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Impedance and Amplifier Power Output Q. Will my 250-watt, 2-ohm amplifier drive a pair of 4- or 8-ohm speakers? And what would the amp’s power output be into a load of 4 or 8 ohms? —Daniel Cones, Bartlesville, Okla. A. The fact that an amplifier can work P safely into a 2-ohm load does not preclude it from successfully driving 4-, 8, or even 16-ohm speakers. The 2-ohm rating indicates the minimum speaker impedance that the amp can drive without damaging its output stage or triggering protection circuitry. The power available will be at its maximum when the amp is connected to 2- ohm loads and will be reduced as the load impedance increases. Because a number of design characteristics are involved, there is no way to provide you with exact wattage ratings at different load values. However, if your amplifier is a typical Class-AB design, you can estimate, albeit very roughly, that its power output will be halved with each doubling of load impedance. For truly ac curate ratings, consult your owner’s manual or the amp’s manufacturer. Defining Dielectric Q. I have heard the word “dielectric” used from time to time. Does it have some thing to do with capacitors? What is it? — Name withheld A. Yes, indeed, a dielectric is the insulating part of any capacitor. (Capacitors, devices for storing energy in an electric field, are used in DC circuits to store and re lease energy, such as a high-voltage pulse of current; in AC circuits, they can be used to block DC.) It is made up of two conductors separated by an insulator—the dielectric. When an electric current flows into the capacitor, a force is established between the two conductors separated by the dielectric. The dielectric material can be mica, paper, polyethylene, beeswax, or even air. The tuning capacitors in old analog radios are a good example. There is a series of stationary metal plates and a second set of plates that is free to move and interleave, or mesh, with the fixed plates. The two sets of plates, although very close to one another, do not touch. This is an air-variable capacitor. The air serves as the insulator, or dielectric, be tween the two conductors. The tuning capacitors in analog portable radios are similar except that mica replaces the air dielectric. The plates do touch the mica, but it doesn’t matter because the mica prevents the plates from shorting out. The amount of capacitance in a capacitor is determined by a variety of factors, including the size, shape, and number of plates, their spacing, and the characteristics of the dielectric between them. Air has a dielectric of 1; mica’s is much higher, between 4 and 9. If the dielectric increases, so does the capacitance (and vice versa). The use of dielectrics other than air can, and does, permit the construction of large capacitors requiring little space. Car Subwoofer Connection Q. Here in Australia, I bought a Sony car head unit (the XR-C750) because of its subwoofer output (two female RCAs, one red, the other white), but the installation guide doesn’t quite answer my questions. Do the two RCAs represent left and right subwoofer outputs? And if so, how do I sum them to mono? I want to connect them to a power amp, then to Alpine Bass Engines (transducers) under the front seats of my son’s car. Summing both channels to mono will give both seats the same sub-bass effect. —Senen A. Silvestre, via e-mail A. When I consulted Mark Weir, Sony I Electronics U.S. car-audio product manager, he told me that your head unit, the XR-C750, is a 1997 model (replaced this year by the XR-C8200) and that the red and white female RCAs do indeed represent the respective right and left subwoofer outputs. However, he pointed out that virtually all modern car stereo amplifiers (those built in the last decade or so) are self-bridging de signs, with separate left- and right-channel input jacks and internal mono-summing circuitry. By connecting the positive and negative speaker leads from your subwoofer (the Alpine Bass Engine) between the amplifier’s positive (-i-) terminal of the right- channel speaker output and the negative (—) terminal of the left channel, the subwoofer will automatically receive a summed mono output. (Some older car stereo amps may have a summing switch that will need to be set to the mono position.) In any case, do not use a Y-adaptor; connect the stereo subwoofer RCA connectors from your head unit to the left- and right- channel input jacks on the power amp. According to the May/June 1997 Car Stereo Review product directory, the rated impedance of the Alpine Bass Engine is 4 ohms. Because you plan to run two in parallel, the combined impedance presented to your amp will be 2 ohms or less. Check the maufacturer’s specifications for your power amp to ensure that it has low-impedance drive capability to 2 ohms and that it will remain stable doing so. Upside-Down Speakers Q. Some speakers I like from NHT use an upside-down driver array, with the tweeter mounted below the woofer. The location of my Boston Acoustics speakers puts the tweeters too high: The woofers are at ear level and the tweeters are well above that. Would inverting the speakers (to bring the tweeters to ear level) also invert the music presentation, placing the bass frequencies above the high frequencies? And what other effects might I expect? Could it damage the crossover or internal parts? —August Timmermans, via e-mail A. You won’t damage the crossover or anything else by turning the speakers upside down, nor will the music presentation be inverted. But the speaker’s overall dispersion characteristics will certainly change. If your speakers are elevated on a bookshelf or near the ceiling, inverting them, as you suggest, might well improve high frequencies at your listening position by bringing the axial response of the tweeters closer to ear level. Speaker designer Ken Kantor, of Vergence Technology and the cofounder of NHT, notes that (all other things being equal) you’ll get the most satisfactory frequency balance listening to most speakers on axis. When he designs a two-way speaker, he chooses the woofer first, then the tweeter, considers their respective directional characteristics and sensitivities, selects the crossover frequency and slope, and then calculates the constructive and destructive effects of the drivers’ combined responses with the crossover in the circuit. Kantor explains that sometimes mounting the tweeter below the woofer yields the smoothest combined response from the two drivers at the listening position; other times, the more common arrangement of woofer below tweeter produces the best response. “You gotta make ‘em work together,” according to Kantor, who then goes on to add that “you’re at the mercy of the crossover.” Why not try experimenting with your Boston Acoustics speakers? You can’t destroy anything, and you’ll learn something along the way. To Poke or Not To Poke Q. Without warning, my amp will clip (right channel first), then go into protection when reproducing fairly loud pas sages that contain sudden peaks (e.g., an explosion in a movie soundtrack). The same thing happens if I just crank it up. The amp has three different voltage rails and taps into the appropriate rail (depending on demand), drawing its power mainly from the transformer and two smallish capacitors in order to produce its maximum output of 375 watts per channel. I suspect the transformer first and the capacitor (right channel) second. Because the amp’s warranty has long expired, I thought I’d try fixing it myself I own a multimeter and am not afraid to poke around inside; I’m just not sure where to poke and what to test for. Where would you begin, and what would you look for? —John lost, via e-mail A. If your amplifier has served you well P and this problem has arisen only recently, I can rule out the transformer as a primary cause. Power transformers are rather stable devices; a winding can short out, or open, but usually that’s it. Before you start poking around inside the chassis, you should have a schematic and a service manual that describes the logic used for the voltage-rail selection. I would start by replacing the filter capacitor, but my gut feeling is that the problem lies in the logic circuits used for selecting the appropriate voltage rail. That said, there may be nothing wrong with your amp. It’s possible that the pro gram material you’re using may be too demanding for your amp to handle at the listening levels you prefer. Low frequencies place the greatest demand on an amp. Some of the explosions heard in soundtracks can have low-frequency components below 30 Hz. They may not sound very loud, but the power level required to reproduce them can be very high, especially if your speakers are acoustically inefficient. Ask a dealer or a friend if you can borrow a powered subwoofer that has internal low- and high-pass filters. Then you can direct the low frequencies to the sub, keeping them out of your main speakers. If your amp’s premature clipping ceases, you’ll have your answer. In any case, do not use a Y-adaptor; connect the stereo subwoofer RCA connectors from your head unit to the left- and right- channel input jacks on the power amp. According to the May/June 1997 Car Stereo Review product directory, the rated impedance of the Alpine Bass Engine is 4 ohms. Because you plan to run two in parallel, the combined impedance presented to your amp will be 2 ohms or less. Check the maufacturer’s specifications for your power amp to ensure that it has low-impedance drive capability to 2 ohms and that it will remain stable doing so. (Audio magazine, JOSEPH GIOVANELLI, Aug. 1998) = = = = |
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