AUDIOCLINIC (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.

Turntable Considerations

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.

Subwoofer Principles

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 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 magazine, JOSEPH GIOVANELLI, May 1997)

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