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by JOSEPH GIOVANELLI Foldback Q. My new microphone mixer has one control I don't understand: "Foldback." What is it used for? -Steve Moroukian, Edina, Minn. A. This control is used in live-performance sound reinforcement to regulate the volume of the on-stage "foldback" speakers through which musicians monitor their own performances and the sound mix reaching the audience. (The term is more common in Britain; Americans tend to say "monitor.") Some of the signal, in other words, is folded back to its starting point, the stage. This is necessary be cause the main speakers, which face the audience, are usually located so far from the stage that the performers cannot hear them clearly. The signal controlled by the pot you mention usually goes to a jack or jacks marked "foldback," whose output would feed external power amplifiers. Connecting an Auto Power Amp Q. I would like to connect my car-stereo system to a booster amplifier, to obtain extra output power. The system, as it stands, has a power output of approximately 10 watts per channel. None of the speaker leads are at ground potential. The unit does not have preamplifier outputs. What is the correct way to connect a power amplifier to the speaker leads of my present equipment? -Barry E. Lyons, Sandy Hook, Ky. A. I presume that you mean none of the speaker leads are at the same potential as the car's chassis ground. This is the case with most car-stereo units today, and boosting such units is a fairly common practice. Two kinds of add-on amplifier are available. Simple boosters (sometimes combined with equalizers) have inputs only for speaker-level signals from low-powered amplifiers like yours. Many power amplifiers accept both preamp level and speaker-level inputs, by means of either separate input connections for each signal type or an input-sensitivity switch or control. If your system delivers a true 10 watts per channel, as measured at some reasonably low level of distortion (1% or less), then simple boosters may not accomplish much for you since they don't put out significantly more power than that. Power amplifiers, however, are available with outputs of several hundred watts, if that's what you want. More likely, though, your system puts out less power than you think. If its specifications actually say 10 watts per channel, odds are that this figure is reached only at a distortion level of 10%. (If a car amplifier's power specifications don't include the distortion level, you can usually properly assume that it is 10%). The amplifiers built into the vast majority of car-stereo units typically deliver far less than 10 watts per channel at low distortion. Your unit's lack of preamp outputs suggests that it may be either an older unit or a low-priced one, making such high power even less likely. A word of caution: Amplifiers with floating grounds (which, judging from your letter, your unit might have) can be damaged by connection to amplifier inputs which are grounded. Check with the manufacturers of your stereo unit and your prospective booster or power amplifier to make sure this will not be the case in your system. If you do have this problem, however, floating-ground adaptors are available to deal with it. Check with a competent car-stereo installer. Damage from Lightning Q. We had a severe thunderstorm a few nights ago. The next day, upon turning on my system, the main power indicator light on my turntable came on as usual, but the platter would not turn and the strobe speed indicator did not operate. Is it possible that a lightning bolt could have done damage? If so, what can I do to avoid such damage again (besides unplugging everything)? -Teddy Herbst, Swiftwater, Pa. A. Lightning could well have dam aged your turntable, even if all switches were turned off (your letter doesn't specify). A direct hit or a "near miss' can result in sufficient voltage arcing across the open switch contacts to get into your equipment and mess it up. If your neighborhood has overhead a.c. wiring, your equipment is very susceptible to such damage. Some protection can be provided by using surge protectors. Still, a close call will likely wreck the surge protector. Therefore, though you may not like to do it, if an electrical storm is likely you should pull the plugs on all delicate equipment. If you use an a.c. junction box or power strip to supply power, to your sound system, you only need to unplug one item, the junction box. If a timer is included, be sure that the timer you use has battery backup so that all its programs will be held during the storm. Loudspeaker Controls Q. I have a question concerning midrange/tweeter controls found in loudspeaker systems. Why do they al ways have a boost/cut range, with a "flat" setting at some midpoint on their L pads? As far as I know, such a passive network cannot possibly boost any signal that it receives. I must assume that the "flat" points on any speaker's L-pad are simply the points where the high-frequency output is attenuated to match the woofer's output. Therefore, when the L pad is turned up to boost the high-frequency driver's output, it is, in fact, not boosting at all, but simply allowing the high-frequency driver to be driven by the full wattage being directed to it, without dissipating any of that wattage in the pad. Am I right about this? -Stan Wirth, Cleveland, Ohio A. You are correct about the way L pads operate. Midrange and tweeter units are designed to be more efficient than woofers so proper acoustical balance can be obtained. Of course, we could design a loudspeaker whose woofer was more efficient than the other elements, but the power lost in the pads would be excessive. While passive circuits cannot amplify signals, they can nevertheless boost a given frequency range over other portions of the spectrum. Where this is done, there are capacitors associated with the resistors. Perhaps the aim is to compensate for a tweeter which is not "flat," perhaps it is to accommodate personal preference. In any event, an active network will, if desired, amplify any boosts or cuts, but these are produced by passive devices (resistors, inductors and capacitors). (Audio magazine, Feb. 1986, JOSEPH GIOVANELLI) = = = = |
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