独立电唱机前置放大器英语资料Word文档格式.docx

1、t lie, and that you cant improve something by adding distortion of any kind.Both camps have valid points. Most of us have heard expensive gear with spectacular specifications, but were left cold by the too good to be true, almost clinical sound of such equipment. Similarly, most people will agree th

2、at just because something has tubes in it doesnt make it worth listening to. Turn on one of those old All-American Five table-top AM radios if you want a striking demonstration of just how bad vacuum tube equipment can sound.Perhaps a way of reconciling the two viewpoints is to consider the distinct

3、ion between musical equipment, and reproduction equipment. For musical gear, the individual frequency, waveform and phase distortions are part of what defines the sound of, say, a Fender tube amp. Just as no- one would try to define a fineStradivarius violin with specs and distortion figures, so als

4、o it would be specious to argue that a certain tube guitar amp has over 12% THD （total harmonic distortion） at 35 watts.Reproduction equipment, on the other hand, has always been expected to give a perfect rendition of the signal applied to it. Sounds good in principle; if such a thing existed, we s

5、hould be able to exactly and perfectly reproduce everything from a grundge band to the New York Philharmonic, making the reproductions indistinguishable from the original performances.But therein lies the rub. The overall sound of a stereo system depends so heavily on the room its in, the speakers,

6、volume level, personal preference, and a host of other fuzzy variables that a perfect reproduction system cannot be said to exist even in this day and age, and probably never will. Add the fact that most if not all recordings are electronically sweetened to some degree to make them sound good （as op

7、posed to being an exact copy of the performance）, and arguments for the clinical reproduction approach lose credibility.Carried to extremes, an ideal reproduction system wouldnt even have any controls, except perhaps for selecting inputs. If you think about it, personal preference is the only reason

8、 why stereos have volume controls, equalizers, and other adjustments to let us customize the sound to suit our very individual ears and brains.It might be best to view the reproduction gear as a continuation of the same process that started with the construction of the instruments used in the perfor

9、mance. What we ultimately hear is the sum total effect of everything from that original instrument design, to the way it is played by the artist, through the entire recording, mixing and distribution process, to the gear we use to play it, and how weve set its controls.There is, therefore, a conside

10、rable difference in design approach between theinstrument and reproduction categories. An earlier design （The Real McTube） documented a vacuum tube pre-amplifier for use as an adjunct to electric instruments. The design approach was largely empirical, and the emphasis was on highlighting the unique

11、distortion characteristics of the vacuum tube. This article explores the reproduction aspect; the design approach was quite mathematical and precise, and the emphasis is on controlling the characteristics of the vacuum tube.Vinyl vs. CD Similar arguments are ongoing regarding vinyl records vs. compa

12、ct discs. The CD camp points at the CDs accuracy, definition, and clarity, while vinyl lovers bemoan the CDs lack of warmth and claim that conventional records sound morenatural.This project gives you the opportunity to explore these subjective and controversial debates, letting your own ears be the

13、 judge. You may find that there are some recordings that sounds better through the tube pre-amp, and others that benefit from the improved definition （whatever that is, technically speaking） that good solid-state gear can offer.2: DESIGN PHILOSOPHYThe design philosophy was to improve on vintage tube

14、 designs by incorporating refinements normally only associated with solid-state gear. Prime among these is the use of differential input stages, in which the inverting input is usedstrictly for feedback. Another is the use of direct coupling between stages, a technique once quite common in oscillosc

15、opes, but rare in audio gear. The resulting circuits are thus closely related to the operational amplifier, and used in a similar fashion.In addition to the quasi-opamp idea, some other design concepts used in theproject are:Use of readily available parts wherever possible. For this reason, tube typ

16、es 12AX7A and 12AT7A were employed.Direct coupling was used in crucial parts of the circuitry. More about this later.Flexibility in use was a prime consideration. While this articles details the use of this circuit design as a magnetic （RIAA） and ceramic phono pre-amplifier, there is virtually no li

17、mit to the number of possible applications with suitable modification of the feedback networks.A Word About Negative FeedbackIt is perhaps an unfortunate accident of history that the term negative feedback was used to describe the linearization technique of applying a portion of the output signal ba

18、ck to the input, in opposite phase to the applied signal. It gives the impression to audiophiles with enough knowledge to be dangerous that negative feedback must somehow be a bad thing. Ive seen diatribes against the evils of negative feedback, yet in the same breath such critics extol the virtues

19、of ultra-linear output transformers. The joke is that such transformers achieve this linearization by applying local negative feedback to the screen grids!Perhaps a better moniker for would be dynamic compensation or something on that order. Consider the following characteristics ofwell-designed neg

20、ative feedback systems:A: Precise control over closed-loop gain and minimization of gain drift withcomponent aging.B: Considerable reduction in distortion and noise introduced within a givengain block.C: Reduction of effective series output resistance.D: Precise control and stabilization of frequenc

21、y characteristics. Extensionof high-frequency response where needed.E: Reduces or eliminates the need for careful component matching3: HOW IT WORKSThe sections that follow step through the principles of operation of this design. Since it is quite different from traditional designs, I suggest that yo

22、u follow through this at least once. If the fine points are over your head, dont worry about it; hopefully enough of the information will stick to give you at least a working understanding of the circuit.There are in-line illustrations of the schematic, repeated as necessary. （If you have caching tu

23、rned on in your browser, youll only have to actually download the images once.） These Figures （schematics） are shown here in reduced size. Click on the image to enlarge it, preferably by right-clicking and opening the image in a new window. Full-size, printable versions are also available in the Res

24、ources section. It is highly recommended that you print out these files and have the diagrams at hand as you study this document, especially if you plan on actually building yourself one of these units.Note that in the schematic diagrams, only one channel is shown; the other channel is of course ide

25、ntical. Please note that part numbers refer to the various sections as follows: （Well be assuming the left channel in this discussion.）1-99Common parts （power supply, etc.） Also tube designations.100-199Left phono preamp200-299Right phono preamp3-A） HIGH-PRECISION PHONO PREAMPLIFIER: OVERVIEWConside

26、rable effort was taken to design a preamp that is as clean as possible tosatisfy discerning audiophiles, while maintaining a relatively low parts count to satisfy limited budgets. It should be noted at this point that, while the discussion is in the specific context of an RIAA phono pre-amplifier, t

27、his design lends itself well to a wide range of other uses.This universal preamp module sports a high gain （about 60 dB） and differentialinput stages that allow for differential input （as for balanced mic preamps） as well as either inverting or non-inverting single-ended applications. Frequencyrespo

28、nse shaping and gain setting are accomplished using the appropriate feedback networks.The schematic diagram of the phono/mic preamp is shown in Figure 2. If built on a PCB, the phono preamp equalisation is accomplished using a small daughtercard directly on the main board; if you use the point-to-point wiring approach,these networks can be wired on separate terminal strips for easy modification ifnecessary.Figure 2 P