Scott-LK-72-Assembly-Instructions 电路图 维修手册.pdf

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1、 INSTRUCTION LEADER IN AUDIO ENGINEERING AND Dfier peafed in 1947, H. H. Scotts Model 210A. This re- markable instrument introduced a score of unusual design features, which today are accepted and used by all manufacturers. The engineering innovations intro- duced in the 21 0A are typical of H. H. S

2、cott, a company which has continued to pioneer in designing and pro- ducing new and better audio instruments. The excep- tional design and careful manufacture that went into the 210A have been proven over the years_ . most of them are still in service, after more than 14 years. They wort perfectly a

3、nd compare favorably to products available today. H. H. Scotts philosophy is not only to develop 1:ew and l;etter instruments, but also to produce equipment that will last. There is no built-in obsolescence in H. H. Scot; products. These brief notes try to indicate how this is accomplished by using

4、the latest example of the H; H. Scott engineering laboratories, the LK-72. What is the function .,of a Power Amplifier and a Preamplifier Control Center The purpose of a high fidelity power amplifier is to take weak signals, increase their strength tremendously, and I convert them into watts of powe

5、r necessary to move the cone of a speaker. It must do this without adding, subtracting, or altering the contents of the original signal in any way. Such alteration of the origi- nal program is referred to as distortion. The purpose of tlte preamplifier control center is twofold. First, it takes the

6、exceptionally small output frofn the conventional magnetic phono pickup (3 to 16 millivolts in most.cases) and steps it up to a level where it can be properly accepted by the power amplifier. The outputs of FM tuners, complete tape recorders, and crystal pickups are sufficiently strong so they do no

7、t need this preliminary preamplification and can be fed directly into the power amplifier. The . second fnction of the preamplifier contrl ter IS to pemut the user to vary the sound to smt . . taste. It permits him to increase or decrease the ble, bass, and volume. It permits him to select a A de rn

8、ge of p,rogram sources,_ reuce record scratch, ,.;.:.,id adJUSt record or tape equahzat10n. Your LK-72 offers on one compact chassis, four pieces of precision equipment. There is a preamplifier control center and power amplifier for the left channel (Channel A), and a preamplifier control center and

9、 power amplifier for the right channel (Channel B). It is actually a remarkable accomplishment in engineering design, to have such outstanding laboratory perform- ance on so simple and compact a unit. How does the Complete Amplifier Work PREAMPLIFIER ST AGE The very weak signals coming from the magn

10、etic cartridge or tape head are fed into Mag High or Mag Low Inputs. They go to the first duo-triode 12AX7 tubes (Vl for the left or A channel, V2 for the right or B channel), where preamplification occurs. Any noise or hum that creeps in at this critical stage will be amplified along with the progr

11、am material and will be audible in the resultant sound. To avoid intro- ducing noise, special pre-selected low noise tubes are used. Hum is usually caused by the 60-cycle AC used to heat the tubes. On all H. H. Scott amplifiers, DC is used instead of AC, so there is no possibility of hum ap- pearing

12、 from this source. This DC is derived from the selenium rectifier, SRI, on your LK-72. In addition to the preamplification, equalization is also accomplished. Equalization involves correcting for the bass cut and treble boost introduced on all phono- graph records and prerecorded tapes to allow thei

13、r manufacture. The equilization characteristics of H. H. Scott amplifiers is always within 2 db of the record manufacturers (RIAA) specification. Naturally, any distortion introduced at this first stage would be amplified along with the music in the subse- quent stages. To keep distortion down to vi

14、rtually unmeasurable amounts, negative feedback is employed in every stage of your LK-72. Feedback is a method whereby part of the output signal is fed back into the input. Negative feedback indicates that the resultant input signal is reduced in strength. While this causes a loss in gain, it also v

15、irtually eliminates distortion. First Audio Amplifier Stage The preamplified signal goes to the first audio stage, another itlO triode 12AX7 (V3 for Channel A, V4 , . laliiiliiiiiiiiiiiiii=J-=-: -:-=-=-=-=-=-=-_j for Channel B). The signal from tuners, complete tape recorders, crystal cartridges, an

16、d other high level pro- gram sources being much stronger, bypasses the pream- plifier stage and goes directly to this first audio stage. Here the signal is further amplified and modified (if desired) by the tone controls. H. H. Scott uses feed- back-type tone controls, giving the advantage of low di

17、stortion, of not affecting the midrange response, and of being fiat when the control is in the flat position (flat position is 0 on the front panel, indicating that there is no tone control boost or attenuation). The first audio amplifier stage also includes the low- frequency cutoff filter. The hum

18、an ear cannot hear signals below 20 cycles per second. However, a good turntable or record will contain noises of lower fre- quency. These noises, .which you cannot hear, are capable of being reproduced by the amplifier. Often these subsonic sounds will generate such enormous signals that they will

19、waste much of the amplifiers power and adversely affect its ability to reproduce the desirable program material._ The special filter used by H. H. Scott eliminates all these subaudible noises and permits the amplifier to concentrate its power in the audible range of 20 to 20,000 cycles per second. S

20、econd Audio Stage and Phase Splitter The pentode-triode 6GH8 (V5 for Channel A, V6 for Channel B) provides an additional stage of gain, and splits the phase for the push-pull output stage. The advantages of a push-pull output stage are higher power and lower distortion. To obtain the advantages of t

21、his system, it is necessary to have some means of inverting the phase of the signal so that one tube in the output stage will be exactly out of phase with the other (in other words - one will be pushing while the other is pulling). It is also essential that the phase splitter be able to send signals

22、 of exactly equal strength to each output tube. In most circuits this is accomplished by having special AC balance controls which can only be set with expensive laboratory equipment. In the LK-72, the AC balance controls are eliminated by using a true self-bal- ancing phase splitter with special mat

23、ched components. The DC balance controls are easily set by following the simple instructions given. No test equipment is needed. To further reduce distortion, negative feedback is ap- plied to this stage as well. Output Stages Two type 7591 pentodes are used in push-pull for each output stage ( Chan

24、nel A are V7 and VS, .Channel 2 B V9 and Vl0). The way these output tubes are operated .is typical of H. H. Scott standard design procedure. A tube manufacturer usually rates his tubes according to the average amount of heat dissipation they can handle. It is standard procedure for most engineers to

25、 use this design center in their circuits or even go beyond it to squeeze the maximum amount of power out of their amplifiers regardless of how this may affect tube life. H. H. Scott amplifiers operate all tubes, par- ticularly the output tubes, well below manufacturers design center. This assures l

26、ong tube life and superior performance. On the LK-72 for example, the output tubes are operated at 75% of manufacturers average. There are other benefits from this conservative opera- tion of the output tubes. Variations in tubes are not critical. It is not necessary to match the tubes to get low di

27、stortion. In addition, bias adjustments are not needed because of the conservative operation. The bias forthe output tubes is fixed (as opposed to the less de- sirable self-biasing of some amplifiers) and is derived from the selenium rectifier, SRI. The output tubes supply the necessary power to dri

28、ve the speaker, but unfortunately the output impedance of the tubes will be high, while most speakers are of low impedance. In order to match these impedances, it is necessary to use output transformers. You can have the most elaborate and complicated amplifier cir- cuitry in the world, but you will

29、 not have a good ampli- fier unless you have good output transformers. Unfor- tunately, good output transformers are very expensive. One of the indications of a good output transformer is its size and weight. Only a large well-designed trans- former can supply low distortion and clean resPonse from

30、the lowest audible frequencies to the highest. The oversize transformers on your LK-72 do just that. The most conspicuous aspect of an amplifiers per- formance is its power output. The LK-72 is rated at 40 watts per channel. You may notice that other ampli- fiers ( many less expensive) with smaller

31、transformers also claim similar or greater power. All amplifiers are rated at 1000 cycles, a frequency at which you do not need a particularly good amplifier to get good power measurements. The vital frequencies are those between 20 and 50 cycles. Here is where you need the Power, and here is where

32、only quality amplifiers can give it to you. The Institute of High Fidelity Manufacturers (IHFM) recognized this problem and has evolved a new amplifier measurement -the power band. Sim- ply, the power band represents the range of frequencies over which the amplifier can supply a specified amount of

33、power at the amplifiers rated distortion. The LK-72 has a power. band extending down below.19 cycles and. over 25,000 cycles. In fact, at 20 cycles the LK-72 will produce over 40 watts of music power ( 36 watts, steady . state) ! ! This is a meaningful specification of the per-.,_,. formance of this

34、 remarkable amplifier . ._ , An amplifiers stability is also important. An amplifier - should only reproduce signals fed to it, not originate tS -wm:at ,. signals of its own. Many amplifiers under certain_ con- ditions will start to produce a powerful supersomc os- cillation. This can happen when th

35、e amplifier is used with electrostatic speakers (which act as a capacitive load), or if the amplifier is operated with no speakers attached (no load). In extreme cases this oscillation is capable of burning out an amplifier in a few minutes. H. H. Scott amplifiers, like the LK-72, are completely sta

36、ble under all conditions -no load, resistive, induc- tive or capacitive load. This means that if the speaker wire should accidentally be disconnected, you need have no fear of damaging your amplifier. We have often ref erred to the desirability of low distortion. A wide frequency response, ample pow

37、er, and low hum and noise are important but cannot com- pensate for high distortion. The LK-72 will produce its rated output of 40 watts (IHFM standard) at less than 0.8o/ total harmonic distortion. This is important, hut even more important is the distortion at normal listening levels. Many amplifi

38、ers will exhibit reasonably lo,v distortion at full output, but actually turn out to have as much or more distortion at normal listening levels of I to 5 watts. This is self-defeating. At normal listening levels, the total harmonic distor- 3 tion of the LK-72 is below 0.05%, which means it is so low

39、 it cannot be accurately measured with even the finest of laboratory instruments! The translucent sound from your LK-72 is proof of this low distortion . Power Supply In order to supply the necessary voltages to all the stages of the amplifier a rugged power supply is needed. The husky oversize powe

40、r transformer along with the GZ34/5AR4 rectifier and SRI selenium rectifier com- prise the heart of the power supply. An important characteristic of the GZ34 is its slow warmup. This gives the other tubes and components an opportunity to reach normal operating condition before the full force of the

41、high voltages and currents are transmitted by the rectifier. The electrolytics ( condenser cans) are all specially designed and conservatively operated. These are other factors in the long life you can expect from the amplifier. Heat can be a problem in any high power amplifier. Under normal conditi

42、ons you can expect the amplifier to use about 200 watts of electricity. This will generate a considerable amount of heat. The heat has to be rapidly transferred away from the output stages where most of it is generated. If it is not, the life of the tubes and other components will be greatly curtail

43、ed. One of the ways the heat is carried away is by convection to the air. The many holes in the chassis are designed to encourage air circulation which well perform this fuQc- tion. Equally important is the use of electrolytic alumi- num for the chassis material. Most amplifier chassis are steel whi

44、ch is a less expen- sive material. Unfortunately steel is not a good con- ductor of heat so such units will have their output stages overheating even though the rest of the amplifier is cool. Aluminum is an excellent conductor of heat, so it rapidly conducts the heat away from output stages to other

45、 parts of the chassis where it can be readily transferred to the air. In fact you may notice that your aluminum front panel is warm to the touch. This means that the heat is being transported away from the heat sources and that your amplifier is operating safely. Another advantage of the aluminum ch

46、assis is that it is nonmagnetic so acts as a shield against induced hum from the power transformer. Conventional chassis made of steel, which is magnetic, are much more prone to this type of difficulty. Conclusion There are many ingredients involved in designing a high quality complete stereo amplif

47、ier. Howevr, this discussion has attempted to show, compromise 1s not one of them. Instruction for the Model LK-72 80 Watt Complete Stereo Amplifier The LK-72 is a complete two-channel stereo ampli- fier consisting of dual 40 watt power amplifiers and dual preamplifiers on a single chassis. It emplo

48、ys the same engineering concepts and features that have made H. H. Scott amplifiers the standard in the industry. Con- servatively designed, the LK-72 will bring you years of listening pleasure. IMPORTANT: Every effort has been made to insure that this kit, when assembled, will perform perfectly. In

49、 order to achieve this result, you must read all of the instructions and follow them precisely. Let us repeat . READ ALL INSTRUCTIONS CAREFULLY . FOLLOW THEM EXACTLY. Unpacking your Kit-Pak Figure 1 demonstrates the step by step procedure in unpacking your kit, as well as the extremely simple procedures involved in setting it up as a convenient work bench. With the Kit-Pak you can work on any table in the house. When it is time to stop working for the evening, all you have to do is dose t