Lux Audio MB-3045 Power Amplifier 电路图.pdf

《Lux Audio MB-3045 Power Amplifier 电路图.pdf》由会员分享，可在线阅读，更多相关《Lux Audio MB-3045 Power Amplifier 电路图.pdf（5页珍藏版）》请在收音机爱好者资料库上搜索。

1、80 Lux Audio MB -3045 Power Amplifier MANUFACTURERS SPECIFICATIONS Power Output: 50 watts average into 8 ohms. (Note: This is a mono unit; a stereo pair was tested.) THD: 0.3 per cent. CAUTION ; WfrtN OUT PI) CONSULT MANUAL FOR ALIONMk.NI IM Distortion: 0.3 per cent. Frequency Response: 10 Hz to 40

2、kHz, -1 dB. SIN: 95 dB. Damping Factor: 16 at 1 kHz, 8 ohms. Input Sensitivity: 0.7 V. Dimensions: 141/2 in. (36.83 cm) W x 91/2 in. (24.13 cm) Dx6% in. (17.15 cm) H. Weight: 33.4 lbs. (15.18 kg). Price: $495.00. The Lux MB -3045 is a mono tube power amplifier rated at 50 watts into 4, 8, or 16 ohms

3、. Built in the classic vacuum - tube amplifier style, its apparent that great emphasis has been placed on the sonic and electrical performance. A strong steel chassis supports all major components (the MB -3045 weighs 33.4 pounds). Viewed from the front of the unit, the unusually massive power and o

4、utput transformers sit at the right and left rear corners of the chassis. Between them are three large aluminum, electrolytic capacitors and the power supply filter choke. Directly in front is the row of tubes, which consists of two miniature types, two larger size drivers, and the output pair, whic

5、h are seated in sockets sur- rounded by a circular ring of vent holes. A sturdy metal cage covers the top of the unit to protect the amplifier and the user from tube breakage and burns, respectively, while main- taining good ventillation. All circuitry, mounted on two printed circuit boards, and wir

6、ing is concealed below the chassis. The semi -fixed d.c. bias, balance, and hum balance controls are also mounted below the surface of the chassis which deters excessive tampering with the controls though making adjustments somewhat inconvenient. A power switch, a.c. socket, and fuse post are mounte

7、d on the rear panel along with the input and output connectors and the input level control. The only component visible on the front panel is the small, but bright neon pilot light. Circuitry Since the introduction of audio power amplifiers, audiophiles have heard about esoteric tube circuits designe

8、d by some high powered engineer for his home system-that surpassed all others. Circuits for such amplifiers seen by this reviewer have tended to use more sophisticated and com- plicated techniques to allow the designer to simultaneously optimize the greatest number of variables, such as power out- p

9、ut, linearity, and response at the extremes of the audio band. However, the Lux MB -3045 appears to be the last word in tube power -amp circuit design, and little expense has been spared to make it so. Figure 1 shows the circuitry to be fancy indeed. Two stages of differential amplifiers provide a g

10、ood deal of voltage gain and supply the out -of -phase signals required to drive the out- put transformer in the usual push-pull fashion. Next, a high voltage differential -driver and cathode -follower stage is employed to provide the very large signal required to drive the grids of the output tubes

11、. (Lux comments that 200 volts, rms, are required at the power tube grids for full power.) The cathode followers are direct coupled to the single pair of out- put triodes that controls virtually all the signal power. Most engineers familiar with such vacuum -tube design will probably be wondering at

12、 this point what sort of triodes Lux has found that are capable of such relatively high power out - AUDIO November1977 AmericanRadioHistory.Com RadioFans.CN 收音机爱 好者资料库 82 Fig. 1 -Schematic diagram of the MB - 3045 amplifier. 1. -,-11- e O 2 N CV O e 2 N VM/ r 3 ._/vvf_ p/ 2 N N 0) M M Q rn + N LL- a

13、 O AUDIO November 1977 AmericanRadioHistory.Com RadioFans.CN 收音机爱 好者资料库 0.2 0.1 o at 0.2 q 0.1 2 r 0 0.2 0.1 020 80HM 4 OHM 16 OHM 50 100 500 1K FREQUENCY - Hz Fig. 2- THD versus frequency for 4-,8-, and 16 -ohm loads at 50 -watts output. 5K 10K 201 puts with acceptable linearity. In fact, they did

14、not find any, so in cooperation with another Japanese firm (NEC) they designed and manufactured their own 8045G output triode and special 6240G high voltage driver. In addition, a special transformer has been designed to get the best match between the new tubes and the load, which employs amongst ot

15、her things, quadrafilar wound primaries. It is most likely that Lux has chosen to use the triode out- put tubes for two reasons. Not necessarily In order of im- portance, they are 1) inherently lower output impedance than pentodes, and 2) inherently better linearity than pentodes. By realizing these

16、 two goals, the amplifier will then by nature drive complicated loads such as speakers more gracefully and generate less distortion, with all other things being equal. The ratios employed in the output transformer were not measured, but if it can be assumed that all four primaries are in a 1:1:1:1 r

17、atio from the knowledge that all four primaries were quadrafilar wound, then perhaps some insight can be had on the operation of the output and driver stages. The first conclusion from this assumption is that the entire output stage is operated near unity voltage gain due to the localized feedback t

18、hrough winding B. Winding A then provides a d.c. current path for the cathode followers and simultaneously operates them closer to constant current operation. Mean- while, the appropriate phase of winding C tends to maintain the cathode followers plate -cathode voltage signal swing to 150 125 100 .0

19、75 050 025 02 W RESIDUAL NO SE .006% IW IOW 50W 20 100 1K FREQUENCY - Hz 10K 55 20K Fig. 3-The MB -3045s power charac- teristics. Top, SMPTE IM versus output, 8 -ohm loads; Bottom, bandwidth for 1.0 per cent THD, 8 -ohm loads. a minimum. Reducing the signal modulation of both current and voltage the

20、n should result in improved linearity in this stage. However, the entire 200 -volt signal must still be pro- vided by the first 6240G. By bootstrapping the 68-kilohm plate -load resistor from this stage to winding C again, the tubes operating current change with signal is reduced. In this case, the

21、connection to the C winding yields better large signal linearity and increased voltage gain. The input stages are straightforward differential amplifiers similar to those used in other tube power amplifiers. One noticeable distinction is the use of winding Dto apply local differential feedback to th

22、e second stage cathodes through the 27-kilohm resistors and the 47-pF capacitors. A summa- tion of signals from winding D and the 16 -ohm tap are used for the overall loop feedback, which when divided down by the 120 ohm resistor, is conveniently applied to the inverting input of the first stage dif

23、ferential amplifier. No a.c. balance control was deemed necessary in this circuit, as the inherent balance in the differential amplifiers and the various local feedback loops serve to maintain symmetrical drive to the output transformer. D.c. bias and balance are provided by the usual voltage divide

24、r arrangements, but in this amplifier bias voltage must be applied to the grids of the driver tubes instead of the out- put tube grids. Within the power supply is the bias voltage divider, which varies the potential on the point marked with INPUT INPUT LEVEL- . OUTPUT COM. 4 H 16 mm M63045 MAfwJFACT

25、VRED tv V% CoftlbRAr.ON .t/WAN OEMAY AC 5A 01s# POWER off on AC.t av.6ow 83 AUDIO November1977 AmericanRadioHistory.Com RadioFans.CN 收音机爱 好者资料库 OPIPIplein Aille 0110 ehlIONI Fig. 4-MB-3045s SMPTE IM residual at 50 watts, Top; and 15.8 watts, Center; with input at bottom. an asterisk; it is nominally

26、 -120 volts. The 30-kilohm balance pot gives the necessary range of adjustment to compensate for the variations in tubes and resistors so the unbalanced d.c. current in the output transformer can be reduced to a minimum. Due to the two stages (cathode follower and out- put tube) and the miniscule de

27、generation from the 10 -ohm cathode resistors, this design has less bias stability than most other tube power amplifiers. Lux recommends that the bias and balance be adjusted whenever replacing output tubes. In the opinion of this reviewer, the best performance will be ob- tained if they are occasio

28、nally adjusted as the tubes age (perhaps every 1000 hours) and also in the unlikely event that Bb the 6240G (V4) tube must be replaced singly. Measurements THD measurements are shown in Fig. 2 for 50 watts into 4, 8, and 16 ohm resistive loads. As can be seen from the curves, distortion was almost i

29、ndependent of load resistance. SMPTE IM distortion is shown in Fig. 3 as a function of output power for the 8 ohm load. The Lux curves for this measurement show a gradual rise below 0.3 watt, which was not detected in either unit tested. At powers above 50 watts, distortion rose gradually until clip

30、ping occured. For a 50 watt output, Lux claims a blanket 0.3% maximum THD or IM, and the amplifiers had no trouble staying well below this figure even after many hours use. Figure 4 is an oscilloscope photo of the IM distortion residual at 50 and 15.8 watts, with the input signal for reference, on t

31、he top, center, and bottom traces, respectively. Keeping in mind the delay in the distortion analyzer, the negative going peaks in the residual correspond to the Fig. 5-MB-3045s frequency response at 1 and 50 watts. (Note break in curves at 30 Hz and 3 kHz.) IW IW I I .411.- y 50W -. 50W OUT, - / FU

32、LL GAIN WATT FULL GAIN I WATT 8d8 BELOW FULL GAIN l 2 10 MI ( 10K FREQUENCY -1e TOOK 200K 1- I I ! Fig. 6-Response of the MB -3045 to a 10 kHz square wave into an 8 ohm load (Top), and into an 8 ohm load paralleled by 2 F. Scale is 10VIdiv. negative and positive signal peaks, which can be considered

33、 third harmonic distortion. The center trace also shows some evidence of circuit imbalance or second harmonic. Although the distortion gets more complicated at higher power levels, this general sort of residual does not show any strong discontinuity in the behavior, as crossover distortion might cau

34、se. The lower curve in Fig. 3 shows the power bandwidth capabilities of the amplifier. Notice that the roll off at the low frequency end is quite gradual. The 1- and 50 -watt fre- quency responses in Fig. 5 show that this unit actually put out 50 watts at 12 Hz with only modest distortion visible on

35、 the scope. This is outstanding performance for a tube amplifier and could only be achieved by using a very good output transformer with lots of muscle behind it. At the 1 -watt power level, the response is within +0, -0.5 dB from 8 to 20 kHz. These tests are performed with a high quality function g

36、enerator having a constant 50 -ohm output impedance. With this low impedance source and with the input level control full up, 1 dB peaking Is apparent at 100 kHz. Since only a handful of preamps present a power amplifier a source im- pedance this low at 100 kHz, this peak will probably never be exci

37、ted in normal use. However, all types of behavior such as this should be kept in mind if any problems arise. Transient phenomena in the MB -3045 tended to be somewhat trying to measure since the observed effects could not be easily isolated. For example, the amp did not exhibit a well-defined slew -

38、rate limitation. Without applying an input signal of larger magnitude than that required to clip the amplifier at 1 kHz, increasing the test frequency to the point of slew -like distortion showed a slew rate of 6-10 volts/microsec. However, the 10 kHz square wave response shown in the upper trace of

39、 Fig. 6 shows the output signal ex- ceeding 25 volts/microsec! There should, therefore, be no ob- vious problem with musical program material. The lower curve in Fig. 6 shows the same 10 kHz square - wave test, but with the additional load of 2 microfarads in parallel with the 8 -ohm load resistor.

40、This ringing is similar to that observed under the same conditions with most transistor amps, but in this case is probably caused by some tuning with the leakage inductance of the output transformer. Clipping into resistive loads was graceful, like most tube amplifiers. However, the manner in which

41、this amplifier clipped into reactive loads was really outstanding. The waveform took the appearance of very soft peak compres- sion and limiting, and only with very large input overloads did any asymmetry occur. AUDIO November 1977 AmericanRadioHistory.Com Every cartridge sounds best with the right

42、Icad. BLt who wo -ries about that little detail? Vie do. ThEts why the AGI 511A has corvenient sockets for cartridge loading ca- pacitors. The sonic improvement with the right ones may sl-ock you. Elim nating RFI problems can make a big improve- ment, too. Its the fast RF you dont hear that can caus

43、e stewing induced distortion you will hear-the result of an amplifier responding slower than its input signal. Only the 511A has an ultra -high speed preamp-250 V/s-that solves this problem. RFI is then passively PHONO INPUT PLUG-IN 1. CARTRIDGE TCx LOADING I CAPACITOR HIGH PASS FILTER FEEDFORWARD.

44、CONVENTIORAL LOW DISTORTI TN, LOW NOISE, BUT SLOW OP -AMP INVERSE IRIAA NETWORK ULTRA -HIGH SPEED SUMMING OU-PUT AMPLIFIER NEGATIVE EEDBACK AGI BI -AMPLIFIED PHONO STAGE Write for the fui story-theres lots more AUDIO GENERAL Dept. A-11, 1631 Easton Roai, Willow Grove, Pa.19090 For the record, the da

45、mping factor was measured to be an even 30, which held up surprisingly well through the audio midrange. Gain at 1 kHz was found to be exactly 27.5X or 28.8 dB. This is slightly higher than most amplifiers in this power range and may be useful in some systems. Listening Tests A ten -year -old trend i

46、n speaker manufacturing, trading effi- ciency for other improvements such as size and price, has been matched by the large increases in amplifier power ratings. With only one exception, the wave of amplifiers over 100 watts per channel has been limited exclusively to tran- sistor designs by financia

47、l considerations and consumer ac- ceptance. It would be natural to expect that for this reason alone the MB -3045 would be rather disadvantaged in the modern audio environment. It is a pleasure to report that this is not so. Driving a pair of low -efficiency, 8 -ohm loudspeakers in a small- to mediu

48、m -size room actually gave the impression of power. Classical recordings with good dynamic range and rock records were regularly and cleanly reproduced at high volume. One transistor amplifier capable of WO watts/chan- nel into 8 ohms was incapable of achieving the same subjec- tive levels. But ther

49、e is obviously more to sound reproduction than just power. Articulation and depth of imaging from a pair of these Lux amps was very good. Very complicated symphonic passages could be torn apart instrument by instrument from the listeners seat with surprising ease. The pleasing but unrealistic bass often associated with tube power amplifiers has been replaced in these units with the extended and commanding low end that one has come to expect only