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

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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