Qsc - CX Application Guide 电路图.pdf

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1、29 Application Guide C O N T R A C T O RA M P L I F I E R S RadioFans.CN 收音机爱 好者资料库 1 Table of Contents Distributed line principles .3 Sidebar: Ohms Law .4 Why 70 volts?. 4 Sidebar: Transforming voltages and impedances .5 “Natural” voltages.5 Designing the distributed sound system.6 Loudspeaker cove

2、rage and placement . 6 Sidebar: Reverberation and RT60.7 Sidebar: The Inverse Square Law .8 Determining power levels .9 Calculating total power requirements .11 Fitting amplifier power.12 Using components with different line voltages .13 Sample applications the voltage on the distributed line is ind

3、eed an audio signal and will modulate as the audio itself does. If you connect a voltmeter across a 70-volt line, you will seldom actually measure 70 volts except on audio peaks. If the audio is muted, you will measure zero volts. Distributed-line amplifiers are designed to produce maximum power at

4、the line voltage. For example, a 70-volt amplifier will produce its maximum power at 70 volts, regardless of whether its a 50-watt, 150-watt, or 700- watt model. What will differ from one power rating to another is the amount of current the amp can put out, as you can determine by using Ohms Law. (I

5、f you need to brush up on Ohms Law, see the sidebar on the next page.) Thus, a 70-watt amp is designed to put out as much as 1 ampere at 70 volts, while a 350-watt amp will be able to produce up to 5 amperes at that voltage. Com- pare that to regular low-impedance amplifiers, whose power ratings are

6、 directly related to the maximum voltages the amps can put into 8, 4, or 2 ohms, so that a higher-powered amp has a higher output voltage for a given load than a lower-powered amp does. For instance, an am- plifier rated at 100 watts into 8 ohms can put out Example of a loudspeaker connected to a di

7、stributed line through a transformer Example of a 70 volt distributed line 4 as much as 28.3 volts, as determined by Ohms Law, while an amp that does 200 watts into 8 ohms can put out 40.0 volts. This is where the true concept of “constant voltage” comes in; it helps simplify system design by conver

8、ting one of the variables into a constant value. But you cant just connect typical 8-ohm speakers across a 70- volt line because theyll want to draw about 625 watts each. How then do you plan and control the amount of power to each speaker when you have a defined maximum line voltage? The answer: th

9、rough transform- ers. Each speaker has a transformer that converts the line voltage to another value (almost always lower) to actually drive the speaker. Taps on the transformer allow you to select the power level the speaker receives when the line voltage reaches its maximum of 70 volts. It is some

10、what analogous to AC electrical service, in that you can plug a 100- watt appliance and a 50-watt one into outlets carrying the same 120 VAC; you dont have 120 volts for one and 85 volts for the other. Regular low-impedance amplifiers are perfect for systems with one, two, three, or four speakers pe

11、r amp channel, with each speaker getting the same amount of power. But if you need to power more speakers, or provide different power levels to some or all of them, you would often have to do some complicated series-parallel calculations and wiring. And even then if a speaker fails, is removed, or m

12、ust be added, it would alter the power distribution among the rest. A distributed line elimi- nates the need for such calculations and considerations. It lets you forget about impedances. And it also lets you substitute amplifier models as needed without having to re-calculate power distribution amo

13、ng the loudspeakers. For example, if expanding a distributed speaker system or increasing some power taps requires you to upgrade a 150-watt 70-volt amplifier to a 200-watt model, you can do so without re-calculating or reconfiguring all the other speaker taps, although you would have to match the g

14、ain of the new amp to that of the old one. Why 70 volts? If 70 volts seems like an odd number to become a de facto standard for distributed line voltage, how about 70.7 volts? Thats the actual figure used in design of distributed lines, although it suggests a lot more precision that you should hope

15、to measure on an audio voltage. The number 70.7 came about for two reasons. First, as weve seen already in this book, many loading and impedance calculations involve squaring the voltage. The approximate square of 70.7 is 5000, which was easy to remember and work with in the days before pocket calcu

16、lators. The second reason is that versions of the National Electrical Code (NEC) before 1999 classify signal circuitry of 100 volts or higher as Class 1, requiring a higher grade of wiring. Settling on 70.7 volts allowed a distributed line circuit to be deemed a Class 2 circuit, with a safety margin

17、 of exactly 3 dB to allow for loading variations, audio peaks, etc. Distributed line voltages other than 70 volts are common in some areas and applications. In Europe, 100-volt lines are prevalent instead of 70 volts. And in the United States, 25-volt lines are common in public school buildings. In

18、applications where distributed lines have to run very long distances, 140- and 200-volt lines carry the audio power at a high ratio of voltage to current (a high-impedance line, in other words) to minimize losses due to wire resistance. voltagecurrent resistancepower Ohms Law Nearly two centuries ag

19、o a German scientist named Georg Ohm discovered that the current through a load is directly propor- tional to the voltage across it, and also inversely proportional to the resistance of the load. This relationship is called Ohms Law, and the scientific community honored ohm by naming the unit of res

20、istance after him. In its basic form, Ohms Law is expressed as the equation E = I R where E is voltage (in volts), I is current (in amperes), and R is resistance (in ohms). You can also use Ohms Law to calculate the power in the load, which is equal to voltage times current. The properties of power,

21、 voltage, current, and resistance are all interrelated, so if you know the value of any two of them, you can calculate the other two. This Ohms Law “wheel” shows how to solve for any of the four properties. 5 1 A 1 A 1 A 1 A Amp: 800 watts 0.08 ? 8W 8W 8W 8W 8 8v 100 A Spkr 1 Spkr 2 Spkr 99 Spkr 100

22、 Total load impedance0.08 “Natural” voltages Some power amps designed for powering direct low-impedance speaker loads have power ratings that make them suitable for driving distributed lines, too. An 8-ohm load draws 625 watts at 70.7 volts, so an amp rated for 600 to 650 watts into 8 ohms is often

23、termed a “natural” for driving a 70-volt line. This relationship works for other line voltages, too, although they are rare for 100 volts and higher: 25V7580 watts 8150160 watts 4300320 watts 2 70V600650 watts 812001300 watts 424002600 watts 2 100V12001300 watts 824002600 watts 448005200 watts 2 140

24、V24002600 watts 848005200 watts 4 200V48005200 watts 8960010400 watts 4 Transforming voltages and impedances Imagine driving a system of 100 8-ohm speakers at a low power (say, 8 watts each) with a single amplifier, like you might need to do in an office buildings paging system. How would you do it?

25、 Connect them all in parallel, perhaps with 00 AWG cable to wire them all together, and find a power amp that can do 800 watts into 0.08 ohms, which comes out to 8 volts and 100 amperes? Thats not practical, not the least because no such amplifier exists! Or would you use a 70-volt amp with a rating

26、 of 800 watts or better, and put a transformer on each speaker to provide the desired power level? Then the amp has to put out 70 volts at 11.4 amperes, for an equivalent load of 6.13 ohms. Thats much more reasonable. A speaker transformer steps the line voltage down to a lower level to drive the sp

27、eaker. In doing so, it also steps up the speaker impedance, so that the line itself sees the speaker/ transformer combina- tion as a relatively high impedance. For example, the transformers in this example have an 8.75:1 voltage step-down, converting 70 volts from the distributed line to 8 volts for

28、 the speaker. Into an 8-ohm speaker, that will produce 8 watts. The ratio of the impedance step-up is equal to the square of the voltage ratio in the other direction. Therefore, the 8-ohm impedance of the speaker driver will be multiplied by a factor of 76.56, resulting in the line seeing a theoreti

29、cal impedance of 613 ohms. (The actual figure will be somewhat less because of the transformers insertion loss.) The importance of this phenomenon is that the high impedances allow you to connect many speakers25, 50, 100, etc.in parallel on the line, which you would not be able to do with speakers a

30、lone in a practical way. 1 A 800-watt 70V amp 8W 8W 8W 8W 8 70v 8v 613 70v 11.4 A Spkr 1 Spkr 2 Spkr 99 Spkr 100 1 A 8W 8 70v 8v 613 Sp Total load impedance6.13 6 Designing the distributed sound system There are several main steps in designing a distributed sound system: Determining loudspeaker cove

31、rage and placement Determining power levels for each loudspeaker Choosing the right amplifier Loudspeaker coverage and placement In placing loudspeakers in a distributed system, the goal is to provide coverage effectively but economically. An effective coverage would be one where the sound from the

32、loudspeakers is not only audible, but also intelligible, wherever needed. An economical coverage would tend to be one that achieves the goal using the fewest loudspeakers necessary. A loudspeaker in an enclosed area produces two sound fields. The main one is the direct fieldsound coming directly “li

33、ne-of-sight” from the loudspeaker. Primary and secondary reflections can also be considered part of the direct sound field, as long as their delays are short enough to psychoacoustically reinforce the original sound. The other is the diffuse field (sometimes called the reverberant field), which is s

34、ound that you might call “post direct.” This diffuse field of reverberation is sound that has bounced around the room, reflecting off surfaces such as floors, walls, tables, ceilings, etc., until it is absorbed by the air, other objects, and the room itself. The diffuse field is comprised of multipl

35、e sound wave fronts traveling in different directions, each taking a slightly different length of time to arrive at the listener (or microphone). As a result, a common characteristic of the diffuse field is “image smearing,” which reduces the intelligibility of the sound. Thus, to keep intelligibili

36、ty high, you should maximize the ratio of direct field to diffuse field. As the Inverse Square Law dictates (see the sidebar on page 8), the direct field sound falls off as the distance from the loudspeaker increases. The diffuse field is also sub- ject to the Inverse Square Law, but moving away fro

37、m one reflective surface often moves you towards another; as a result, the intensity of the diffuse field usually doesnt vary significantly throughout a room. The graph at right shows the direct, diffuse and combined sound fields (direct and diffuse, summed) of a single loudspeaker in a large, fairl

38、y reverberant room. Closer to the speaker, the direct field is much stronger than the diffuse field; intelligibility here will be very good to excellent, but it will drop off as you move further away. At the critical distance, DC, the direct and diffuse fields are equal in intensity, and beyond DC t

39、he diffuse field overpowers the direct. At this position a person speaking clearly though the sound system might be heard, but not clearly enough -35.0 -30.0 -25.0 -20.0 -15.0 -10.0 -5.0 0.0 110 Critical distance,DC Diffuse (reverberant) sound fieldDiffuse (reverberant) sound field Direct + diffuse

40、fieldsDirect + diffuse fields Direct sound fieldDirect sound field Distance from loudspeaker, in meters Relative SPL (referenced to 1 meter from loudspeaker) 203040 Intensity of direct and diffuse sound fields in an enclosed space 7 Center cluster Under-balcony fill to be understood: this is the “wh

41、at did he/she say?” syndrome. In this example, DC is approxi- mately 15 meters. Not all reverberation is detrimental. A controlled amount, either natural or added electronically, can enhance the aesthetics of speech and, to a greater extent, music. However, adding elec- tronic reverb on a sound syst

42、em designed for mostly utilitarian purposese.g., paging or re- corded background musicis very rare. The less reverberant the room, the less intense the diffuse sound field will be, and DC will be greater. Conversely, increased reverberance of a room will shorten DC and sharply reduce the intelligibl

43、e coverage area of the sound system. Increasing the power to the loudspeaker is not a remedy, because the increased direct field in turn excites the diffuse field. The result is the relation- ship between direct and diffuse fields will stay about the same. And to make matters worse, the likelihood o

44、f feedback through an open micro- phone increases. Solutions to maximizing sound system intelligibility in a difficult room include: controlling the reverberant nature of the room through acoustical treatment of reflective surfaces, and architectural means. controlling coverage by using loudspeakers with directional qualities (Q) that will help keep sound on the audience and off the walls and other surfaces. using many low-powered loudspeakers close to the audience instead of one (or a few) centrally located high-powered loudspeakers. This