AUDIOCLINIC (Jun. 1986)

Equalization with LPs and CDs

Q. When I bought my CD player, I was a bit disappointed with the sound on some of my discs. The frequency response was not what I could obtain with LP versions of the same records.

To my ears, the CD versions had a peaky midrange (3 kHz) with loss of warmth. I then remembered an article in Audio that suggested a boost of 3 dB in the 120-Hz range along with a cut of 3 dB in the 2- to 4-kHz range for CDs with this problem.

The other day I was searching through my record collection, and found a record with the label "Digital." This LP never sounded right because of too much warmth and lack of mid range. It was then that I thought of using the mirror image of the CD-correction curve. That did the trick!

-Marc Ferland, Montreal, Que., Canada

A. As with any other program source, you will like the sound of some CDs and dislike that of others. As you have found, you can, with a slight adjustment of some equalizer controls, make a given program source sound better. Using an equalizer in this way is more important, to me, than using it merely as a tool for producing an over all flat frequency response for a given listening position.

Static Solutions

Q. In your December '85 column, reader Andrew Hinds raised the issue of static. Your answer merely scratched the surface with regard to solutions. Having spent the last 20 years in the computer industry fighting this stuff, I would like to offer the following suggestions:

1. Check if your wall outlet is grounded. If you are not the technical type, get a qualified person to do this for you. Should the outlet not have a ground, get one installed. It may be expensive, but it's worth it.

2. Ground all your equipment. Be cause most audio equipment has only a two-wire wall plug, you will need to run extra wires from your components to the wall outlet/receptacle.

3. Install a grounded anti-static mat in front of your equipment. This means wiring the ground to the mat. Most computer stores carry these or will direct you to a source.

4. Use anti-static spray on your car pet. Commercial sprays, although good, smell horrible, do not last long, and are expensive. I prefer a 50/50 mixture of fabric softener and water.

Apply in a fine spray. Not only is this cheap and long-lasting, but it will also make your carpet smell nice.

The presence of static has wounded many computer systems, causing untold damage. In audio components, static, at its worst, can lead to premature equipment failure. At best, it can reduce one's listening pleasure.

-Lutz E. Moeckel, Costa Mesa, Cal.

Groove Skipping and Compliance

Q. I recently switched from a cartridge of relatively high tracking force to one requiring somewhat less force; the compliance of the new cartridge is higher than that of the old one. For some reason, my new cartridge often skips over the opening grooves, whereas the first one did not. Why aren't cartridges uniform as far as compliance and tracking force are concerned?

-Henry C. Moski, Branford, Conn.

A. The most likely cause of your groove-skipping problem is a mis match between your new cartridge and your old arm. Every arm/cartridge combination has a resonant frequency which is a factor of the cartridge's compliance and the total effective mass of the cartridge and arm. By raising the compliance without lowering the mass, you have moved the system's resonance down, probably be low the desirable 8- to 12-Hz range. As a result, record warps (which are mos pronounced at the record's outer edge) and, perhaps, vibration caused by footfalls in your room can now ex cite this resonance. Possible solutions include switching to an arm with lower mass (difficult and expensive), and switching to a cartridge with lower compliance. Your cartridge's manufacturer might possibly offer a stylus for your cartridge with lower compliance than the one you have now, but it may not have as desirable a tip design.

As to why tracking force and compliance are not standardized: All else being equal, a cartridge with high compliance and requiring a minimum tracking force will tend to have better transient response than cartridges of lower compliance which require a greater tracking force. Obviously, given the same stylus tip configuration, cartridges which require less tracking force will also produce less record wear than those requiring higher tracking force. Cartridges with high compliance are more expensive to produce and are sometimes unsuited for some discs. Further, these cartridges are more readily damaged.

In order for a cartridge to have high compliance and to track at as low a force as practical, some trade-offs must be made, often in terms of reduced output voltage. Thus, high-compliance cartridges may prove unsatisfactory when used with preamplifiers or receivers having insufficient voltage gain and/or signal-to-noise ratio to accommodate them.

Signal Loss at Low Volume

Q. At low volume, regardless of program source, both channels of my receiver cut in and out-with lots of static. The receiver will operate normally for about 5 to 10 minutes before the onset of this condition. I can temporarily restore the receiver to normal operation by quickly turning the volume control way up and then turning it down to its original setting. I then can listen for another 5 to 10 minutes before the cycle starts again. The receiver works properly at volumes above back ground-music level.

I have had this receiver in the shop several times. Each time, the technician believed that the receiver was merely dirty. All the controls were cleaned, but these "repairs" had no effect.

-Jon T. Satterwhite; Rossville, Ill.

A. The condition you have de scribed could be the result of so many different factors that I cannot pinpoint any one as the likely cause. The interesting thing is that the condition occurs on both channels. This tends to sup port the idea that there is a defect in the power supply, which is certainly common to both channels. But I cannot explain how the quick rotation of the volume control could shock-excite the power supply into proper operation.

Can it possibly be that there is an IC (other than a voltage regulator in the power supply) common to both channels? You will need a schematic of your equipment to determine this.

In any event, you need to trace the signal through your receiver. Hopefully, you will locate the earliest stage which exhibits volume loss. Even here, the effects of feedback could obscure the true nature of the problem. Use a second amplifier to trace the signal.

You will likely need a service manual so you can follow the path of the signal through the receiver. Obviously, you must monitor your receiver as well as the "test" amplifier so you can hear the reduction in volume-maybe in both amplifiers and maybe not, depending on whether you have connected the test amplifier to a stage of the receiver which exhibits this volume drop.

Of course, you can also watch for changes in any of the various power-supply voltages.

Another possibility is that the system becomes unstable and breaks into oscillation. The frequency of such oscillation may be too high to hear, but it should be detectable on a 'scope or by noting an unexpected a.c. voltage. A defective filter capacitor may lead to unwanted coupling among various audio stages. This, in turn, can produce oscillation.

If you are unable to perform these tests, find a competent service shop.

Perhaps a factory service center has found your problem to be a common one, with your make and model of receiver, and it may well have a cure.

Bridging a Power Amplifier

Q. I am confused about how to bridge a stereo power amplifier for monophonic use. Can you enlighten me on the subject?

-Edgar A. Gorospe, San Diego, Cal.

A. To bridge an amplifier in this fashion, a monophonic signal must be fed to each of the amplifier's input channels, with one channel receiving the signal out of phase with respect to the other. This out-of-phase condition causes one channel's "hot" output terminal to go positive while that of the other channel goes negative, producing a greater voltage swing than either channel could manage on its own. To do this, essentially places the two out put circuits in series, just as you would place two batteries in series in order to increase their output voltage. The speaker is connected between the two hot terminals rather than between one hot terminal and ground (as is done for normal stereo use). Some amplifiers incorporate the necessary bridging (phase-reversing) circuit, thus requiring only one input cable. Others require external bridging circuits, while still others cannot be bridged without damage.

Results vary. Most commonly, bridging the amplifier will cause it to deliver exactly twice the rated power of either channel when used normally. But some amplifiers can deliver more than twice their single-channel rated power when bridged, while others may deliver a bit less.

Consult your amplifier's manufacturer about the advisability of bridging, and to find out what output power may result and what impedance restrictions, if any, may be imposed for bridged operation.

About CDs and Players

Q. Before I buy a CD player, I need to know more about the product. If CDs are better than conventional records and tapes, why? Would a reason ably discriminating listener really be able to notice this improvement? Why is the term "digital" used? How is a disc made? Is there more than one size? What features should I look for in a CD player? Is there a version which can both record and play? Finally, are CDs recorded directly from the music source or are they copied from records and tapes?

-P. Y. Williams; Marble Falls, Tex.

A. No matter what I tell you about CDs and the machines used to play them, the only way you will know if you like the music they produce is to listen to them. As with anything related to music listening, our perception of CDs is subjective. However, I can say that most listeners do hear a distinct improvement, while some critical audiophiles feel that LP records, despite the measurable advantages of CD, sound better in some respects.

There are some objective facts I can mention. It is possible to record more low frequencies on CD than is possible either on tape or on phonograph records. The signal-to-noise ratio on CDs is better than on phonograph records or cassettes. Wow and flutter are virtually unmeasurable. Given all of this, you can see that the dynamic range of music on CD can be extremely wide.

The term "digital" refers to the fact that music is encoded on the disc as a series of binary numbers (ones and zeros) representing its amplitude measured every 0.0000226 second. These binary digits (1.4 million per second) are represented by tiny pits, too small to be seen by the naked eye, which are read by the finely focused beam of a small laser. The original recording from which the CD is made may have been either digital or analog; if the latter, it is converted to digital form for the CD.

The discs come in only one size, 4.7 inches (12 cm) in diameter. The manufacturing process is complex, involving the creation of a master from which the tiny pits are molded, the molding of the disc, and the coating of the disc with a reflective aluminum layer and then a protective lacquer on which the label information is stamped. Only the lower side of the disc is read by the laser, but the disc can still hold up to 74 minutes of music. The present Standard specifies that these discs be recorded only on one side.

Most CD-player features relate to convenience rather than sound, helping you locate and access specific points on the disc and set up pro grams of tracks you wish to play. Have a dealer demonstrate these to you so you can determine for yourself which ones you want. Machines which can record other types of optical discs are still comparatively rare and quite ex pensive. It is anticipated that CD player/recorders for the home will be avail able in a few years.

Compact Discs are made from the record companies' master tapes, and therefore have higher fidelity than if they were copied from the records and tapes released for sale. (There are exceptions: CDs of historic performances made before studios began using tape recorders. Even these are probably made from tape dubs of the original master discs, with ticks and pops edited out and possibly other enhancements made.) Since CDs cannot be edited (among other reasons), they are not used for studio recording.

(Audio magazine, Jun. 1986, JOSEPH GIOVANELLI)

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