JBL Technical Note - Vol.1, No.16 电路原理图.pdf
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 elements. 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 be 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 Delivered 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, 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) provided 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 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 deliver 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 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. 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 maximum 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 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 somewhat 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 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, we 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 output 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, since 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 frequency 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 loads. 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 however, 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 one-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, 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 variation 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 corresponding 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 be aware that high-efficiency loud- speaker systems, such as those generally used in sound reinforcement work, present the greatest load variation to an amplifier, due to the relatively high back EMF these loudspeakers produce. The situation becomes even more critical when stereo amplifiers are bridged for mono application. In this mode of operation, the two amplifier sections are operating in series, and the output voltage is effectively doubled, making it pos- sible for the amplifier to deliver large currents. Specifications for bridged operation will clearly indicate the minimum load impedance and the maximum safe power which can be delivered to that impedance. It is important that such precautions be carefully followed. The Onset of Distortion in Amplifiers: In many professional applications it is difficult to ensure that an amplifier will never be driven into distortion. It is thus essential that we know how a given amplifier will recover from momentary overloads and what the audible effects of these may be. Certain kinds of speech and music signals may be subjected to clipping of the waveforms with little or no audible effect. The key here is the crest factor of the signal. 1. Crest factor: Crest factor is the ratio of the rms value of an audio waveform and its peak value. As shown in Figure 4 the crest factor of a continuous sine wave is 1.4, or 3 dB. The crest factor of a square wave is unity, or 0 dB. Many audio signals 2 r m s Zm i n Zmin rated for maximum allowable current draw from amplifier 2 *-rms Maximum power = Er m s Maximum current = * * min Z (ohms) Figure 4. Crest factors in audio signals. 20 log (1/.707) = 3db Peak value (1) rms value (.707) Peak it can be heated to the point where it literally burns up; or, it can be mechanically stressed to the point where it is perma- nently rendered inoperative. The two failure modes are generally independent of each other; however, under certain operating conditions, one mode may aggravate the other. 2. Determining the thermal limit of a transducer: Thermal power limits for a given transducer model are determined statistically. A sufficiently large sample of the transducer in question will be constructed and subjected to increasing input (bower, typically in 1 dB level increments and allowed to reach thermal equilibrium after each increase. The applied signal is shaped pink noise, and the spectrum is controlled so that the transducers are not unduly stressed mechanically during the entire process. 1. Thermal limits: Under continuous operation, a loudspeaker will heat up to some temperature and remain at that temperature in a state of thermal equilibrium. Heat is removed from the device at the same rate it is generated internally. Voice coil resistance is the major source of heating, and under normal operation the loudspeaker can withstand tem- perature rises up to the design limits of the materials and adhesives used in its construction. Temperatures in the range of 200 degrees C (400 degrees F) are not uncommon. When heated, the voice coil resistance rises, bringing with it a host of performance compromises. The resistance rise, however, acts to protect the transducer to a limited degree and does not materially affect the transducers power rating. Interested readers are referred to Technical Note Volume 1, Number 9, for a discussion of power compression in low- frequency transducers. Power input is carefully monitored by observing rms values of voltage and current, and the test is carried out until all of the samples experience failure due to the burnout. It is clear that not all of the transducers will burn out at exactly the same power input, and what the test usually shows is a normal distribution, or bell curve, as shown in Figure 7. In this example, most of the transducers fail in the range of 200 watts. Should this transducer be rated at 200 watts? Not necessarily. To ensure field reliability the manufacturer may want to rate the model at 175 watts since nearly all of the sample can meet this rating. Such a conservative rating may be good for the manufacturer in the long run, since the demanding user will generally be satisfied. However, in the advertising ratings game, the manufacturer may be at somewhat of a disadvan- tage when competitive transducers are compared on a unit cost basis. 4 10 dB Input Power (watts) 10-to-1 duty cycle (B) Continuous rating (A) Figure 7. Distribution curve for power failure. 5. Displacement power ratings: Sample size = 20 For constant applied voltage the displacement of a cone will double for each halving of the driving frequency. A limit occurs at the resonance of the driver, below which the displacement will remain constant for all frequencies. If an unbaffled low-frequency transducer is driven with a signal in the range of its primary resonance, its displace- ment can be quite large. If driven with a power input approaching its thermal rating, damage may be done due to mechanical stresses. The suspension may be deformed, and the voice coil made to rub against the pole piece or top plate. Once these conditions occur, the transducer is likely to fail abruptly. 3. Factors affecting thermal capacity of transducers: There are two key points here: materials must be capa- ble of withstanding high temperatures, and heat must be removed as efficiently as possible. While the search for high-temperature adhesives is ongoing, the effective removal of heat remains the major design challenge for transducer engineers everywhere. Henricksen (2) describes in detail a number of techni- ques for maximizing heat sinking away from voice coils. In general, large diameter voice coils dissipate heat better than smaller ones, since there is more wire in the voice coil, and a greater area of the coil is exposed to the top plate and pole piece of the magnetic structure. We often see fins molded into the outer portion of the magnet structure. This gives the impression that there is significant convection cooling effort under way, but such is rarely the case. 4. When do thermal ratings apply? The thermal power rating of a transducer applies any time it can be assured that voice coil motion will be restricted to its linear range. In low-frequency applications, this implies that the transducers are mounted in properly ported or horn-loaded enclosures and that their response is electrically rolled off below enclosure resonance or horn cutoff. For mid- and high-frequency cone transducers the nature of program virtually ensures that voice coil excur- sions will be small enough to be quite linear. For high- frequency compression drivers excursion limits must be carefully noted, and the high-pass slope should be at least 12 dB/octave. In cases of bi-amplification it will be essential to place a blocking capacitor in series with the driver in order to avoid the effects of very-low-frequency turn-on transients. These can easily destroy a compression driver diaphragm by causing it to undergo very large excursions. Since compression drivers are normally padded down relative to low-frequency system requirements, a resistor, normally equal to three times the nominal impedance of the driver, can be shunted across the driver, affording added protection by shunting the reactive components of the impe- dance peaks below horn cutoff. Thus, the transducer designer must impose certain power drive limits on low-frequency drivers based on the frequency of the input signal. But long before the transducer has reached the danger point in its excursion, it will have reached an undersirable level of distortion, and it is this which we want to identify as a displacement limit for the transducer. By general consent in the
