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Conversion Confusion Q. If I understood it right, an article I read said that the A/D converter in a CD player converts the info on the CD to digital. I thought that the info on the CD was already digital? A. The information recorded on a CD player is, indeed, digital. An A/D converter turns analog signals into digital before they’re recorded on a CD, so a CD player doesn’t have an A/D converter. It does have a D/A converter, to turn the digital signals back to analog during playback. In all likelihood, the writer wrote the opposite of what he meant (every writer does, once in a while), and the editor failed to catch the error (every editor does, once in a while). Audio Mode Selector on VHS Hi-Fi VCRs Q. Some VHS Hi-Fi VCRs have an audio mode selector. This switch usually enables one to select the Hi-Fi stereo channels or the linear track (which is usually mono). If I set this switch to mono and record a stereo program, will it be recorded stereophonically on the Hi-Fi track? Will it be in mono on the Hi-Fi track? Or will that track simply be blank? A. The audio mode selector affects only the VCR’s output; it has no effect on the input and recording process. Therefore, a stereo program will be recorded in stereo on the Hi-Fi tracks regardless of this switch’s setting. However, if the switch is set to mono, you will hear only the linear mono track until you set the switch back to stereo again. Those few VCRs that were de signed to record stereophonic linear tracks may be an exception to this. Another Way to Avoid CD Damage In the February 1996 “Audioclinic,” David A. Taylor stated that he sometimes cannot play CDs that had once played perfectly. You suggested that his problem might be dirt on the disc or on his CD player’s optics. I suspect that the problem could be the technique he uses when removing a CD from its case. What I do is press down on the hub of the case with my index finger while removing the CD with my other fingers and a thumb. This reduces some of the case’s grip on the CD, which lessens the bending stress applied to the disc during removal. Theoretically, the disc’s aluminum layer is less likely to be damaged this way. I have no data to back up this theory, but I have used the technique successfully for many years. -- -- Replacing the Amp in a Powered Subwoofer Q. Is there a way to replace the amplifier in a powered subwoofer with a more powerful amp? A. Before replacing a powered sub I woofer’s amp with something more powerful, check with the subwoofer’s manufacturer to see how much power the driver can handle; it may be designed to handle only the power its original amp provides. Further, the original amp may have incorporated the crossover or some of the subwoofer’s controls, in which case you’ll need to provide substitutes. Even if you can get around those potential problems, you may not be able to find a more powerful amp that fits the space provided. If you don’t insist on your new amp’s being built into the cabinet, you can use an external amp. Just remember to remove the original amp and connect the wires that formerly came from the old amp’s output to the output terminals on the new amp. -- -- Mysterious Music Q. My system includes a CD transport and an outboard D/A converter. I don’t know how I stumbled on this, but I found that, with the D/A off but the phono volume turned way up, I can hear music very faintly being played in the transport. I tried changing interconnects, but the situation did not change. Could this music be coming in via the power line? I didn’t try filtering the line. A. Is the music you hear on a CD that’s currently playing? And does your CD player have analog as well as digital outputs? If it does, and if those outputs are connected to an input on your preamp (or the preamp section of your amp or receiver), your mystery signal might be getting into your system via leakage in the source selector. (This is more likely to happen with a CD player than with other sources, be cause CD players have higher output voltages.) If this is the case, disconnecting the cable carrying this signal should cure the problem. Or are you hearing music coming from the transport itself? If so, and if it’s from a CD that’s playing, then something in the transport is causing it. If it’s unrelated to the current CD, your transport may be picking up and demodulating a radio transmission (more likely AM than FM); grounding the transport might cure this. Radio-frequency energy from a local AM or FM station could also be getting into the sensitive phono circuitry of your system, and it could indeed be entering the system via the power line. If your mystery music is audible only when there’s no other signal and you have the volume turned up fully, there is no need to go to extremes in order to eliminate it. I wouldn’t worry about interference you can’t hear under real listening conditions, though I would be concerned about interference you can hear under realistic but extreme conditions. -- -- -- Power-Line Filters Q. My system sounds poor, and I’ve been told that’s because of RF signals in my power line. I am considering buying a power-line filter/conditioner to clean this up. On the other hand, a salesman told me that the odds are only 50-50 that using the filter will solve this problem. A. Whereas some sound systems are virtually impervious to RF and other electromagnetic interference that may be in the power line, other systems are very susceptible to it. It HI’ is getting into your sound system via the power line, it is certainly possible that the filter/conditioner will remove it. But a lot of the interference that finds its way into a sound system comes via other routes. If this is the case, the filter won’t do anything for you. Since it’s hard to know whether the filter will help you, see if a dealer will let you try one overnight. By the way, there are many other possible reasons for your system’s unsatisfactory sound. -- -- -- Speaker-Impedance Limits Q. If my receiver calls for loudspeakers whose impedance is no lower than 8 ohms, is there any way I can use a 4- or 6-ohm speaker? What will happen if I try this? A. If the owner’s manual for a power amplifier or a receiver says that 8 ohms is the lowest impedance that should be connected to it, the safe bet is to follow those instructions. Many that carry such a warning do so primarily to make UL happy and will work fine with lower-impedance speakers. Unfortunately, there’s usually no way to find out other than by trying it. Here’s the scoop. The lower the impedance of the speaker load, the more current will be drawn from your receiver’s output transistors. Raising current flow heats up the output stage. Too much heat will make these transistors fail, usually causing the failure of other components. This will require a costly repair. If parts are no longer available for your receiver, you may even have to replace it. These problems usually occur only if you run your receiver close to its maximum power output. You can’t tell exactly when you’re doing that, but a high volume-control setting or distorted sound should serve as a warning sign. So unless you plan to use your receiver only for background listening, you’ll probably want to make sure that any speakers you use that have low impedance also have high sensitivity, so you can get ad equate volume without overdriving your receiver. If you have your heart set on a loud speaker whose impedance is, say, 4 ohms, you should seriously consider replacing your amplifier or receiver with one that is specified to work with such loudspeakers. If your budget won’t permit that, keep things as they are and enjoy the sound produced by your present system. -- -- -- Setting Tone Controls Q. Is there any general rule determining which bass- and treble-control settings are best? If not, how can I find out which are the right settings? A. There is no one perfect setting for tone controls; you must experiment. But it helps to know what they are for. Bass and treble controls have several purposes. They help you deal, crudely, with frequency imbalances in your audio system or your room’s acoustics. They’re more helpful in compensating for frequency imbalances in individual recordings, and they can help you adjust the tone of even well-balanced recordings until they sound more to your liking. If your system lacks a loudness-compensation control, adding a bit of bass when you turn the volume down will make the softened music sound more natural and not so thin. Try listening to a variety of recordings with your tone controls switched out or turned to their “flat” positions (usually, pointing straight up), until you’re familiar with how your system sounds without them. Then turn the bass and treble controls, one at a time, all the way up and all the way down, to hear what they can do. After that, experiment with subtle increases in bass and treble. (If you have to use your tone controls’ extreme positions to get good sound, there’s usually something wrong.) The more expensive the equipment, the less likely it is to have tone controls. That’s because many audiophiles feel that these controls introduce unacceptable distortion levels and phase shifts. I do use them, how ever, because I think the good they can do when I need them far outweighs any potential ill effects. -- -- -- Matching Main and Center Speaker Sensitivity Q. What happens if the sensitivity of my main front loudspeakers is 5 dB greater than the sensitivity of my center speaker?—Name withheld A. It isn’t too important that the sensitivities of the main and center speakers in a home theater system be matched. Just as a balance control takes care of imbalance between the left and right channels, your system’s center-channel level control should take care of the difference—assuming, that is, that your system’s center level control has enough range to eliminate this imbalance. However, if these speakers are mismatched to such a degree in sensitivity, they’re probably mismatched in other ways and will sound different: Voices and other sounds, for example, may change tone unrealistically as they move between the center and flanking speakers. -- -- -- Headphone Impedance and Tape Deck Meters Q. My cassette deck has no headphone jack, so I’ve connected my headphones (which have their own volume control) to its line output jacks. When I connect the ‘phones to the deck, the reading on the deck’s meter is a lot lower than when my deck is feeding my receiver. And the higher I turn the head phones’ volume control, the lower the reading becomes. Can I damage the ‘phones or the deck by connecting things this way? A. Because your meter is connected, directly or indirectly, to your deck’s line output, anything that drops the voltage at the output jacks will lower the meter reading. Adding a high-impedance load, such as the input of a receiver or amplifier, does not drop this voltage appreciably. But your headphones have a much lower impedance (probably on the order of 8 to 35 ohms, as opposed to several thousand ohms), so they do drop this voltage. The impedance of your headphones varies with the setting of their volume control; raising the volume lowers the impedance. No damage is likely to occur to the head phones or to the deck, though it can occur if your deck has a direct-coupled output circuit. When such circuits are loaded too heavily, they can overheat. This can, if continued, damage the equipment. With such a large impedance mismatch between your ‘phones and your recorder’s output jacks, you’re probably not getting enough level when you listen to the deck through headphones. A far better arrangement is to connect the headphones to your receiver’s headphone jack, if it has one. You could also use audio transformers to match your headphones’ impedance to that of your deck’s output or try to find head phones that have higher impedance. (Audio magazine, JOSEPH GIOVANELLI, Jul. 1996) = = = = |
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