Audioclinic (Jan. 1985)

Letter Writing

I try to reply to every "Audioclinic" letter--yes, even those which do not find their way into this column.

Unfortunately, however, some letters never get answered. Not counting possible loss in the mail, the major reason is that their authors do not enclose stamped, self-addressed envelopes as requested in each column. And there are many letter writers who are in such a rush to pen their thoughts to paper that they never put their return addresses on their letters. Had they sent the return envelope, properly filled out, I would at least have a way of tracking their addresses down.

Even if the envelope does have a return address, it is better to put your address on the letter itself. The volume of my mail is quite heavy. If an envelope strays away from a letter, I may fail to answer the letter or may waste precious time in attempting to head off such disasters.

These simple precautions will enable me to serve you better.

-J.G.

Litz Wire and Amp Shut-Down

Q. Many loudspeaker cable manufacturers use one variation or another of something known as "Litz" wire.

What is it? I understand that this wire has low d.c. resistance, and I have been told that, because of this low resistance, the amplifier with which it is associated will shut down. Any comments on this?

-Carlos Estrada, Jr., Tampa, Fla.

A. Litz wire is stranded wire, with each strand in the bundle insulated from every other strand. This wire has been used for years in such items as high-quality loop antennas and i.f. transformers. Losses caused by "skin effect" are kept low in these r.f. applications.

As designed for loudspeaker cables, Litz wire has low d.c. resistance.

That low resistance cannot shut an amplifier down, unless its two conductors are shorted together. However, many premium speaker cables, probably including some Litz types, have high capacitance, and this can shut some amplifiers down-permanently, in a few cases.

Such occurrences are rare and probably becoming rarer as amplifier and cable designers have grown aware of the problem. But it still would make some sense to find cut the cable's capacitance; check with your amplifier's manufacturer about it, unless you know someone who already uses that amplifier/cable combination successfully.

Center-Tuning Meter Problem

Q. The test equipment 1 have on hand consists of a Radio Shack FET VOM and an Eico Model 482 20-MHz oscilloscope. My receiver is not in warranty. Its center-tuning meter does not work; it remains centered as I tune across the dial. At one time this was an intermittent problem, but now the meter never operates. The receiver works fine in all other respects. Can I make repairs, given the equipment I have on hand, or do I need more elaborate equipment?

-J. D. Williams; Jr., Waycross, Ga.

A. The first thing to look at in a case like yours is the meter itself. The next component to examine is the IC driving it. Because the problem was at first intermittent, you should initially check solder connections of all components and wires related to this circuit.

Meter circuits of this type operate as follows: The detector, when a station is properly tuned, will produce 0V. If a station is mistuned, the detector will produce a d.c. voltage having some given polarity. If the station is mistuned to the opposite side of center, the polarity of the d.c. voltage at the detector will be reversed. The IC meter driver derives its input from the detector. At this point there will be a time constant whose purpose is to smooth out instantaneous voltage changes resulting from the received station's audio modulation. This keeps the meter from jiggling, especially at bass frequencies.

The IC is connected directly to the center-tuning meter. The test equipment you have will be more than adequate for troubleshooting this circuit.

Impedance of A Two-Way Speaker System

Q. Not long ago I was in an audio shop looking at loudspeaker systems.

One of them, consisting of an 8-ohm woofer and an 8-ohm tweeter, caught my attention. The tweeter was wired in series with a nonpolarized 4-µF capacitor. I asked the salesman what the impedance of this system was, and he said if was 8 ohms. I asked, "How can this be when the two speakers are in parallel?" and the salesman replied, "The capacitor keeps the impedance of the system at 8 ohms." How can the impedance be 8 ohms when the speakers are wired in parallel?

-Richard Roy, Dalhousie, N.B., Canada

A. The salesman was correct. The impedance of that loudspeaker system is 8 ohms.

You are right in saying that, when two loudspeakers are wired in parallel, their combined impedance must be different from that of a single speaker.

In the case of the system you have described, however, there is a capacitor in series with the tweeter. A capacitor must not be considered merely as a piece of wire. Because of the electrical properties of a capacitor, the tweeter is effectively not wired into the system for low frequencies; it is "reconnected" at high frequencies. Of course, there is nothing in this simple circuit to "disconnect" the woofer at high frequencies, so its effects will still be present.

Chances are that the impedance of the woofer rises at frequencies above the point at which the tweeter begins to operate. The tweeter and its associated capacitor will result in a falling impedance at the crossover point and above. Of course, this drop is limited by the impedance of the tweeter. Much depends on the characteristics of both the woofer and the tweeter, but it is possible that, in the vicinity of the crossover frequency, the impedance will dip below the 8-ohm, nominal value. However, we still can say that the impedance of the system is 8 ohms nominal.

Headphone Sensitivity

Q. I have a question concerning headphone impedance. My receiver is capable of 70 watts output per channel, and I am using a CD player with fixed headphone volume. I have two sets of headphones, one having 150 ohms impedance and the other, an electrostatic/dynamic combination, having an impedance of 400 ohms. I must advance the volume control considerably past its midpoint if I am to get sufficient volume from the 400-ohm phones as compared to the setting of that control when using my 150-ohm phones. Will I damage my amplifier by running it at such a high volume? (The amplifier is not clipping at this volume control setting.) Neither pair of phones, when connected to the headphone jack on the CD player, provides enough volume.

The fixed output level from the CD player is 5.1 V at 600 ohms and, I believe, 0.66 V at 8 ohms. Should not there be sufficient output from the CD player with 5 V driving the headphones?

I am, therefore, confused about the impedance ratings of headphones. I have seen some phones rated at 50 to 70 ohms which work fine with my player, but they are inexpensive and do not provide good audio quality.

-Frank Fabian; San Francisco, Cal.

A. The problem isn't the headphones' impedance; it's their relative sensitivity. As with loudspeakers, some headphones require more-sometimes much more-power than others to achieve a given listening level. Electrostatic phones, in particular, require a lot of signal, as the amplifier's output is sometimes used to supply polarizing high-voltage power for the electrostatic transducers as well as to supply a driving audio signal.

I do not believe you will damage your amplifier when operating it in the way you have described, especially because it is not clipping. Because of the circuitry in the amplifier, the high impedance of the phones and the fact that loudspeakers are not connected during headphone operation, the amplifier is not being called upon to supply power so much as to supply voltage. To be on the safe side, check with the manufacturer to see if the output circuit can operate with a minimal load and at advanced volume control settings which could drive the system to near clipping.

Record Cleaning Mistakes

I have had considerable difficulty eliminating extraneous noise when playing records. There were two reasons for the problem, both having to do with the way that I have used my Discwasher in the past. Because anybody could make the same errors, I present the following to keep others from falling into the same pits.

First, always use the Discwasher in a well-lit area. I do not know how much dust I left on my records in the past simply because I could not see it. Second, make sure that the brush used to clean your discs or your stylus is itself clean, for otherwise it's like trying to clean a window with a greasy rag.

Also, after putting the required drops of fluid on the pad, use the side of the bottle to smooth it evenly over the record's surface.

Boy! Was I glad to find out that the flaw in my music was not in my audio system.

-Brian Campbell; Lindenhurst, N.Y.

Hearing Loss

After having reread "A Sense of Loss" (Audio, July 1983), I felt compelled to share an experience with fellow readers.

Just about a year ago I visited a local doctor to have my left ear cleaned of wax, which sometimes clogs that ear and temporarily causes a hearing loss. The doctor used a water stream device which, I believe, burst my eardrum because of too high a pressure. Although I still could hear, frequencies below 300 Hz were lost and I was in severe pain for several days.

Last winter, a second doctor (an ear specialist and a micro-surgeon) patched the hole in my eardrum, and I regained some of my low end. I have a low-level, constant ringing in my left ear, but this is masked for the most part by everyday ambient noise. I am learning to accept the noise. I am fortunate that no greater damage was done and that modern medical technology was able to minimize my loss.

I have written this to make others aware of our gift of hearing, so that they will not take it for granted. I have a higher regard for my hearing now, having been through this experience. We go to great lengths to obtain our high-fidelity systems in order to enjoy our music. Let us not forget the component we cannot replace.

-Joseph Ombres; Toms River, N.J.

I get lots of mail from readers wondering if their music systems can provide enough sound level to take advantage of the wide dynamic range of CDs. If you are already listening to music at sound levels approaching the threshold of pain, you do not need, and should not want, more sound level-whether from digital discs or from any other sound source. I have received some letters from teenagers who have lost hearing sensitivity just by playing their music at high sound levels.

Bass, Midrange and Treble

Q. A typical crossover network may have a 400-Hz crossover point between bass and midrange and a 5-kHz crossover point between midrange and treble.

Is there a numerical point in the 20 Hz to 20 kHz audio spectrum where bass ends and midrange begins? Is there a numerical point where midrange ends and treble begins? Or is everything so gradual that there are no exact points assignable to these divisions?

-David Baldwin; Colonial Heights, Va.

A. There is no real, physical point separating bass from midrange and midrange from treble. After all, the spectrum is one continuous sweep, with no breaks or discontinuities which audio equipment designers can use as dividers. You can see, therefore, that the terms bass, midrange and treble are arbitrary.

The only way to determine where crossover is to take place in a complex speaker system is to study the characteristics of the drivers to be used. The crossover point for a woofer would be chosen so that no peaks or serious dips took place in that portion of the spectrum assigned to it. Thus, if a woofer's inherent roll-off started at about 600 Hz, with serious peaks occurring at about 700 or 800 Hz, we would cross it over no higher than 400 Hz, so the effect of the peaks would be reduced to virtual inaudibility by the combined driver and crossover roll offs. The midrange speaker chosen to work with this hypothetical woofer would have to have a low-frequency limit below the 400-Hz crossover point.

The crossover between midrange and tweeter would be treated similarly.

(adapted from Audio magazine, Jan. 1985; JOSEPH GIOVANELLI)

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