JBL Technical Note - Vol.1, No.16 电路原理图.pdf
《JBL Technical Note - Vol.1, No.16 电路原理图.pdf》由会员分享,可在线阅读,更多相关《JBL Technical Note - Vol.1, No.16 电路原理图.pdf(11页珍藏版)》请在收音机爱好者资料库上搜索。
1、Technical Notes Volume 1, Number 16 Power Ratings of JBL Loudspeakers and JBL/UREl Amplifiers Introduction and Scope of this Technical Note: JBL and UREI manufacture both loudspeakers and amplifiers, and it is the companys obligation to give the user guidelines for proper matching of these system el
2、ements. The task is not as easy as it would seem, because there are many modes of both loudspeaker and amplifier operation, depending on the specific application. In this Technical Note we will examine both amplifier and loudspeaker rating methods in detail. Both tutorial and real world cases will b
3、e examined, and the reader should, after digesting this Technical Note, be able to choose the correct loudspeaker and amplifier power class for a given job. First, let us look at traditional amplifier and loudspeaker ratings. Amplifier Power Specifications: 1. Basic Rating Methods: A. Power Delivere
4、d to Nominal Load Impedances: In the days of tube amplifiers, the tradition developed of rating amplifiers by measuring the continuous average sine wave power the amplifier could deliver into a nominal load imped- ance, consistent with some maximum allowable level of dis- tortion. As a general rule,
5、 loudspeaker impedances were assumed to be integral multiples or sub-multiples of 8 ohms. Life was made relatively simple through the use of output transformers, and this meant that the amplifiers power out- put capability was constant over the entire range of imped- ances (4, 8, and 16 ohms) provid
6、ed the correct taps were used on the output transformer. Loudspeakers rarely present the ideal 4, 8, and 16-ohm loads which their specifications imply, but these errors were usually overlooked, inasmuch as the amplifiers were rela- tively tolerant of load variations. Power P P is transformed so that
7、 it can be delivered in full to any of the rated impedances (16, 8, or 4ft) B. Maximum Voltage Swing into Minimum Load Imped- ance: With the advent of transistorized amplifiers, alternate rating methods have arisen. These amplifiers are normally operated without output transformers, so they cannot d
8、eliver full power to a wide range of loads. Such amplifiers usually have a maximum output voltage swing which they can deliver, and this value is related to the positive and negative voltages delivered to the output transistors by the power supply. This maximum output voltage swing can be delivered
9、to a load as long as the maximum current rating of the output devices is not exceeded. Even momentary excess output current can cause instant failure of the output devices, and it is customary in transistor power amplifiers to incorporate some kind of internal current limiting as a failsafe feature.
10、 Thus, we can rate a transistorized amplifiers output capability by stating its maximum output voltage swing (in volts rms) and the minimum impedance across which this voltage can safely be applied. For a resistive load, the minimum resistance is given by: R = (E rms)/0 rms) where Er m s is the maxi
11、mum voltage swing the amplifier can deliver, and lr m s is the limiting output current capability. It was an orderly world; the user would simply connect an 8-ohm loudspeaker rated at 50 watts to the 8-ohm out- put tap of a 50-watt amplifier, and that was all there was to it. The loudspeakers rating
12、 of 50 watts implied that it could, over its operating bandwidth, safely handle the output power of a 50-watt amplifier, and few, if any, problems ensued. Figure 1 shows the rating method in detail. Taking into account complex loads such as loud- speakers, the amplifiers limiting current may be some
13、what different than in the case of a purely resistive load. In the interest of completeness, this method of output rating would include both minimum resistive and minimum complex loads for the amplifier. Figure 2 shows the basis of this output rating method. 1 Figure 1 Maximum power into any design
14、impedance. 16-a Tap 8-0 Tap 4-0 Tap Figure 2. Maximum voltage swing across mininum load impedance. JBL and UREI have retained the traditional rating method for amplifiers, inasmuch as there is a wide universe of transducer hardware whose ratings are consistent with the older method. As an example, w
15、e will give the power output ratings of the JBL/UREI model 6290 stereo amplifier: Rated Power (20 Hz-20 kHz) 8-ohm 300 watts (per channel) 4-ohm 600 watts (per channel) 16-ohm bridged 600 watts 8-ohm bridged 1200 watts What these ratings tell us immediately is that the same voltage swing at the outp
16、ut can be accommodated with either 8 or 4 ohms. The 4-ohm load will draw twice the cur- rent, and therefore produce twice the output power. What about operating the amplifier into a 2-ohm load? Some professional amplifiers carry such a rating - or, rather, a derating. In our opinion this is risky, s
17、ince the load will fall below 2 ohms at some frequencies. We would rather see designers maintain nominal loads no less than 4 ohms. Similarly, in the bridged mode the nominal load must be no less than 8 ohms. 2. Real versus theoretical loads: An 8-ohm resistor pro- vides a constant load over the fre
18、quency range, while a loudspeaker does not. There are no simple loudspeaker loads; they are all reactive to some extent, and a typical example is shown in Figure 3. This curve shows the steady- state magnitude of impedance over the frequency range of interest. Figure 3. Resitive and loudspeaker load
19、s. A - 8-ft resistor B - 8-0 loudspeaker Frequency (Hz) When we label Curve B as 8 ohms, we are implying that the average load is somewhere around 8 ohms. At reso- nance peaks the value is considerably higher, while at some points in the mid-band it is a little lower. The general impli- cation howev
20、er, is that on music or speech program the average load seen by the amplifier will be about 8 ohms. But the real situation can be more complex. Recent studies (1) have shown that complex loudspeaker loads can, under specific transient drive conditions, produce dynamic loads which can be as low as on
21、e-half the steady-state min- imum value! Thus, a nominal 8-ohm low-frequency loud- speaker with an impedance minimum of, say, 6.2 ohms, may, under the right drive conditions, present a momentary dynamic resistive load to the amplifier of 3.1 ohms. Such factors as these are rarely considered per se,
22、and they are often responsible for triggering current limiting in amplifiers in systems where conventional design wisdom has indicated that no problems exist. Since it is virtually impossible to specify an amplifiers input signal, the only way to guard against the adverse effects of dynamic load var
23、iation is to design into the system the capability of coping with one-half the steady-state minimum impedance. Thus, if an 8-ohm load presents a steady-state mini- mum value of 6.2 ohms, it should be powered by an ampli- fier capable of handling a load as low as 3.1 ohms, with current capability cor
24、responding to the maximum rated out- put voltage of the amplifier. This is an extreme requirement, and not many systems have been designed to satisfy it. Many amplifiers are audibly unstable under such operating conditions, while others may handle such transient signals in stride. The reader should
- 配套讲稿:
如PPT文件的首页显示word图标,表示该PPT已包含配套word讲稿。双击word图标可打开word文档。
- 特殊限制:
部分文档作品中含有的国旗、国徽等图片,仅作为作品整体效果示例展示,禁止商用。设计者仅对作品中独创性部分享有著作权。
- 关 键 词:
- JBL Technical Note Vol.1 No.16 电路原理图 Vol No 16 电路 原理图