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(Audio magazine, July 1996)...
The April issue’s “Jazz & Blues” section included a review of Terry Evans’s CD, Puttin’ It Down, released by AudioQuest. Why wasn’t there any mention that an LP version is also available?
Today’s tenuous vinyl “renaissance” includes state-of-the-art reissues of classical recordings and rock music as well as new rock/pop releases (from Bruce Springsteen, Green Day, et al.). These LPs are as much an improvement in quality and sound reproduction from previous LPs as present CDs are compared to the early atrocious-sounding ones. Mass-readership audio magazines, such as Stereo Review and Audio, typically have not reviewed new vinyl albums.
Vinyl album sales were prematurely killed off in the United States, starting in the mid-’80s, as part of the marketing strategy for CD—something that did not hap pen in Europe. is the recent vinyl renaissance likewise to be killed off, this time by mainstream audio magazines deliberately ignoring an alternative format?
Already, Reference Recordings and AudioQuest have stopped general release of their recordings on vinyl, the former citing a total absence of reviews of its classical and jazz vinyl releases by audio magazines. AUDIO is doing readers a disservice—those readers, that is, who have not been duped into believing that CDs provide the only legitimate listening experience.
In the case of Puttin’ It Down, we didn’t know an LP version was available. Arid most labels send us only the CD version of a release for review, even if a vinyl edition is also available. Reviews should be relevant to any release format with respect to musical content, and though CD and vinyl releases would tend to sound somewhat different, they should be close enough in most cases for many of the comments on sonic character to apply substantially to the LP as well as the CD.
I don’t think the theory that the LP was purposely killed off holds much water, by the way. Sales of LPs had already started to decline before the Compact Disc was introduced. Nonetheless, record labels didn’t show much enthusiasm for CD in the beginning, and it was some time before the new discs stopped being a curiosity in record shops. Stocking policies of record stores pretty much followed the demand curves, which meant that eventually most didn’t want to bother with LPs anymore. They may have overshot the market a bit when they dumped vinyl altogether, but I see no reason to think any grand marketing conspiracy was involved.
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Rolling Your Own Can Be a Drag
The remarks about designing one’s own loudspeaker (“Fast Fore-Word,” May) reminded me of an experience I had that should serve as fair warning to anyone so inclined.
It was 1966, and I hadn’t yet quit my day time engineering job to go into the audio business full-time. Discos were just opening up, and I wanted to bring high-fidelity techniques into the commercial sound field. For some reason, no one was doing it. At around the same time, I was beginning to write equipment reviews, mostly of loud speakers and phono cartridges, for Audio. So I thought I knew something about speakers.
One day I came across a pair of terrific-sounding electrostatic tweeters and began playing around with them, together with some cone midranges and very large cone woofers. Eventually I came up with a tri-amplified loudspeaker system that sounded pretty good to me. All of my audiophile friends who auditioned the thing loved it. Then I invited two acquaintances, who were expert speaker designers in their own right, to audition the system. (I have omitted their names for their protection.) The demonstration consisted of playing two of these hybrids, which were placed in corners be hind an acoustically transparent curtain in my basement. The experts were so impressed that, at first, one of them wanted to pay me for the design so that he could market it. When I told him that it was just from playing around and that I had gotten lucky, he raved about how great the thing sounded and encouraged me to do something with it commercially. The other expert said the system sounded as good as anything he’d heard to date. I thanked them for their encouragement.
A few weeks later, I got a job to build a disco sound system in a small Manhattan club. Since I wanted hi-fi sound and not public address, I decided to use eight Acoustic Research AR-3a speakers, which were, at the time, considered to be good, compact, full-range hi-fi systems. They were to be powered by four McIntosh MC- 275 amplifiers, so each speaker system would have its own 75-watt amplification channel. The eight speakers were to be in stalled around the dance floor, near the ceiling. This was before subwoofers and tweeter arrays, mind you.
Before installing the speakers at the site, I thought it might be interesting to compare them with my home-brewed concoction that everyone had liked so much. I placed the AR speakers behind the curtain, next to my own systems. The Ails were in two four- speaker arrays, each with two speakers on top and two on the bottom. I used pink noise and a sound-level meter to adjust the amplifiers so that the ARs and my own speakers would sound equally loud. Even when I played them quietly, the ARs blew my systems away! Shortly afterward, I dismantled my own speakers, and no one heard them again.
Before designing loudspeakers, one needs to know that it’s much more of a science than one might suspect, and good science requires proper knowledge and tools (plus, an anechoic chamber helps). Our egos often interfere; we tend to think that our ears are as good as, if not better than, somebody else’s. But before your friends in spire you to go to the trouble of designing and building your own speakers, consider just going out and buying them. Then you can spend the time you’ve saved listening to good music. I’ve since taken my own advice and have never regretted it.
--Al Rosner; President, Rosner Custom Sound Long Island City, N.Y.
"AUDIO CLINIC": Answers to Common Audio Queries
AUDIO CLINIC (Audio magazine, May 1997)
Output Tube Glow
Q: I am 15 years old and am trying to learn about audio and how equipment works. I know that vacuum tubes are like light bulbs; over time, their filaments burn out. I also understand that if you leave a tube amplifier on all the time, the fidelity will improve. Naturally, I had to experiment. I left my amplifier on overnight to see if this made any difference in sound quality. I woke up the next morning, and to my surprise, the output tubes were a hazy, glowing purple. The audio sounded the same as if I had not left the amplifier on. What made the output tubes in my amplifier glow purple
A: I certainly can appreciate how a 15-year-old can be an audiophile and an experimenter. I was about that age when I cut my first disc recording, and I was hooked. A year or so later I designed my first sound system, complete with two turntables and a mixer. I sold it to a very happy client!
Now, back to your problem of glowing output tubes. I could tell you that the tubes glowed because they were angry over not getting enough rest. I know you wouldn’t believe that, so I’d better get serious.
It would be wonderful if vacuum tubes could be completely evacuated of gases, but it is almost impossible to obtain a perfect vacuum. Some of the remaining gases are embedded in the cathode. Heat can force some of this gas to boil off the cathode’s surface and enter the electron stream. The high voltage present between the plate/screen and cathode will ionize the gas, resulting in the glow that you described. (This is the same principle that underlies the operation of neon signs.)
Because of the higher filament current and greater power dissipation within the output tubes, they run hotter than voltage amplifier tubes. This is why output tubes are more likely to glow than other tube types.
Some tubes are poorly evacuated, and these will glow almost immediately after warm-up. If the tubes are old, gases will have boiled from the cathode and they will glow more than newer tubes. Also, if the grid bias voltage is too low, the tubes’ plates and screens will draw excessive cur rent. This, in turn, will cause more heat to be developed, and hence more gases will form, helping to create the glow. Incidentally, the excessive current will ruin the tubes.
The matter of a tube-equipped amplifier sounding better when it is left on continuously rather than turned off after each use has been debated for a long time. I have not discovered any scientific reason why leaving a tube amplifier on should improve its sound. There might be some truth to the idea that tubes will last longer when they are left running, because the heating and cooling of their filaments contribute to their gradual deterioration.
Q: I want to upgrade the sound from my turntable. I bought a new cartridge, but will I get improved sound by using better interconnects than those supplied with my turntable? The owner’s manual for the turntable says that it should be cleaned and lubricated every two years. I’ve had it for 20 years without servicing it and have noticed no ill effects. Do you think I should get it serviced
A: It’s amazing that you’ve had no problems with your turntable after 20 years of use. Even though it appears to be running well, you may be surprised at how much less rumble or wow and flutter you’ll hear after lubricating it properly. The oil reduces friction, which results in smoother rotation of the various parts.
The owner’s manual should tell you how to lubricate the turntable. You should be able to do it yourself; if you can’t, take your turntable to a reliable service shop. Usually the platter’s main bearing requires some oil every few years. The instructions may recommend a particular lubricant, but 10-weight oil or a fine machine oil will suffice. Don’t use common household oil, because it leaves a waxy residue.
The capacitance of phono cables is the main characteristic that affects the sound of a moving-magnet cartridge (some are more susceptible to such loading effects than others). Cable capacitance is usually stated in picofarads per foot of cable; when its total capacitance is added to that of the preamp’s or receiver’s phono stage, it should equal the total capacitance called for by the cartridge maker. Check the cartridge specs and those of your preamp or receiver. Even if your old cables had the proper capacitance for your previous cartridge, that capacitance may not be appropriate for the new cartridge. You can easily adjust capacitance by trimming or lengthening the cables between the turntable and the preamp.
CD Player to Multiple Receivers
Someone asked a question about connecting his CD player to three receivers. Although you answered the question well, I have some additional suggestions that may be helpful. As a custom installer, I have done this type of work many times. Two approaches have worked well for me.
To send only the CD signal to two receivers, the simplest approach is to use a standard Y adaptor (for each stereo channel) to two sets of shielded cables leading to the two receivers. You mentioned the problem of possibly shorting inputs on some receivers. By soldering a 1-kilohm resistor into the hot line leading to the receiver with the shorting in puts, the shorted contacts will no longer pose a problem to the other receiver or to the CD player.
However, a distribution amplifier is the best solution for a multiroom or multizone installation where many receivers are to be used. Such a device accepts one input, divides it electronically, and distributes it to multiple outputs. The outputs are buffered, and the signal is not degraded. Niles Audio (800/289- 4434) makes a one-in, six-out stereo distribution amplifier (Model ADA-6) and a similar video amplifier (Model VDA-6) that sends a composite video signal from one VCR or laserdisc player to as many as six TVs or VCRs. Also, Xantech (800/843-5465) has two audio/video distribution amplifiers, the AV 426 and the AV-61. The best source for information about these products is an experienced custom installer. If you cannot find one, call the Custom Electronic Design and Installation Association (CEDIA, 317/599- 5850) for the names of qualified installers in your area.
Q: Is the operation of a subwoofer based on air pressure or vibration? Is a bigger driver better than a smaller one? Is the structure of the cabinet important for quality sound? Is it true that placing fiberglass or foam in the cabinet will improve the subwoofer’s sound quality?
A: A subwoofer uses both vibration and air pressure to reproduce low-frequency sounds. The cone of the sub’s driver vibrates back and forth quickly to produce higher frequencies (e.g., 100 times per second for a 100-Hz sound) and relatively slowly for low frequencies (e.g., 20 times per second for a 20-Hz tone). The alternate compression and rarefaction (expansion) of the air surrounding the speaker cone generates changes in air pressure that cause our eardrums to vibrate at the same rate as the woofer’s cone. This information is sent to the brain, which interprets the signals from the eardrum as “sound.” (More specifically, in the case of Jurassic Park, you’d hear the thumps of the dinosaur’s feet as it lumbers towards the terrified passengers inside the Jeep.)
The larger the area of the subwoofer’s cone, the more air that it can move for a given excursion. This means that a small- cone woofer must move farther forward and backward in order to displace as much air as a larger cone and thus produce the same output. There is a practical limit to how much excursion can be achieved with out excessive distortion, however. But this can be overcome with multiple drivers; in fact, some designers have obtained excellent subwoofer performance by using two long- excursion small-cone woofers in a single enclosure.
Ideally, a subwoofer cabinet, like any speaker enclosure, should not vibrate at all; only the woofer’s cone should move. If the cabinet vibrates, it will tend to radiate sound the same way the cone does. It won’t radiate a lot, but it may be enough to alter, or color, the tonal character of the sub- woofer’s sound, making it inaccurate. That’s why high-quality speakers and sub-woofers often have internal braces to minimize vibration of the cabinet walls.
Various linings or fillings—made of foam, fiberglass, or other absorptive materials—are often used in speaker cabinets to kill reflections within the cabinet that might reemerge through the cone and color the sound. Such fillings can also affect driver loading, however, so it is not a good idea to add such fillings to a speaker after the fact.
Adjusting Tape-Head Azimuth
Q: I decided to readjust my tape deck’s head azimuth. The record and play heads of this deck are on a single plate, so I have to tweak only one screw. I used a five- year-old tape, The Simpsons Sing the Blues, because I had no other that I knew to be in correct azimuth. I set the azimuth so the tape sounded right and then compared it to another copy. The newer copy sounded much brighter, and I could hear subtle details that were muffled on the old tape. Again I adjusted the azimuth until I got the new tape sounding as bright as possible. Then I played other tapes and had to readjust the azimuth again! What is going on? Which of my tapes has the correct azimuth?
A: The tiny gap in an analog tape head must be exactly perpendicular to the tape for best high-frequency response. If the azimuth is off by a minute of arc either way, you’ll get degraded sound. Moreover, the tape you play during azimuth adjustment has to have been recorded on a ma chine whose head azimuth was correctly set. In either case, if the gap of the play back head does not line up with the magnetization on the tape, high-frequency losses will result. If a tape has been recorded with an incorrect azimuth, it can be played back properly only by misadjusting the playback head to compensate for the azimuth error.
Your problem is to determine which of two copies of the same prerecorded tape has been recorded with the correct azimuth.
Worse, both may suffer from azimuth error—we just don’t know. The quickest way to determine proper tape-head azimuth is to obtain a good test tape.
Because test tapes are often hard to obtain and expensive, I made an azimuth- alignment tape of my own. I started with what I hoped wasn’t a dangerous assumption, that the azimuth of most commercially recorded tapes is dead accurate. I then checked enough tapes to determine if most of them agreed in azimuth. In other words, if the sound was similarly clean and unmuffled from all the tapes, then I could adjust the heads on my machine and most tapes would sound good with my guestimate set ting. Some tapes fell outside the range, so I eliminated them. However, I found that many tapes worked well for a given azimuth setting. I chose one as the reference tape and set it aside, playing it only for azimuth adjustment; I bought another copy of that tape for my music library.
If your preamp or receiver can be set to mono, do so and then play your tapes. When the azimuth for a given tape is right, switching from mono to stereo will result in virtually no change in high-frequency response. All you should hear is the sound stage widening as you switch to stereo. (Use the preamp’s or receiver’s volume control to compensate for level differences when you switch from mono to stereo; otherwise, the louder signal may sound subjectively brighter.) If your tapes all sound good in mono, then it is likely, by the law of aver ages, that they are all correct in terms of azimuth and that your cassette deck’s play head is properly adjusted.
Tape decks that have separate play and record heads are more difficult to adjust because both heads must be correctly aligned. Adjust the play head as described above. Next, make a recording of high-frequency noise. You might want to use a pink-noise signal from a test CD or FM interstation noise (turn off the muting), and I recommend using a C-60 cassette (the base film is thicker and is less likely to stretch, which could skew the azimuth of the test tape). As you make this recording, adjust the record head’s azimuth for best high-frequency response while you monitor the sound off the tape from the play back head. You should switch between mono and stereo to be sure that you have this head properly aligned. Now you can use your pink-noise tape as an azimuth- alignment reference.
Speaker Cones vs. Discs
Q: Why are dynamic speaker drivers (woofers, subs, etc.) cone-shaped and not flat or circular discs? What are the disadvantages of each? I know that Precision Power makes flat-diaphragm speakers for car audio systems. What gives?
A: The main reason for making a speaker diaphragm in the shape of a cone rather than a flat disc (circular or rectangular) is for purposes of stiffness and rigidity. It is easier to control irregular flexing of a cone than it is of a flat disc (bending or flexing of a diaphragm produces resonances that cause tonal colorations and distortions). If you imagine the physics of a speaker’s motion, you’ll understand why.
Consider the speaker’s task: It must create sound by producing pressure waves from a vibrating source, the driver’s diaphragm. To accomplish that, it has to alternately compress and rarefy air molecules—and do so rapidly (e.g., a woofer diaphragm must move back and forth 300 times per second to produce a 300-Hz sound). A dynamic speaker is really a reciprocating air pump driven by an electromagnetic motor (the voice coil/magnet assembly), attached to the apex of a cone.
However, let’s assume for the moment that instead of a cone, we attach the voice coil and magnet to the center of a flat disc, held in place at its edges by a border of flexible rubber (the surround), the latter attached to a rigid frame. If you imagine the voice coil rapidly pumping back and forth, it will tend to displace the center of the flat disc before the larger area near its outer circumference—to bend rather than move uniformly, unless, of course, we make the disc of very stiff material. That’s the trick. If we use a rigid metal or hard plastic, then the diaphragm may become too heavy, and the voice coil/magnet will require too much power to move the flat disc back and forth. On the other hand, if we attach the voice coil to the apex of a dense, stiff-but-light paper cone, and perhaps treat the cone with a thin lacquer spray or plastic coating to in crease its rigidity, then the cone will follow the vibrations of the voice coil with greater uniformity and less bending than a flat disc driven at its center.
Flat diaphragms are nothing new; electrostatic speakers are, of necessity, flat, as are planar magnetic drivers, and they can sound superb. In the ‘80s, Sony developed a flat, square aluminum-honeycomb-diaphragm woofer. To drive it uniformly, four voice coils were used. It worked fine but was costly to manufacture. And for a number of years, Phase Technology has made speakers that use flat diaphragms made of light but rigid expanded-foam plastic.
There also are sound reasons to alter the shape of a diaphragm, such as to control the dispersion and diffraction of high or low’ frequencies. For example, large flat diaphragms produce serious beaming of highs, which has prompted some designers of planar speakers to shape diaphragms in a gentle curve, to better disperse the highs and alleviate the beaming.
Insofar as applications in car sound, when you consider that space within a car is scarce, particularly in the doors and the dashboard, a flat diaphragm makes sense. A compromise must be struck between efficiency and practicality.
A Phono Cartridge Phono Inputs Can’t Take
Q: I came across a three-speed turntable that can play my old 78-rpm records. However, its crystal cartridge produces terrible overload and distortion when I connect it to my amp’s phono input. How can I adapt the crystal cartridge to the magnetic phono input of my amp?
A: Crystal (and ceramic) cartridges do fl not require standard RIAA equalization and are relatively high-output devices (that’s why your cartridge is overloading the magnetic phono input). You should there fore connect this cartridge to one of your amp’s auxiliary inputs. However, you’ll need to advance the volume control significantly because the output of the crystal cartridge won’t match that of high-level sources, such as a tuner or a tape deck. I would have suggested replacing the crystal cartridge with a magnetic one, but the of most old turntables that used crystal or ceramic cartridges were not shielded well enough to permit hum-free operation.
AUDIO CLINIC (Audio magazine, July 1996)
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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 al ready 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 sub- woofer’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.
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.
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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.
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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.
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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.
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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.
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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.
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--Subwoofer Crossover Considerations--
Q. I have read that a weakness in a subwoofer/satellite system could be a gap between the upper frequency limit of the woofer and the lower frequency limit of the satellite speakers.
I have considered adding a 12-inch sub- woofer to my existing loudspeakers. The frequency response of the speakers that would then become the satellite systems extends down to 60 Hz. The subwoofer’s response is 29 to 180 Hz. Because its 180 Hz overlaps the frequencies of the satellites, would these speaker systems blend all right? Is there a gap?
A. If your subwoofer could not handle frequencies above, say, 40 Hz, there would be a gap between its upper cutoff and the 60-Hz cutoff of your smaller speakers. But you have an overlap, the opposite of a gap.
That’s not too good, either. The frequencies in the overlapped region, 60 to 180 Hz, may be overemphasized, because both speakers are producing them. Or you may get cancellations, due to phase differences between the two speakers’ outputs in this frequency range.
Luckily, overlaps are easier to cure than gaps. Just add a crossover to channel low frequencies to the woofer and everything else to your satellites. In your case, the crossover should be set to something like 100 Hz. This should permit both the satellites and the subwoofer to operate over the flattest portions of their response curves.
--Playing Records Monophonically--
Q. I have a large collection of LPs dating back to 1953. My new receiver, unlike previous units, does not have a button for mono. I can’t play my monophonic discs monophonically, and I don’t get as good sound because of this. Is there anything I can do to restore this option?
A. Monophonic records usually do sound better when played monophonically. Any vertical information picked up by a stereo cartridge playing mono records will include only rumble (some of which may be on the record itself), some distortion caused by pinch effect (which drives the stylus upward, especially at high frequencies), and noise. Since this information shows up as a difference between the channels, you need only cross-connect the channels to eliminate it.
The easy way is to connect two shielded Y adaptors in series between your turntable and your preamp. You need one Y adaptor with two female RCA jacks and one male RCA plug (such as Radio Shack #42-2436) and one with a female and two males (such as Radio Shack #42-2435). If this hookup gives you hum problems, try detaching your turntable’s ground lead from your preamp. If the problem persists, carefully trim back the insulation from one of the two RCA plugs that goes into your preamp and then sever the shield surrounding its center conductor—just be careful not to cut into that conductor or the insulator surrounding it.
A more elegant way, which would save you the trouble of plugging and unplugging things, would be to install four RCA jacks and a single-pole, single-throw switch in a small metal box. Wire the left-channel jack of one pair to the left jack of the other, and do the same for the two right-channel jacks. Wire the switch between the center terminals of one pair’s left and right jacks. With the switch open, signals pass through the box unchanged. When you close the switch, the two channels are shorted together, and the signal becomes monophonic. Connect your turntable leads to one pair of jacks, and run a cable from the other to your preamp. It’s a good idea to leave the box in plain sight when you set it to mono to remind you to switch it back again for normal stereo listening.
--CD Timing Accuracy--
Q. I recently purchased a new CD player and have a question about its timing accuracy. Considering all of my experiences with open-reel tape decks and cassette decks, I have noticed that few home units approach professional recording and broadcast standards for timing accuracy: 0.1% speed accuracy, or 3.6 S per hour.
It was natural for me, therefore, to continue the practice of checking the accuracy of CD players. Most of the players I have owned have run just slightly under the times shown on the CDs. You can imagine my surprise when I checked the accuracy of my new player and found it ran about 12 S per hour faster than the expected timings!
You may not think this is a matter of any concern, but this was an expensive purchase, and I would think that it should offer better timing accuracy. I would like your opinion in this matter while I await a response from the manufacturer.
A. The timing of the data stream in a CD player is controlled by its internal clock circuits. If you can obtain a service manual for your player, you may find a trimmer that can be used to adjust the frequency of the clock oscillator. If the player is still under warranty, the maker should be willing to adjust the clock.
(After I sent this advice to Mr. Bailey, he replied: My faith is renewed! The manufacturer replaced the clock module with a high-accuracy unit, and the player now times out perfectly.)
--Are My Loudspeakers Any Good?--
Q. I recently purchased a pair of loud speakers based on how they sounded at my dealer. I have no stereo components as such, so I connected these speakers to my wife’s rack system.
At first I was very pleased with the sound, but then the right speaker sounded very “bass heavy.” I took the loudspeakers back to my dealer, and he tested the loudspeakers by connecting them to a high-powered amplifier.
The dealer could not detect any problems with the speakers and told me that my amplifier was probably not powerful enough to handle the dynamics of the music.
So, now what? What should I do if I invest in a high-quality, high-power amplifier and still have this bass problem?
A. It is difficult to answer your question without having been with you when you were evaluating your loudspeakers. About all I can say is that if these speakers sounded as you expected at your dealer and if they now sound poor at home, there may be some problem in your wife’s rack system. As I understand it, the sound was fine at first, and then deteriorated. This may indicate that something has gone wrong with the rack system, though it’s difficult to imagine what could go wrong that would result in over-accentuated bass.
I really suspect that the bass problem was there all along but that until you became accustomed to the equipment, you failed to notice it.
This bass heaviness may well be the result of room acoustics. Experiment by placing the loudspeakers in different parts of the room and at various distances from walls; you may well find a location that solves the problem.
If you have not resolved this problem, see if you can borrow a friend’s system and connect it to your loudspeakers. If you notice a marked improvement in audio quality, you will know that you need better components. If nothing helps, let’s hope your dealer will exchange your loudspeakers or refund your money.
--More Hiss on CDs than LPs?--
Q. My CDs of rock music from the 1960s and early ‘70s contain much more tape hiss than those same recordings on the original LPs. This problem does not appear on my jazz CDs, and many of these are from the late ‘50s and early ‘60s.
Can you explain why this should be so? Also, at what frequency is this hiss occurring? Would knowing this allow me to use my equalizer to cut it down?
A. I don’t have a definite explanation. Possibly, the rock material was so rich in treble that the engineers had to roll the treble off a bit when mastering the LPs, so as not to cause tracing distortion. This would also roll off any hiss in the master tape. As CDs have no treble saturation problems, they’d require no such roll-off.
Many of the master tapes of these recordings were doubtless made before Dolby noise reduction became popular, so they’ll have audible tape hiss. Overdubbing through multiple generations would raise the hiss still higher. Such overdubbing was and is more common with rock than jazz, which might account for the difference. Also, acoustical instruments are more used in jazz than rock, and such instruments don’t have enough highs to force the engineer to cut back the treble when cutting the LP master. Hiss is usually not a single frequency but a wide band of frequencies that overlap the music, so it’s impossible to equalize it out without losing some musical overtones as well.
HISS IS NOT A SINGLE FREQUENCY, SO YOU CAN’T EQUALIZE IT OUT WITHOUT LOSING MUSICAL OVERTONES.
I’d suggest you experiment by rolling off frequencies in the range from 3 to 5 kHz, where the ear is most sensitive to hiss. Attenuate these frequencies as little as possible, because the sonic quality of the music will also be affected. Try listening to an equalized signal from the CD player and an unequalized signal from LPs, and adjust your equalizer until the two sounds match.
Editor’s Note: Extra hiss can also occur on CDs that are made from later tape generations than the master used for the LP, or from tapes that have been improperly stored in the decades between LP and CD mastering. When trying to get rid of hiss, I’ve found it helpful to cut the top most equalizer band or two to reduce severe hiss; this works best on recordings that don’t have much tonal content above 10 or 12 kHz. And the Auto-correlator circuit on some older Carver preamplifiers (it’s not on the current ones) does a decent job of reducing hiss but has minimal effect on the music.
-------------- (Jan 1994) -------------
Minimum Impedance Problems
Q. My receiver delivers 110 watts per channel into 8 ohms. Recently I purchased four new loudspeakers. One pair has an impedance of 8 ohms; the other pair, 6 ohms. I like to run all four speakers simultaneously. The receiver's instruction manual talks about either 12 ohms per speaker minimum or 6 ohms per speaker minimum. I have my speakers connected in parallel. Is that safe? Can I damage any of my components with my present wiring?
A. It seems to me that you can operate your equipment perfectly just as you now have it wired. If I understand you correctly, your manual says you can operate two pairs of speakers in parallel as long as the mini mum impedance per speaker is 6 ohms. One pair of your speakers meets that mini mum, and the other has a higher impedance than that.
If, on the other hand, the combined impedance of both speakers per channel must be 6 ohms, you'll have problems with a parallel hookup. In parallel, your loud speakers will have a combined impedance of less than 4 ohms.
Does your manual's reference to 12 ohms apply to serial connections? If so, I suspect that 12 ohms per channel is the lowest value recommended as the combined series impedance of your two pairs of speakers. In that case, your present loud speakers can run successfully in a series hookup. If the manual actually says each speaker's impedance must not be lower than 12 ohms (unlikely, as few speakers have impedances above 8 ohms), you have a real problem, as none of your speakers has such a high impedance.
If your manual is confusingly written, check with the service department of your receiver's manufacturer to see which hook ups are safe. Assuming both serial and parallel connections are okay, I recommend you operate your speakers in parallel because the damping is better.
If there is any doubt, don't drive your amplifier to maximum. If you run it at half power or a bit less, chances are you will minimize overheating and damage to the output stages. There is little audible loss of volume between full and half power.
"Pops" from the Power Lines
Q. I hear a "popping" sound from all speakers in my audio/video system. The sound is usually just a quick "pop" every 2 to 15 minutes, and I also hear it when a light switch is turned on. What is causing this sound, and how can I prevent or eliminate it? Will my components be damaged by these "pops"?
A. The "pop" you hear is caused by transient voltages on the power line. These transient spikes are often produced when a refrigerator or furnace starts and stops operating. As you have noticed, light switches often cause the sound.
I have never heard of such noise spikes being so loud as to damage equipment. But they are annoying, so you need to deter mine which of your system components is passing these spikes along to its output. Since all your speakers are reproducing the popping sound, look for components that affect all channels. Components that fit this description typically include A/V receivers, preamps, and surround decoders. If all your speakers are fed from the same amplifier, it's also a suspect.
To determine the actual source of the problem, try disconnecting one suspect component and running the system with out it. If the "pops" are still heard, reconnect the component and try disconnecting another one. Keep trying, working closer and closer to the power amplifiers, until the noise is gone. If you're left with only the power amps as likely culprits, check by substituting other amps for them.
Once you know which device is causing the problem, write to its manufacturer and ask what remedies he has for the problem. With some components, the solution is to place low-value capacitors across the feed back resistors, to narrow the bandwidth. Before you try that, let the equipment's maker recommend capacitor values.
Sometimes, especially in homes with old and undersized wiring, the problem can be cured by running a separate a.c. line from the fuse or breaker box for use with the audio system only.
You have observed that one or more light switches are sources of these "pops." If you get a noise spike when your furnace or air conditioner switches on, the relay contacts associated with your thermostat may be another source of the problem. For these sources, you may be able to reduce the level of the noise transients by putting RC circuits across the offending contacts. Such circuits consist of a resistor and capacitor wired in series across the contacts. Use half-watt or larger resistors, and capacitors with working voltage of at least 400 d.c. Try a 100-ohm resistor and a 0.1-mF capacitor for starters, but you may have to experiment to find what values work best.
It's not easy to install an RC network in a refrigerator or freezer, but you should be able to find noise suppressors that can be plugged in between such appliances and the a.c. line. Make sure these suppressors have sufficient power-handling capacity for whatever appliance you use them with.
Audio and AC Polarity
Q. Why do some high-end components have switches to change the polarity of the a.c. line? I can't see how changing the polarity of an a.c. signal can improve the sound quality.
A. Reversing the polarity of the a.c. power-line voltage entering a sound system will not improve the intrinsic audio quality of that system. On its face, it does seem ridiculous to think that by reversing the polarity of an a.c. voltage, the sound of an audio system can improve or deteriorate. However, there still may be some apparent sonic improvement. The power line has one side grounded, and changing the polarity of the power-line voltage affects the action of the ground in relation to the chassis. This can, in some situations, affect the amount of background hum that emanates from the loudspeakers. Even where hum is not apparent, it could be present to an ex tent where it will mask some nuances of the music. When the hum is reduced or eliminated (as a result of reversing the polarity of the power-line voltage or however it was done), the music will take on a more open quality.
CD Track Lockout
Q. Why are there presently no CD players that allow the user to "lock out" certain tracks on a particular CD so that they cannot be played? On any given CD, there is likely to be at least one undesired track Wouldn't it be easier to lock out one or two tracks rather than having to program the player for each CD so that only the desired tracks will be heard?
A. Quite a few current CD players offer such features as Favorite Track Selection, letting you. store the track programs for many CDs and automatically repeating those programs when you replay each disc. Check whether any model you're considering has the memory capacity to handle the number of discs and tracks you'll need. Philips and Philips-owned brands, such as Magnavox, were first with this feature, but other makers have it by now (often under a different name).
If all you want to do is lock out tracks on a one-time basis, look for a player with a "program-out" feature. This lets you pro gram the player either to play the tracks you want or to exclude the ones you don't want, whichever makes programming simpler and faster for a given disc.
Q. Last year, I had my receiver converted from 120-V operation to 220-V operation hey an authorized dealer in Paris. Is this in any way likely to degrade the performance of my receiver?
A. Inasmuch as you had the power transformer replaced by an authorized dealer, the correct replacement part was probably used. Therefore, I believe the receiver's performance will not have been de graded by the conversion.
CD Players, Amps, and Level Controls
Q. Do I really need a preamplifier be tween my CD player and my power amplifier? Can't I just use a high-quality ganged volume control between the player's output jacks and the inputs of my power amplifier?
A. Using a volume control between the output jacks of your player and the input terminals of your power amplifier will work fine. Just for the sake of completeness, some CD players are equipped with their own volume controls; such players will also work well for your application. However, I personally don't like this arrangement, because there are times when I would like to make tonal adjustments. Your arrangement makes no provision for this. A preamplifier also provides switching facilities and input/output connectors for other equipment in a system; your arrangement doesn't (although you could use a switch box).
Misaligned FM Tuner
Q. I have an FM tuner connected both to a dipole antenna and, via switching, to the local cable company. The cable system re transmits our local stations and provides a stronger signal than I can obtain using my dipole.
The tuner is still new, with less than 100 hours of use. I have noticed that the LED center-tuning indicator frequently shows that tuning a little higher in frequency may be required. If I move up 25 khz, the indicator shows correct center tuning, but the signal- strength meter shows a slightly weaker signal. This occurs whether I use my dipole or cable.
I have heard that atmospheric conditions can sometimes cause stations to drift. I've also heard that tuners can drift. Should I have my tuner serviced?
A. Atmospheric conditions have nothing to do with the frequency accuracy of an FM broadcast station. I cannot even imagine how such conditions would "fool" the tuner into a frequency error. I think the real problem is that the tuner's detector is not aligned properly. Let's say that it is supposed to be centered on 10.7 MHz. If, because of misalignment, it happens to be centered on 10.725 or perhaps 10.675 MHz, then the signal meter and the center-of-tuning meter readings will not agree. Depending on the bandwidth of the i.f. system in your tuner, this misalignment may produce some distortion on highly modulated signals.
------ (August 1998) ------
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.
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.
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.