought Audio Innovations in 1991. While other companies were beginning to introduce exotic high-end triode amplifiers, and a prototype “Third Audio” using the then extremely novel 845 bright-emitter triode in push-pull was shown at hi-fi shows in 1990 (never released commercially), the new owners changed their approach by focusing more on the lower end of the valve market. Audio Innovations branched out into kits and even introduced a transistor amp with an old style quasi-complimentary output stage!

In 1996 Audio Partnership bought the brand. This was regrettable as they also bought a number of other once excellent firms like TDL that were then used to brand low grade products. The production of Valve amps ended, and the Audio Innovations brand was used for inexpensive lifestyle systems, shelving and silver tinned wiring sold principally by the Richer Sounds budget retail chain in the UK.

(Dagogo wishes to thank Dimitri van Hoven for providing all product brochure images in this article. -Ed.)
A note about the author:

Rob Harris has a particular interest in the historic development of high-fidelity and audio. His interests outside of audio include philosophy, music, and history/archaeology. He contributes articles to a political magazine and several websites. Rob lives in the Republic of Ireland, and he will be contributing a series of commentaries to Dagogo. – Editor

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Kondo

جمعه 6 ژوئن 2014
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بخونید عالیه :

http://www.goodsoundclub.com/Forums/ShowPost.aspx?PageIndex=1&postID=11835#11835

This material is courtesy to Mr. Shibazaki, the owner of Sibatech Shop in Tokyo, that sell internationally Japanise Hi-Fi equipment.

http://www.sibatech.co.jp/

I hope that Sibatech people are more decent then our american distributor-asshole that trades Kondo in US.

MY THOUGHTS ON HI-FI
by Hiroyasu Kondo, Audio Note Co., Ltd., Tokyo
Translations by Hiro Yoshizumi, Sibatech Correspondent in Calgary, Canada
Part 1 (written early summer, 1999)

Particles and Wave Motion

Albert Einstein said that motion is energy. I believe that motion is sound. I am all the more convinced of my belief when I listen to the swelling mass of sound in the mildle of Wagner’s Tannhauser overture. Especially so when listening to the same music performed by the great maestro, Arturo Toscanini at his last concert of April 4,1954. which sounds as if the particles of the sound were colliding with one another and whirling in a thunderous march. It seems to me that those particles made unimaginable movements. I can easily put myself in that scene where the 87-year-old maestro was giving everything he could to his farewell performance and the orchestra was responding to its fullest capacity.

Sound is said to be propagated as wave motion. This is only true in a large room with no obstruction. In reality, however, sound waves move in much more complicated way, colliding and jostling here and there, sometimes in a whirl. We audio engineers should try to visualize in our head how the sound waves are behaving, which cannot be explained by electrical theory. There seem to be still countless unknown factors challenging audio engineers. The more you think about audio, the deeper it appears to become.

Solemn Sound

Two hundred Zen monks begin prayer at five o’clock every morning at the Soji-ji, the head temple of Zen Buddhism. The monks sitting in the left and right files serenely recite sutra-chanting. What a solemn tone! This daily ritual invites people to the world of nirvana. What should I do if I am requested to express this solemnity with the sound reproduction equipment?

First off, I must think of how to collect the sound. According to the current recording method, a number of microphones would be placed in various locations like chessmen. But I am skeptical about this method, primarily because the more microphones are used, the more emphatically the original sound near the microphones is recorded, thus disturbing the sound wave harmony that most counts. Just imagine the noise emitted from a twin-engine plane. It may sound like “Brrr…”, which is produced by subtle differences in frequency. Most musical instrumients or vocal congregations always produce “difference tone”. It seems to me that this gentle trembling vibrates harmonics, further producing chords and as the result, a beautiful tone. If you want to produce a beautiful tone, you must first think of the mechanism in which it is produced.

Analog Sound Is Digital Sound

The analog disk does not always mean to have analog sound, nor the digital disk digital sound. To me the audio systems now available on market sound like digital even in their analog-processing stage. Each note sounds as sharp and acute as the square wave. It is Iike digital photo prints showing every detail of the object so clearty. Its first impression is superb and its resolution is of first rate, yet I wonder if this is the royal road to audio.

The kind of sound that I want to produce is such that its individual particles might have correlation. It is easy to produce so-caled “mellow” sound by choosing proper parts and circuits, but this “melow” property is tricky. It is a technique to gradate boundary lines. Vacuum tubes of 30 years ago happened to produce such a sound and this trend still continues. If any change ever occurred, it can be said that digital-like sound has been added. I cannot agree to this trend. I want to produce a sound in which its individual particles may radiate energy into the ambient space just like the sun and fuse into one. After all, I think I must go back to the stage of picking up sound.

Sound of Electron

Have you ever seen the movements of electrons? The textbook says that the electrons move around the protons with a furious velocity. Sometimes I can vlsualize those movements very clearly. They are the behaviour of the thermoelectrons. The sound produced by the efficiency-oriented vacuum tubes is heavy whle the one by the vacuum tubes of simple structure is transparent. I imagine that this difference might lie in the relationship of the magnitude of the emission to the plate voltage. The thermoelectrons float around the heater or cathode like a sea of clouds. The bigger the emission, the more the thermoelectrons. Each thermoelectron must be separated from the clouds and be made to reach the plate. This process is related to the voltage applied to the plate.

Let’s think of the structure of the pentode tube. Electrons overflow from the cathode, forming huge clouds. As the primary grid with a fine mesh controlling the electrons stands up nearby, the floating thermoelectrons are at a loss with the electron and grid, both negative. It seems to me that this state might have something to do with the “melow” sound produced by the pentode tube. Then, how can you reduce the floating electrons? In conclusion, there will be no alternative but to adopt a coarse mesh for the grid like VT-C and raise the plate voltage, using the directly heated triode. But now the heater becomes shaky, rattling the electrons, which is to affect the sound. How difficult audio is!

Part 2 (written in summer, 1999)

Sound of Transformer Part 1

Those who have studied electricity have little knowledge of transformers, because school usually teaches only industrial-use power transformers. Therefore, audiophiles or audio engineers are forced to study transformers through Hi-Fi specialty magazines. Even those publications do not give much space to articles on transformers. Is this an indication that audio transformers are a history? Certainly, the transformer has some inherent problems such as mu-linearity of magnetic core, distortion of exciting current, and Barkhausen noise. Fastidious theoreticians cannot stand these kinds of problems. Cost-conscious engineers try to avoid transformers and design circuits consisting of only capacitors and resistors. Transformers were excluded from electrical circuits. I think this is wrong. I still believe that high-quality transformers produce nice tone.

I can quote a number of good examples. At broadcasting stations, for one, sound signals are transmitted through tens of transformers from their entrance to exit. If the transformer is the root of all problems, TV and FM stations are to be delivering terrible sound. But actually their sound is not bad. Why is that? I want to answer this question.

Sound of Transformer Part 2

It really interests me how tone quality changes when the sound passes through a transformer. The transformer can be regarded as a filter with time constant in the low and high frequency ranges. This explains why audio engineers were greatly concerned over expanding the frequency response in that prime era of transformers. Even today I fear to leave a transformer on for a long time, because I remember that over-heated power transformers caused fire from time to time. It was quite a task to manufacture high-quality transformers when both iron cores and wire materials were of inferior quality. As the result of having experimented with many different transformers I can classify tone quality into two categories: soft tone and hard tone. A major factor deciding tone quality lies in materials of wires and cores. Let’s start with core materials. There are permalloy for small signals and silicon steel for medium and large signals. If a certain kind of core is good for all, this would be the most ideal. In reality, however, a proper core must be chosen depending on the initial responding speed of magnetic flux in the small range and the maximum magnetic flux density. Permalloy and silicon steel produce respective different tones.

Sound of Iron Core

Iron cores are used for transformers. As the signal transformer has a voluminous winding, signals directly pass, even without the iron core, in the high frequency range. Problem is the low and medium frequency ranges. Here lies a problem inherent in the transformer. The property of the iron core is first judged by its hysteresis curve. But this is merely a criterion, because measuring is done on the winding iron core. Next, it is judged by how much magnetic flux can pass through it and where the saturation point is. While these procedures are enough for power transformers, further studies are required of for audio transformers. In case of the power transformer transmission of minute signals does not have to be considered. In order to transmit minute signals an iron core is needed that sensitively reacts in a very low magnetic field. To this end, a core named “nickel core” containing 45 to 78% nickel is usually necessary. A problem with the nickel core is its low density of the maximum magnetic flux. Furthermore, there are so many different kinds of nickel cores, each producing different tones. Generally speaking, the less the nickel content, the harder the tone. On the other hand, the tone produced by silicon steel (OPT) tends to be soft. The tone has little clear boundaries, because magnetic flux does not occur in the small signal range.

Rendezvous of Transformer and Silver Wire

There are many factors that might decide tone quality of the transformer. In particular, winding materials play an important role. In comparison of tone quality between OPT for the single-ended amplifier and that for the push-pull amplifier, the former seems to produce clearer outline of tone. I can think of various factors to explain this difference, but there is one thing that nobody has so far taken notice of. It is whether there exists any magnetic field generated by the direct current applied to the winding material, depending on OPT for the single-ended amplifier or that for the push-pull amplifier. It seems to me that this magnetic field generated by the direct current might make the difference of tone quality between the two winding materials. In other words, this is the relationship between the magnetic field and the behaviour of the electron. Experience tells that when you wind wire around a horseshoe-shaped magnet and send signals through it you would notice a more conspicuous change in tone quality due to the difference of wire materials than when there is no magnet. Repeated experiments show that the “silver wire” produces less changes. The “copper wire” usually make the tone coarse. If you wind silver wire around the transformer the conventional tone changes dramatically. Now, the statement that the transformer deteriorates tone quality seems to be rootless. This fact cannot be denied just because electric theory has not clarified the relationship between magnetic field and silver yet. Sooner or later, people will start looking squarely at the reality. I might add that direct current magnetic field still remains in OPT for the push-pull amplifier of Audio Note Japan.

Part 3 (written in late summer, 1999)

High Voltage Resistant FET Amplifier

The first product made by Audio Note Japan was a pre-amplifier, for which I used a high voltage resistant FET developed by Mr. Shigeru Terada in cooperation with Shindengen. The field-effect transistor was usually used as a voltage element like a vacuum tube. But because of its big distortion this transistor was not used for an amplifier pursuing a high performance. When used as a voltage element in place of a vacuum tube, FET showed a Vp-Ip curve peculiar to the semiconductors and profusely generated second higher harmonics. Moreover, its low voltage resistance (about 50V) made FET difficult to be used. Then, Mr. Terada developed a FET that could resist 200V. This phenomenal improvement made the job of amplifier designing much easier and expanded ranges of low distortion. I designed a pre-amplifier using this FET, which brought about “Meister-7”, alias “M-7”. The photo below shows the first unit of this model. The large box on the top contains oil-condensers and chemical condensers for the power source. After that, I designed a more compact “M7-II” with low distortion, for which I chose a Cascode circuit in the amplification stage. This circuit contributed to success in reducing leakage current dramatically and at the same time canceling distortion. I used an oil-condenser for the coupling condenser. The discontinuation of production of FET called a halt to the production of M-7, which had reached 100 units. I hear that some units are still being cherished.

Emergence of “Ongaku” Amplifier

Why is the tone of the vacuum tube amplifier favored? One of the factors in terms of the circuit will be its ability to handle high B-voltage. Among the tubes available to us today, “211” can take as much as 1,000V. “211” is equipped with a coarse grid and has very few stray electrons thanks to its low bias, which means that this tube has excellent linearity of Vp-Ip characteristic. In fact, the mu (amplification factor = 4) linearity is flat. Everybody might think that he can design a high performance amplifier only if he uses such a fabulous element. I made a number of 211-S amplifiers, but was not thoroughly satisfied with their tone quality although they showed excellent characteristic. They lacked ‘tenderness’ of 2A3 and ‘depth’ of 300B. After a succession of trial and error I reached a conclusion that the problem had something to do with the tone quality of the circuit elements used for “211” that leaves little ambiguity. Therefore, Mr. Yasuhiro Oishi helped me wind silver wires around the silicon steel. The result was just incredible. What a marvelous sound! Encouraged by this discovery, I next made silver-plated condensers. These inventions produced the kind of tone that nobody had ever experienced before, and Mr. Masahiro Shibazaki of Sibatech Inc. aptly named the amplifier “Ongaku” (music).

Tchaikovsky’s “Pathetique” Symphony

What an introvert music! Its mood is transformed all of the sudden after the second theme of the first movement. I tried to make my own interpretation of this music. A young man starts on his journey of life with anxiety. He fights to the limit with himself and the world. A glimpse of light rescues him and he wins. But there is little time left for even a transient relief. Now he must confront underground spirits. The roaring timpani spellbinds listeners with fear. In time he reconciles with the spirits and calmly falls into sleep with a deep sigh. This symphony is full of strange orchestrations. Brass instruments always follow bass clef of woodwinds. Low string instruments sound groaning throughout. A thunderous fortissimo is followed by an impossible pianissimo. Ritardando and accelerando are alternately and obstinately repeated. High performance techniques are required of the orchestra, and preposterous technology is demanded of the sound reproduction equipment. I seriously wonder if mediocre amplifiers and speaker systems can fully reproduce the consideration that Tchaikovsky intended to pay to that delicate music. At this point of time I am confident that my amplifiers, speaker systems and cartridges can convey the exquisite shades of the music more deeply and faithfully than any other equipment. My equipment stands on its own.

“211” and “300B”

The performance of “300B” is almost comparable with that of “211′. Both were American inventions at the time when that country was still enthusiastic in making of consumer products. In a way ‘300B” is easier to use, because it only requires 400V for B power. It was Japanese audiophiles that made the world recognize the superb tone of “300B”. If you make a 300B amplifier you would understand that it has a unique tone. Someone said that the secret lies in in its structure in which the filament is hung down. You will see a spring hang the filament. Remember that the echo machine was of spring-type. That echo machine was relived inside a vacuum tube. When you hit the glass, it sounds somehow convincing. The material of the heater makes the difference between “211” and “300B”. “211” is filled with thorium to increase the strength. Mind you, this tube had been intended for military tanks! The difference of materials for the heater affects the tone of the amplifier. The tone of the 211 amplifier is crisp and firm. I should add that Golden Dragon’s 300B shows satisfactory electrical characteristic. I am proud to say, because that tube is equipped with the tungsten invented by Japan’s most advanced technology.

Part 4 (written early autumn, 1999)

How to Listen to Toscanini

LP records were invented around 1949, when tape recorders were put into use for broadcasting. Experiments in stereo recording started around 1955. The first half of the 1950’s saw a dizzying pace of innovations in the music world as well as tremendous advance in quality of sound reproduction. In particular, the invention of LP records can be compared to that of Edison’s wax-coated cylindrical gramophone in its impact. LP records were invented by CBS Laboratories’ Peter Goldmark and other engineers. When I visited CBS Laboratories in 1970’s I saw the first cutter of LP records that was proudly on display. The cutter was a lathe-improved machine, thus being called “Lather”. CBS approached their rival RCA for the new invention. CBS and RCA were the world’s leaders of music recording at that time. David Sarnoff, the then president of RCA immediately instructed his engineers to develop compact, light discs, and as a result EP records were born. In time the application of LP and EP records was divided up in such a way that LP records were used for long-playing music and EP records for short-playing like popular music. I hope readers understand the background of such a phenomenal era. History repeats itself. You never know if large-size records might be revived in the future.

Episode 1

“I am eager to attend your performance at Bayreuth as Fuhrer of The Third Reich.” “My music is not for a devil.” These are the words in the letters exchanged between a modest Hitler and Great Maestro Toscanini. 1930’s saw a number of serious problems smolder the world. The Empire State Building was about to shake off its shameful nickname, “The Empty State Building.” You will notice interesting street scenes if you walk in Duomo of Milano in the evening. People are engaged in feverish debating here and there. Certainly, Italian people like to debate on anything. Toscanini courageously stood up for democracy at those difficult times. In a later year, said Albert Einstein, “You are not only the world’s greatest conductor but also proved your noble character in fighting fascism.” Toscanini left his footmarks of historical importance in the European music community in 1930’s. In January, 1937, he made a crucial decision, which led him to working exclusively for an American broadcasting station.

Socony Oil Company

It came to my notice that a Toscanini’s CD which I recently purchased gave a credit to Socony Oil Company as its sponsor. Toscanini who had wanted to retire moved to America very grudgingly. Therefore, they treated him with their senses honed. To RCA money was no object in founding a world-class orchestra for The Great Maestro. Socony Oil Company became the sponsor for the newly-founded orchestra’s broadcasting. Remember, the American prosperity was brought about by her infinite underground resources. We should consider it quite lucky that irreplaceable music assets were left behind. About the same time an epoch-making radio receiver that was to be called “Superheterodyne receiver” was invented by Edwin H. Armstrong, an American engineer. The new receiver made great strides in improving the receiving quality, but the reproduced sound was not entirely satisfactory yet. So-called magnetic speakers were prevalent at that time. RCA worked very hard to make magnetic speakers reproduce Toscanini’s sound at its best form with the result that the notorious 8H Studio came into being. Reverberation was minimized in the studio. If you listen to Toscanini’s historical performance of Beethoven’s Fifth Symphony you will hear individual instruments distinctively. But you will also realize that it was recorded in a studio of dead sound. A Great Maestro’s plaque is still displayed at the entrance of the 8H Studio.

Ribbon Microphone

The microphone setting that looks unique from today’s standards consists of 3 microphones in the upper location: the main microphone in the center, the spare one at the right end and the one for shortwave at the left end. They are all ribbon microphones made by RCA. Ribbon microphones are still in use. For example, they are used for AM broadcasting, because they are most appropriate for recording vocal sound clearly. A ribbon microphone is constructed in such a way that an ultra-light diaphragm is hung down in a strong magnetic field. It pliably follows soundwave just like willow leaves rustling in the wind. Trouble with it is that thanks to its extreme softness rather than its lightness the oscillatory frequency resonance is very low and the low frequency characteristic rises up. Besides, there is no electric generation seen in the horizontal direction of the microphone. Here you will notice the tone characteristic of the reproduction of Toscanini’s music. He arranged the first violins at the left of the stage and the second violins at the right, by which the expanse of strings and balanced harmony would be materialized in a concert hall, but the 8-letter directional microphones were not as effective. Presumably, that is why the microphones were set up very close to the ceiling. A ribbon microphone has its frequency characteristic only up to about 7KHz, thus cutting off thrilling high tone. However, it beautifully records the sound of low strings and percussion instruments.

Part 5 (written in July, 2000)

Intelligence of Mankind – Cutter Head

The cutter head is a device indispensable to production of analog discs. The world’s first cutter head with which Edison recorded “Saint’s Name had the same structure as the SP soundbox. With the advent of the stereophonic recording the cutter head made astonishing progress. As is usual, a race for technical dominance occurred between the V-L method and the 45-45 method. In the early stage there was a so-called mono-compatible stereo in which mono cartridges were used for reproduction. This was invented by an economy-minded European. In no time Westrex, a subsidiary of the then world’s top electric manufacturer came out with the 45-45 method. The difference of mere 45 degrees gave the decision in favor of the 45-45 method. Just remember that sound grooves are cut horizontally in the monaural recording. In the 45-45 method “L” and “R” are cut downward with an angle of 45 degrees respectively. Harmony signals are recorded in the horizontal grooves while difference signals in the vertical grooves. The respective signals of “L” and “R” that are not independent form a vector resultant. The signals are never divided up. When you look at the grooves of an analog record through a microscope you see nothing but uneven protuberances. It is indeed amazing that these grooves contain information of all the instruments of an orchestra. However, you might say that you do not need to take such a trouble as analyzing sound waves just to enjoy listening to music.

The cutter head is able to present complex music information in a visible form. See the the photo below. This photo shows the grooves of a stereo record. Notice the circles area. The zigzag line representing horizontal signals shows a harmony signal that is monaural, and the amplitude shows the volume. Furthermore, the area where the lines are thin or thick represents vertical signals, or “difference signals” , showing the expanse of sound. Then, where are “L” and “R” signals? They are contained in the walls of the grooves. Their look is just like the Hakone mountain range. The frequency of horizontal and vertical signals is recognized by the number of plus and minus cycles per second. The standard of describing a signal strength in terms of speed is 5 centimeter per second, which means that sound grooves are cut at the rate of 5 centimeter per second.

Westrex 3C

Let’s look at the structure of the venerable Westrex 3C cutter head. As it requires a fairly big power to cut wave forms on a soft “lacquer disc”, its cantilever has about 5mm thickness. Its characteristics feature minimum resistance and extremely big mechanical resonance. The engineers had to decide where they should set the center of the resonance. Music information is centered in the midrange and human sense of sound is centered at 1KHz. So, let’s set the resonance point at 1KHz. Thus, a decision of historical importance was made. Naturally, the tone quality at this kind of characteristic was poorer than that of the telephone. First of all, the frequency had to be made flat. The motional feedback (MFB) helped to solve this problem, but at the same time posed other problems. The first problem was uncontrollable frequency characteristic in the range beyond the ability of MFB. The second was power required of the lower and higher ranges. At the recording of pop music the sound of drums and cymbals directly recorded “on mike” presented very difficult conditions. But these problems were solved in a unique way.

Cutter Amplifier

There is a vacuum tube called EL-156, which was developed for the cutter amplifier. Thanks to EL-156, power was raised from 60W to 100W. RIAA characteristic demanded power. This characteristic that should be correctly called the reverse RIAA characteristic rises as much as 20dB at 20KHz. If, therefore, 1KHz requires 10W, 20KHz needs 100W. Ampliers as powerful as 600W are available today. It is incredible that a power of 600W goes through that small drive coil even if momentarily. Some music records such a huge current as 50A on the ammeter. So, the engineers used a circuit breaker, then the sound got murky. They found that it was caused by “chattering” at the contact of the circuit breaker. You never know what will happen if as much as 50A of current runs.

I should add that only two companies were capable of manufacturing the cutter machine in the world. The cutter head was produced by Westrex, Neuman and Ortofon, but the cutter machine only by the said two. Westrex is an American manufacturer while Neuman a German manufacturer. Their products are distinctly different in tone characteristic. One of the solutions for compensating the problem of the cutter head was “half-cutting”, whereby the disc revolution is reduced by half and so is the tape speed of the taperecorder.

Part 6 (written in autumn, 2000)

Sony C-37A

Not many people will deny that Arturo Toscanini, Wilhelm Furtwangler and Bruno Walter are the most prominent conductors that the 20th century has produced. It may be interesting to know how these conductors behaved towards “Machine”. Bruno Walter drove a car himself in an era when the automobiles were regarded as a status symbol of high society. Arturo Toscanini had a chauffeur named Emilio. It was to Wilhelm Furwangler that Richard Straus declared “Never will I ride on a car you drive.” In his later years Bruno Walter lived in the balmy Los Angeles and actively made recordings with John McClure. They used multi-microphones, among which Sony C-37A was most frequently adopted to effectively produce from a less than 50 orchestra the same volume as a full member orchestra. Walter said, “this microphone conveyed my music.”

Neuman M-50

It may be safely said that the principles of microphones and speakers will not change forever even in the digital age. Current microphones were first produced around 1950. Particularly noted was Neuman’s condenser microphone. Germany, though defeated in World War II, retained her engineering ingeniousness. One year after Sony introduced CU-1 (C-37A), Neuman M-49 was born to conquer the recording studios all over the world. With the advent of the stereophonic age Neuman microphones gave birth to the “brilliant” recording tone. I never forget that characteristic “mellow” tone. Neuman next brought out M-50. Its structure was about the same as that of M-49, but surprisingly, they crammed the microphone unit in a globe-shaped ball. As a result, smooth frequency characteristic and controllable medium/high tone were achieved. My physics teacher used to say, “Round off the angles.” The same may apply to audio.

Ribbon Microphone

Microphones are basically either non-directional or 8-figure directional. Therefore, there is no rationale in pursuing “uni-directional” for microphones. Usually the back of vibration element is covered, which explains why the tone produced sounds kind of murky. The ribbon microphone, in particular, due to its audio tube called “labyrinth”, cannot avoid bouncing or reflecting effect, thus producing characteristic murky tone. Just ask people which they think clearer in tone, ribbon or condenser type. 99 of 100 people will go for condenser type. You may then think ribbon type is now histroy, but it still survives in some areas. AM radio is one. The gist of AM radio is voice. That is why ribbon type that has good separation is appreciated by AM radio people. A certain ribbon microphone captured my attention. It was Aiwa VM-18. As its ribbon area is small, so is the substance. As a result, it is very compliant to incoming sound even in the high tone range. It is doubtful the frequency characteristic is rightfully flat. It sounds more natural to me if it is a little raised around 10KHz when “off-mike.”

Improving of Ribbon Microphone

The ribbon microphone has such a structure that a duralmin foil of about 30mm in length, about 5mm in width and about 10 micron in thickness is hung in a strong, criss-cross magnetic field. The ribbon is notched so it may move freely. While I studied the tone of the ribbon microphone I discovered that there were two problems intrinsic to the ribbon microphone. One problem is the noise of the ribbon itself. The ribbon microphone uses a ribbon made of duralmin that is anti-corrosive. The ringing noise of duralmin cannot be shaken off. This noise is what you hear when you crush aluminum foil. That is a minute split resonance. That understood, I tried silver foil of 3 micron. Silver foil did not emit that stimulating split resonance. Another problem lies in the matching transformer. Vibration is produced by ultra-low resistance. A transformer is indispensable to raise the generating voltage to make the small resistance practically usable. However, it is this transformer that prevents the tone of the ribbon microphone from fully performing its merit. For the core of the transformer I chose a material in which magnetic flux goes through in ultra-low magnetic field. For winding coil there was no alternative but silver coil. Transmission characteristic in the minute level of the silver coil totally changed the tone of the ribbon.

The material courtesy to: Sibatech, Tokyo:http://www.sibatech.co.jp/

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اهمیت ارتباط Power Supply با مدار تقویت یک آمپلی فایر از دید طراح Metaxas

چهار شنبه 6 ژوئن 2012
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برندی هست به اسم Kostas Metaxas که آمپلی فایر ترانزیستوری میسازه و مطلب جالبی در مورد ارتباط منبع تغذیه با استیج های تقویت داره. قبلا هم اشاره ای داشتم به نظر nutshell در مورد وضعیت نویز سوئیچینگ درمنبع تغذیه آمپلی فایر ها .

تجربه هایی که من در شنیدن صداها داشتم نشون میده هم منبع تغذیه و هم وضعیت برق سیستم خیلی مهم هست.

مجله تی ان تی هم مصاحبه ای داشته با طراح این برند:

http://www.tnt-audio.com/intervis/mase.html

متن زیر نوشته این طراح در مورد منبع تغذیه هست :

PDF Format

The most significant difference between VALVE and TRANSISTOR circuits is the amplifier/power supply regulator circuit interaction which is far more critical in Transistor circuits.
To illustrate this phenomenon using the the most basic amplifier gain stage, let us assume that our input signal is a 1.0 Volt peak to peak, 1 kHz sine wave and the amplifier stage has a gain of 10 so that the output voltage is 10 Volts peak to peak. For simplicity, resistor R is the load resistor which dictates the overall gain.
If this was a VALVE amplifier, the high voltages which are typical in valve circuits (from 200-400 Volts DC) would result in a valve of around 50,000 to 100,000 Ohms for resistor R.
The equivalent transistor amplifier using much lower voltages (from 12-15 Volts) would have a substantially lower value of R between 200 Ohms-100 Ohms. Therefore the power supply used in the transistor amplifier is virtually directly linked to the transistor amplifier circuit compared to the isolation of over 50,000 Ohms in the Valve circuit.
If we study this basic circuit to analyse the behaviour of the input signal we can see that the 10 Volt output signal not only appears on the output of the amplifier circuit, but in fact also travels through resistor R towards the power supply.

If we assume that the regulator impedance at V+ is around 2 Ohms just for the purpose of this illustration, then let us study the amplitude of the 10 VOLT sine wave as it goes through R and returns back to the OUTPUT of the TRANSISTOR circuit and VALVE circuit.
In the VALVE circuit, the 10 VOLTS goes across the 50,000 Ohms R towards the power supply impedance of 2 Ohms and the 10V signal is attenuated 50,000/2 = 25,000 times.
Therefore 10V/25,000 = 0.0004 Volts of 1,0kHz sine wave.
On its way back to the OUTPUT of the circuit it is attenuated by the impedance of the amplifier (say 100 Ohms) which then: 0.0004 Volts/50,000/1,000 = 0.000008 Volts.
So therefore, a 0.000008 VOLTS of out of phase sine wave accompanies the 10 Volts sine wave as out-of-phase distortion on the VALVE CIRCUIT.
In the TRANSISTOR circuit, the 10 VOLTS going across the 200 Ohms resistor R would be attenuated only 10/200/2 = 0.1 VOLTS.
Then on the way back to the output, the voltage is attenuated by: 0.1V/200/1000 = 0.05 VOLTS of out-of-phase sine wave added to the 10 VOLT output sine wave.
If you compare the TRANSISTOR and VALVE circuit, the ‘phase distortion’ is 0.5% for the TRANSISTOR as compared to 0.000008% for the VALVE which clearly demonstrates one of the major reasons for the difference in sound between VALVE and TRANSISTOR circuits.
If we monitored the V+ point of the transistor circuit using an oscilloscope, we would notice this 0.1 Volts, 1.0 kHz signal. If we were to increase the frequency to 10,000 Hz and up to 1.0 MegaHertz the speed of dynamic behavior of the power supply becomes critical. Using a normal I.C. regulator would result in the signal at V+ actually increasing in amplitude as the frequency increases to that at 1.0 MegaHertz the 1.0 Volt sine wave is not over 1.0 Volt!
To fully understand this interaction between the amplifier an power supply, it is necessary to understand the operation of a voltage regulated power supply.
A voltage regulated power supply is essentially a D.C. amplifier (not unlike a normal power amplifier) which instead of having an audio signal at the input which is then amplified to become a larger audio signal at the output, has a fixed D.C. voltage reference at the input which is then amplified and becomes a larger DC voltage of at the output. The output impedance of the regulator, not unlike the output impedance (or “Damping Factor’) of a power amplifier is less than one ohm at D.C.
If we use a 2.0 Volt zener diode as our fixed DC voltage reference at the input of the D.C. amplifier which has a gain of 10, the resulting output voltage is 20 Volts D.C.
The negative feedback loop of the amplifier which fixes the gain of 10 times the 2.0 Volt zener reference is very important because it maintains the output voltage irrespective; of an increase or decrease in the power supply voltage to the amplifier as long as there is a minimum voltage for the regulator circuit to operate (for a 12 Volt regulator, the minimum voltage is 15 Volts).
This is the STATIC performance of a voltage regulator which although important, does not affect the overall sound of the amplifier as much as the regulator’s DYNAMIC performance which is influenced by the speed and ‘open loop gain’ of the regulator.
To understand why the Dynamic performance of a voltage regulator is so important, we need to go back to our basic amplifier circuit and investigate what happens to the 1.0 Hz, 10 Volt output signal as it goes across resistor R and encounters our voltage regulator.
To ensure an absolutely stable D.C. at V+ the residual of the 10 Volt sine wave at the OUTPUT is fed through the negative feedback loop of the regulator to force the amplifier to correct this error by applying an inverted signal identical to the residual sine wave to totally eliminate the residual sine wave at V+. A high speed regulator would therefore treat a signal 1.0 Mega Hertz in the same manner as a signal at 1.0Khz.
The ultimate voltage regulator would effectively have a theoretical output impedance (or ‘Damping Factor’) at V+ of zero ohms at all frequencies as a result of its wide bandwidth before the addition of negative feedback.
In this way, the attenuation of the 10 Volts across the resistor R residual would be complete, and no attenuated component of the 10 VOLT sine could be deflected and return to the OUTPUT of the circuit and cause severe phase anomalies by adding to the new signal presented at the output remember that it would take a few microseconds for the signal to go through the resistor and come back.
This extraneous out-of-phase information which adds to the new OUTPUT signal then destroys TIME/PHASE characteristics of the amplifier circuit.
In real world power supply circuits, the impedance of the power supply increases as the frequency because the open loop gain is reduced at high frequencies and the amount of feedback used to linearise the amplifier circuit and maintain the low output impedance is substantially reduced.
If we go back to our basic circuit and analysed the performance of an I.C. positive voltage regulator (say a LM78LXX from NATIONAL SEMICONDUCTIONS) it would have an output impedance at the pin of its output lead of around 0.2 Ohms from DC to 10kHz, and then an increase to 0.4 Ohms at 20kHz, then 4.0 Ohms at 1 MEGAHERTZ which clearly illustrates the open loop frequency response has a turnover point around 10 kHz.
When you add the normal distance between the regulator output and amplifier circuits which may be as little as 60mm to as much as 200mm in many circuits, the overall impedance in creases 5 to 10 times. Also, to stabilise the operation of this I.C. regulator, it is essential to use an output capacitor for stability.
Clearly, this is not adequate for high performance, high speed transistor circuits. For this reason, we have approached the design of our regulators as PART of our amplifier circuits, rather than make the fastest amplifier circuit and add a slow I.C. voltage regulator with an output capacitor and call it a finished design. Our discrete voltage regulators are designed to have the absolute lowest noise, reject mains ripple, but more importantly to have a speed (1000 V/microsecond) which is a result of its wide band design (an open loop frequency response greater than 500kHz) and output impedance which is an order of magnitude better than any I.C. The regulator stability is achieved without ANY capacitors by varying the ratio between the local and overall feedback of each device.
We position the regulators within inches of the active circuits (in the case of our OPULENCE, the regulator is 3mm! from the active circuits) and the regulator impedance is flat from DC to beyond 5 MegaHertz at less than 0.05 Ohms.
Beyond this electrical design aspect, we listen to the sound of our regulators whilst developing each amplifier circuit to ensure that every component change or substitution produces an audible improvement from the selection of transistors to best biasing currents , choice of voltage references zener and degree of local feedback.

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یک مقاله از نلسون طراح آمپلی فایرهای ترانزیستوری تو مد سینگل اندد

دوشنبه 25 آوریل 2011
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همونطور که میدونید ساده ترین و ابتدایی ترین طرح یک آمپلی فایر چه ترانزیستوری و چه لامپی بر مبنای سینگل اندد هست که در اون کل سیگنال در محدوده خطی فقط یک ترانزیستور یا یک لامپ تقویت میشه.

ترانزیستور و لامپ هر دو هم تو حالت سینگل اندد استفاده میشن و هم تو حالت پوش پول که تو حالت دوم دو لامپ یا دو ترانزیستور هر کدوم نصف سیگنال رو تقویت میکنند.

اکثر ترانزیستوری ها الان تو مد پوش پول کار میکنند اما کسانی مثل نلسون هم هستند که ترانزیستور رو تو مد سینگل بکار میبرند. هم ویتوس و هم ASR و هم گران ترین Audio Note ژاپنی (GAKUOH مدل قدیمی که الان با II جایگزین شده) هر سه پوش پول طراحی شدند و گرچه گرفتن صدای خوب تو مد پوش پول سخت تر از مد سینگل هست اما ناممکن نیست. آئودیو نت ژاپن چنین ساختاری داره اما از پوش پول هم استفاده میکنه :

http://www.stereophile.com/interviews/597kondo/index.html

Audio Note Japan built amplifiers all share the following design criteria:
* All tube
* Class A1 operating points
* Simplest most elegant circuit and power supply design
* Highest materials and component quality
* No phase splitting (except Gakuoh) or deconstruction of the music signal
Depending on models, single-ended or push-pull operation of output stages with Zero or -3dB overall negative feedback.
1) The hallmark of all Kondo power amplifiers is that they are made of pure hand drawn silver wire. The out transformers are wound with age annealed silver wire, all internal signal wiring and ground are pure silver litz braid covered with six coat of polyurethane. There are over 20 pounds of pure silver in each Ongaku.
2) All Kondo amplifiers use special Hi-B cores in their output transformers. The exact composition, as well as the annealing and stamping of the core material are all engineered by and supervised by Kondo. There are no ‘off the shelf’ materials used in Kondo’s output transformers.
3) All Kondo amps have a heavy, pure copper chassis.
4)  All Kondo power amplifiers use ISO TANGO power transformers and chokes, these are the worlds finest.
5) All Silver Kondo amps use hand made silver coupling capacitors made from hand drawn silver foil. These capacitors are made in Kondo’s own factory.
6) The tantalum resistors, designed by Kondo and unique to the Audio Note line, are costly but are no compromise and necessary to obtain the “Kondo sound”.
7) Kondo amps are wired with a special acid free, silver solder strictly selected.

اینم مقاله نلسون :

http://www.passdiy.com/pdf/zenamp.pdf

بخشی از متن نلسون رو آوردم :

10 Watts of Single Stage Single Ended Class A Transistor Amplifier
I. What is the sound of one transistor clapping?
There are two most essential principles to audio amplifier design. The first is simplicity. The second is Linearity.
Einstein said, “Everything should be made as simple as possible, but no simpler.” Simplicity is a common element of the best and most subtle designs. It is preferred for purely aesthetic reasons, but also because fewer elements color the sound less, and lose less information. Many audiophiles, including myself, are willing to sacrifice other areas of performance to achieve the intimacy with the sound available through a simple circuit.
An amplifier should be simple, but it also must be linear. Some measure of distortion in an amplifier is unavoidable and forgivable if it is of a less offensive type, but still it is important that the measured distortion performance be reasonably low. The advantage of a simple circuit is lost if the sound is overlaid with an excess of false coloration.
Many complex topologies have been justified by high quality of measured performance. By objective criteria, this is a perfectly valid approach. There are many applications where the need for measured precision is important and subjective performance is unimportant. Any application where the performance is crucial to obtaining accurate numbers, such as in an MRI field amplifier, should be judged by objective means.
But this is not rocket science; our objective is to make listeners enjoy sound. If we justify this approach by calling it art instead of science, that is perfectly fine, even preferable.
Resolving the apparent conflict between simplicity and objective performance is our goal. Commercially available power amplifiers have as many of 7 gain stages in series. The simplest I know of still has 3 stages. This succession of gain stages is essential to build up excess gain that can be used for negative feedback. The feedback is used to correct the performance of the gain stages. Paradoxically, the extra gain is used to correct the extra distortion of the additional gain stages.
How simple can we make a circuit and still have it perform well?
Obviously an amplifier with a single gain stage will be about as simple as we can topologically create, and we ask the question, “How much performance can we get out of a single gain device?”

II. Single Ended Class A
Only one approach is available for linear performance from such a simple circuit: Single-Ended Class A. It was the topology in the earliest use of gain devices (tubes, of course), but has not been widely employed in the output stages of solid state power amplifiers due to its energy inefficiency. Single-Ended Class A operation has received increased attention lately, primarily from tube enthusiasts, and recently a number of companies have introduced tube Single Ended Class A amplifiers. They are characterized by limited ower, high cost, and multiple gain stages.
I published a 20 watt bipolar Single-Ended Class A design in 1977 in Audio Magazine, and it had four gain stages. Pass Labs has been manufacturing the Aleph series of Single-Ended Class A amplifiers since 1992, and they have three gain stages. I am unaware of other solid state offerings in the US, although I expect that my hegemony will be shortlived, with the imminent appearance other single-ended transistor amplifiers.
Simplicity is not the only reason for the use of the single-ended topology. The characteristic of a single-ended gain stage is the most musically natural. Its asymmetry is similar to the compression / rarefaction characteristic of air, where for a given displacement slightly higher pressure is observed on a positive (compression) than on a negative (rarefaction). Air itself is observed to be a single-ended medium, where the pressure can become very high, but never go below 0. The harmonic distortion of such a medium is second harmonic, the least offensive variety.
It is occasionally misunderstood that single-ended amplifiers intentionally distort the signal with second harmonic in order to achieve a falsely euphonious character. This is not true. Low distortion is still an important goal, and it is my observation that deliberate injection of second harmonic into a musical signal does not improve the quality of sound.
Single-ended amplification is distinct from push-pull designs in that there is only one gain device for each gain stage, and it carries the full signal alone. Linear singleended designs operate only in Class A.

In contrast, push-pull designs share the signal between two opposing devices, one concentrating on the positive half, the other the negative half. This positive negative half of an audio signal is an artifice imposed by the desire to efficiently handle an AC only signal, with no DC component. Most Push-pull Class A designs offer energy efficiency of twice that of most single-ended designs, and they also offer a measure of distortion cancellation.
A well matched push-pull pair of gain devices will have lower measured distortion due to cancellation, and will concentrate the harmonic content into third harmonic and other “odd” harmonics, reflecting the symmetry between the plus and minus halves of the waveform. Operation is possible in Class A, Class AB, and Class B modes. The most linear of these is Class A, in which the circuit will dissipate at idle more than twice its rated output. Push-pull circuits have higher efficiency, and they also have an advantage in being able to source current in excess of the idle, or bias, current, by dropping into a lower class of peration. A Push-Pull Class A amplifier idling at a 1 amp bias current can deliver 2 amp peaks before leaving Class A, and can deliver still higher currents considered as a Class AB amplifier, where one half of the amplifier experiences cutoff, and does not carry the signal for a portion of the waveform. By contrast, Single-Ended Class A amplifiers cannot linearly deliver current beyond their bias point, and they generally must dissipate at idle more than 4 times their rated output. Typical efficiency is about 20% maximum. This tremendous inefficiency alone explains why Single-Ended Class A has received limited attention, although careful consideration of possible circuits reveals that efficiencies approaching 50% are possible. In addition, there are ways in which a Single-Ended Class A amplifier can be operated as a push-pull device beyond its bias point, the assumption being that push-pull performance is preferable to clipping. Pass Labs has received one patent and has an application for another reflecting new developments in this area.

Figure 1 shows a simple example of a Single-Ended Class A circuit. In this case the gain device is a FET, although the concept applies equally well for a tube for bipolar transistor. The input signal is applied at the gate, and the transistor provides current and voltage gain which appears at the drain. The gain stage is biased by some form of impedance which sources the bias current to the transistor.
This impedance might be a resistor, or it might be a constant current source, or it might be some other load, such as a loudspeaker. Because this element carries the DC bias current, it is unlikely that we would want to use a loudspeaker for this, and typically we would want to attach the loudspeaker in parallel with the bias element, in series with a blocking capacitor.
If the bias element is a resistor, we see a typical efficiency of about four percent , This means we idle the circuit at 100 watts and have a maximum output of 4 watts.

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آقای Herb Reichert از تجربه هایش میگوید

سه شنبه 28 سپتامبر 2010
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Comments Closed

مقاله از سایت ETM هست که رومی نقدش کرده هست، فراموش نکنیم فضای Audio تابع تجربیات شنیداری است و اینترنت جایی است که باید ما از تجربیات اشخاص درست حسابی مثل رومی استفاده کنیم:

http://www.goodsoundclub.com/Forums/ShowPost.aspx?postID=10956#10956

http://www.enjoythemusic.com/diy/0709/flesh_blood.htm

The light from the streetlights mixes with the glow of the bright emitters in my listening room. Nyiregyhazi plays Liszt in front of me and my skin seems electric but I am completely relaxed. I let out my breath and my pulse is near flatline. Audio can really be good to us… if we let it. This is why we must seek to create and explore the frontiers of audio. This is why many of you build your own audio equipment.

I started building and designing my own audio because I am a ‘gearhead’ by nature, not because I thought I could do better than the pros. However, one day I took the negative feedback out of an amplifier I had built and my whole body went limp. Big symphonies by Mahler began to expand and breathe. A strong sense of voluptuousness replaced a feeling of tension and hardness. Average and poorly recorded program began to become part of my midnight repertoire. I concluded I was on to something important. I realized that there must be more design decisions, big ones, like this one with the feedback, that impact the basic character of my system. This is the point at which I became “high fever”. I now had a sense of what was possible. I began to engineer with a purpose.

Over the last decade, I looked carefully at each part of the audio amplifier design process. My first rule was: take nothing for granted and keep an open mind. This attitude steered me down many wrong roads; like the six months I spent chasing A/C balance in push-pull amps or looking at pentode driver stages. Big dead ends!

On the other hand, looking into every detail of the single-ended, directly-heated triode amplifier has netted results beyond my wildest dreams. The whole world of high-end audio is changing and believe me, it is the result of the work you and I and our friends are doing in our homes. We are experiencing a ‘new dawn’ in audio, but this new dawn is only the beginning and there is much new work to do. So, I want you to take this design as a beginning and build on it. Apply your own audio ethics and benchmarks.

From the letters and phone calls I receive, I know that many of you believe that if you can get a schematic for a great amp, then go out and buy what you think are some great parts, and put it all together carefully, then you will end up with a great amp. Sorry, it just doesn’t work that way. You might get a good amp by this method, if you are lucky and inspired, but to get a great amp you must suffer. Whenever I build a BAD amp, and I have built quite a few, J.C. always says, “Welcome to the next level.” It is the failures and the mistakes that get you to the other side of the mountain. Each failure makes more clear what you do not want. Each success enlarges your vision of what is possible. The process of building is to wire your mind, to focus your sights on that point on the horizon where greatness lays.

I know the Reichert 300B-SE amp doesn’t look like an unusual or original design. But, on the test bench, this amplifier design sets very high standards for bandwidth, rise time, and distortion. My main design goals were vividness, body, color, and dramatic contrast. I want a vibrant, breathy, gripping sort of presentation. I am an artist, a painter in the painterly, romantic tradition. I want my hi-fi to paint with strong, rich strokes.

What I don’t want is distant, thin, or mechanical sound. Lots of flesh and blood. Lots of drama and empathy and lots of Technicolor and Panavision; this is what I was after when I designed this amp. Each design decision was a trial and error attempt to get to this end. Please understand, this is not a ‘part of the month’ design. Each parts choice has a personal evolution, all yin/yanged to my taste. If you change ANY part, it is your amplifier design, not mine.

Amplifiers are engineered from the output terminals backward. The amp/speaker interface is everything and one’s focus must always be on the ‘black & red’. Amplifiers are always designed for a particular speaker or speakers and any amp designer who says differently is trying to sell you something he doesn’t own. How the amplifier behaves into the chosen speaker load is the first point to consider when making all major engineering decisions. This amplifier was created to power Altec VOTs, Edgarhorns, WE-755As, and Altec 601As. I have also discovered that it works very well with LS3/5As, AR-M1s, Lowther PM-6s, and Audio Note Model 2s & 3s. I used a circa 1946 Altec VOT system to do the initial design work. Final touches were done with the Onken/Edgar system and Audio Note Model 2 & 3 speakers.

Output transformers are the cornerstones of any tube amplifier design. Since 1980, I have tried outputs from UTC, ACRO, Peerless, Dynaco, Fisher, Partridge, Western Electric, Chicago, Hammond, Magnequest, RCA, Audio Note, and God only knows…? I prefer the Tango to any other. I distributed Tango in the U.S. until 1993. Maybe that’s why I began this design with the Tango XE-60-5S single-ended output. I do not care how “romantic” you want to be, you must have speed and bandwidth. An amplifier that is slew limited or unstable outside of its passband will never have good tone character.

I believe the ultrasonic and infrasonic behavior of a SE amp must be carefully examined. Excessive phase shift or ringing in these regions will surely sabotage the amplifier’s potential for greatness. If the output transformer rotates phase more than 40 degrees below 100 Hz, the amp will sound slow and the bass will seem to lag behind, lacking tunefulness . Bass transients will sound dull. Likewise, if there is ringing in the ultrasonic region, the amp will sound hard and hollow.

The proper selection of core material, core size, aspect ratio, and winding technique is far more critical in SE designs. Poor choices lead to soft, lazy, unrefined sounding amplifiers. The Tango line is unique in that it is the product of two decades of continual development. World-wide, there are thousands of SE amps with Tango outputs. The Tango XE-60-5S measured 18-80 kHz, -2dB in this amp. This is at 7 watts! Remember, I do not sell these transformers anymore. I just still love them.

I chose the WE 300B tube after living with amps built around the 6B4G  2A3 , 50 , 45 , 801A, and the 10Y – 10Ys push-pull are still my personal favorite, but even the VOTs like more than 3 watts. Of the available triodes, the 300B plays the most records with the greatest ease and the most refinement. It is voluptuous and elegant. Also, it is the ONLY tube I am aware of with perfect sample to sample consistency. The Western Electric 300B is always quiet and it will last forever. There is a hypnotic quality to the 300B sound that draws me into the performance like no other type of amplifier.

Operating points are next on the design agenda. My best friend and tube maven, J C  Morrison, has already written the book on this subject: run your triodes hot! With pentodes, I like low plate voltages and high current. With triodes I like high plate voltages AND high current. I run the WE 300B at 425VDC on the plate and 80mA standing current. I want deep class A1. I want to tickle the center of the B-H curve and swing very little current across the power supply. Symmetrical clipping and fast graceful recovery are a combination of power tube operating point, driver stage design and power supply engineering. These three elements work in concert and must be designed together with a clear sonic goal in mind. The amplifier’s ability to drive speaker loads with ease and refinement will be seriously handicapped if we make a bad decision here.

I chose the 6SN7 cascade after examining the distortion spectrum and sonic character of the SRPP, transformer coupling (Tango NC-16 and NC-14), 5687 anode follower, the mu-follower, and several variations on direct coupling. With the R-C coupled 6SN7 and 500VDC raw supply the amp ‘locks’ into class A1 with a minimum of A2 voltage. The 12K plate load on the driver stage gives me the rise time and low odd-order distortion product I wanted. Don’t laugh, but I only like the GE 6SN7GTB in this position. You must use GTBs to get the required plate dissipation and the GE version sounds best to me. The first stage is R-C coupled to the driver stage to hold the output tube grid swing to max even with poorly matched tube sections or aging tubes. Direct-coupled designs seem to change their sound over time even though their minimum phase character is highly appealing. In this design, the time constants have been very carefully considered, so don’t go changing any resistor or capacitor values.

The power supply is everything in these little amps and it also seems to be the area where designers agree the least. All the SE 300B amps I have heard sound very different from each other. Some sound (from Tube Amplifiers) more mechanical than the worst solid state designs. The reason, I expect, is wildly different ideas on power supply design. First rule of triode amp design: Solid state rectifiers = mechanical sound. You don’t think so? Then you haven’t really compared. I promise you, IF there is only ONE thing I have learned in ten years of amp design, it is this first rule. A lot of time went into selecting the rectifier tube for this design. Normally, I use the WE 274A/B, but this amp sounded best with the RCA-5R4GY.

The pi-filter is the heart of this amplifier. If this amp sounds better than others, it is probably due to the choke/cap selection on the pi-filter. I don’t like to waste a lot of energy charging and discharging caps. I want a narrow torque curve – high rev supply. Low impedance and minimum storage gives fast recovery and fine texture to the sound. Remember, most amps under 2000 watts are running at or near overload. Let’s make them sound unstressed at overload. Then the music will sound unmechanical. The power transformer should be rated at least 500mA at 800VCT. An even higher current rating is better (I use 750mA) because I want a small value bleeder resistor at the end of the filter. I like to bleed at least 25%% of the total standing current. This stabilizes and regulates the supply. With the small Black Gate caps, the heavy bleed appears to enhance the clipping characteristic.

We want 480 to 500 volts across the output of the pi filter. The choke should have a DCR of less than 300 ohms. I aim for 10-20 Ohms DCR! This is a big chunk of iron, but it is a very important part of the design. The number of Henrys is less important than the DCR rating. Two to ten Henrys is fine. We are looking for a B+ that is fast and linear, but loosely and naturally regulated. Remember, the reservoir and decoupling capacitors are part of their respective stage’s transfer function. This means that they are just as important as the tubes in determining the linearity and character of the amplifier.

For caps there are a few choices. I have tried all the usual stuff: WE oil and paper, photoflash, polypropylene, etc. The Black Gates (47uf at 500vdc x 2) are now my first choice, but only if you play music every day. These are electron-transfer/electro-static and must be kept charged. They take a full 24 hours to recharge when left to discharge. These are not electrolytics. They work more like an electrostatic speaker. There is no electrolyte or electrochemical delay. These caps are super wideband, linear, non-resonant, and quiet. The Cerefine are almost as good. They use a ceramic powder instead of pure carbon like the Black Gates. This ceramic powder allows for a quicker charge up time. Both caps behave very gracefully under A/C conditions. Do not use polypropylene. If you do, this is your amp design, not mine.

I am not going to get into the parts philosophy thing except to tell you I have tried everything I could lay my hands on. My ideas change daily so here are today’s recommendations. No Teflon. No silver plated wire. No MIT multi-caps. No metal film or metal oxide resistors. No polypropylene caps. No Vitamin Qs. No Solen. No REL caps. No Holcos. No metal oxides. No Vishays. No SCR. No MKP-1845s. No solid state diodes. No solid state current sources. No silicon anything. If you use this stuff, you know who’s amp design it is NOT.

Please use Allen-Bradley resistors in the plate circuits. Paralleled resistors, in plate circuits, are quieter and sweeter sounding. Use Audio Note Tantalum resistors in the cathode and grid circuits. On the cathodes of the 300Bs use Caddock 50 watt MP 850s. You can use Caddock MG or MX in the plate circuits if you are a noise freak, but I think the A/Bs sound more relaxed and showcase the wood and brass tones on orchestral music. I have only three recommendations for coupling capacitors and they are all Audio Note. Due to my affiliation with Audio Note you probably won’t take my coupling cap recommendation too seriously, but that’s OK. You lose! If you want even a chance of catching my Ongaku sonically you must use Audio Note silver foil paper in oil coupling capacitors. If you can’t afford these use AN copper foil paper and oil capacitors. There simply are no other choices.

Wire is a separate issue. Wire is a system thing. After twelve years of building amps, I only know two things for sure:

1) Use tube rectifiers

2) You can never have too much silver in the signal path.

go with out food or clothes  but buy lots of records and wire your hi fi in silver.

Audio Note or Kimber silver wire are my first choices for internal wiring of this amplifier. If you can not afford this stuff  just use Carol PVC hook up wire.

Nothing is worse than silver plated copper. Stay away from all Teflon coated wire if you are looking for relaxed natural sound. Believe me the Carol PVC stuff is good for everything in your system from the tonearm to the speaker. If you can not afford silver, and you trust me, try it.

In fact, if you want to build this amp on a budget, try this… you can still say it is my design. Use the Tango XE-20S or the Audio Note outputs. Use the Carol PVC wire, the AN Paper and Oil Caps (Regular type), Allen-Bradley resistors, and Sprague or Mallory power supply and bypass caps. You will lose some of the refinement but none of the naturalness.

If you make substitutions with parts try to avoid plastic, especially hard plastic. Think voluptuous and colorful. Oh yeah, even if you are on a budget, try to use a copper chassis, 2% to 4% silver solder, and high quality ceramic tube sockets.

Please, try to wire the circuit just as given on the drawing, observing the ground points of the cathodes and the PS capacitors. Do not buss the power supply caps. The pi-filter and the 300B cathode resistor should be grounded at the same point. Likewise the 6SN7 cathode resistors, the driver stage bypass condenser and the decoupling capacitors should all go to the same point.

All of this design talk may be for naught. You see, I believe most deeply that the real magic ingredient in any amp design is the wu of the designer. This wu flows from the designer’s hands during construction and raises the effort above the common and imperfect. Therefore it might be best if you design your own amplifier, for your own speaker, based on what you already know and what you think of my ideas. My philosophy rests on the romantic and the expressive. Drama and contrast with the grace and poise of a bullfighter are my audio system goals.

This circuit and these parts choices were developed inside the world of my hi-fi to my taste! If you want an exceptional music reproduction system in your home you must first develop your internal reference for natural sound. Then you must outline your aesthetic and make a series of design decisions that reflect that aesthetic. But remember, you won’t be happy if you acquire your aesthetic from reviews and audio pundits. You must discover your own. Trial and error is tedious and it takes a long time to become a wizard, but I am sure you will look good in the wizard’s cap.

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Class A Class A/B Class B Amplifier

یکشنبه 10 ژانویه 2010
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یک مقاله خیلی خوب از طراح Pass Labs میخوندم که فکر کردم بد نیست اونرو اینجا بگذارم چون عده ای از دوستان دوست دارند مفهوم Class A و Class A/B رو به زبان ساده بدونند و من هم  به ذهنم رسید مقاله نلسون رو اینجا قرار بدم چون هم به زبان ساده توضیح داده و هم به نکته های جالبی اشاره داره.

نلسون (طراح Pass Labs) اشاره داره به اینکه Class A ها غیر از رفتار خطی تر که باعث میشه دیستورشن پایین بیاد یک شاخص مهم دیگه هم دارند اونم اینکه میزان دیستورشن در Class A مقدارش با افزایش توان خروجی تغییر نمیکنه.

نکته مهم دیگه Class A ها اینه که تابع دیستورشن اونها هارمونیک های اولیه رو داره اما Class B ها هارمونیک های بالاتر رو دارند و میدونید هارمونیک های بالاتر برای صدا وضعیت بدتری ایجاد میکنند تا هارمونیک های پایین تر.

مهمتر از همه اینکه نلسون مثل ویتوس و طراح audio Note UK کاملا مخالف فیدبک منفی هست و میگه میشه یک Class B پرتوان ساخت و دیستورشن اونرو با فیدبک منفی آورد پایین اما نتیجه نهایی اصلا مطلوب نیست.

دیروز داشتم به دفتر های فای ام نگاه میکردم که دیدم نزدیک به چهار پنج مطلب نیمه کاره دارم و متاسفانه وقت نشد این مطالب رو تکمیل کنم و نیمه تموم مونده. یکی مبحث Macro vs Micro هست ، یکی DPOLS ، یکی Audio Reviewing و یکی هم تحلیل Audio Note Ankoru مهندس پوینده و مهمتر از همه عکسهایی است که آقای راد زحمت کشیدند و از نمایشگاه مونیخ برام آوردند و من شرمنده ایشون شدم و تا به این لحظه نشد اونها رو تو سایت بگذارم.

برای Spintricity هم باید یه چیزی بنویسم که هنوز کاری براش انجام ندادم.

سیستم شهرام عزیز رو هم گاهی آخر هفته ها میشنوم و به شهرام میگم من هر بار که این صدا رو میشنوم تعجب میکنم و مگه میشه آدم از یک چیزی چند بار تعجب کنه، اصولا یک چیز جدید فقط یک بار باعث تعجب آدم میشه و نه هر بار که تجربه اش میکنی!

راستی آرمن عزیز هم لینک زیر رو امروز برام فرستاد ، ببینید تو قیمت های پایین چه چیزای جالبی پیدا میشه:

http://www.sasaudiolabs.com/

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