Audioclinic (May 1983)

Power Output, One Channel Driven

Q. Why does an amplifier deliver more power with one channel driven than with both channels driven?

-Bob Robinson; Warrensville Heights, Ohio

A. An amplifier will produce more power with just one channel driven be cause the power supply is, under such conditions, not required to deliver as much current as when both channels are driven. The output voltage of the power supply will therefore be higher.

This has to do with what is known as the "regulation" of the power supply.

The better the power supply is regulated, the less the discrepancy between what happens with one channel driven and what happens in terms of power output with both channels driven.

FM "Drop-Out"

Q. I have an FM tuner connected to a rhombic antenna with a leg length of 8 feet. I am using coaxial cable for the lead-in, but with this setup, multipath distortion is very bad.

When receiving close, strong signals, the sound will cut out and the signal-strength LEDs do not illuminate when a lot of bass is present. When there is less bass, these LEDs remain lit, and the signal remains audible. This only happens, however, when the muting is turned on.

The stereo indicator also flickers when a strong bass signal is received.

This happens regardless of the muting setting.

What would cause these problems?

-Mark Cuddeback, Park Falls, Wisc.

A. I cannot say for sure just what is wrong with your reception. It may be that the timing capacitor associated with the muting circuit has opened.

This would mean that if signal strength changed, as it momentarily does with deviation from the center of the carrier during modulation, the set could cut out. Because bass produces the widest swings in the frequency of the carrier, it would happen during loud bass rather than with other kinds of program signals.

It is possible that the tuner is not aligned as it should be or that the muting action is too strong. There may be an internal adjustment you can make, but to determine this or to perform any alignment, I suggest you obtain a ser vice manual or send the equipment to a qualified technician for adjustment and checking.

The fact that the stereo indicator fluctuates independently of muting set ting could indicate that the muting circuit is not the problem. It may also be that there are several difficulties.

All of the symptoms you have de scribed could also relate to severe multipath problems. In that event, orient your antenna to minimize this.

If the tuner is literally cutting out, then it's unlikely to be simple tuner overload. I've never seen overload make that happen, though it may cause distortion and other problems.

FM DX Update

In a discussion of FM reception some months ago, I mentioned the ad dress of the WTFDA (Worldwide TV FM DX Association). This organization is still in operation. Because of reader interest, I am happy to give you their new address: P.O. Box 97, Calumet City, III. 60409.

-J.G.

Record Destruction, or, One for the Books

Q. For a long time now, I have noticed a problem with some "non-audiophile" turntables and phonographs. Using a new stylus ruins my records, I found by switching back that any of the old needles which originally gave me no problem, still gave me none. If I play part of a record with a new needle and play the rest of the disc with one of the "old" ones, I can hear where the "new" needle left off. It sounds as if the grooves are being eaten away. Playing a disc a few times with a new needle can cause it to look and sound like a worn 45-rpm record. This problem is limited to just 45-rpm records-not 33 1/3 albums or 78-rpm records.

In addition, these records cannot be stacked. Attempts to play one 45 on top of another only results in "slipping." Can you determine the cause of this problem?

-Eric Fox, Bay Shore, N.Y.

A. I believe that all of your problems result from the tracking force being too great. Over the years the force has gradually increased. The new stylus, with its smaller tip radius, exerts more pressure on the disc's surface than the worn one does. The ultimate result of this is that a great amount of record wear is produced.

See if you can reduce tracking force to tolerable levels. Look for a spring which may have stretched. Remove one or two turns, reattach the spring, and measure tracking force.

The fact that 45-rpm discs are the ones mostly affected by your system is the result of the turntable not running "true." This results in the tonearm's rising and falling as the height of the table changes. Add this to too high a tracking force, and wear will be accentuated. Because the force exerted by the turntable to force the tonearm up ward is greater at 45 than at 33 1/3 rpm, a greater amount of record wear can be noticed at that speed.

At first glance one would think that record wear would be even more serious when playing 78-rpm discs than when playing 45-rpm discs. The only reason why this is not the case is that the tip radius of the stylus is larger than that used for 33 1/3-rpm discs. There fore, the contact pressure on the disc is less, so wear is less severe than it would be if the stylus was not different.

It may be, too, that the record wear of the 78s is not as noticeable because they are made of a different material than 33 1/2-rpm discs.

Recommended Crossover Frequency

Q. What is meant by "recommended crossover frequency" for loud speakers?

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

A. To begin with, it means that the speaker in question leaves you the option of selecting a crossover frequency, either because the speaker is a separate driver or because it's part of a system which has separate input terminals for each driver so that you can bypass its internal crossover if you wish.

The response curves of the drivers in a multi-speaker system do not butt together like beads on a string, but overlap. The crossover network and the natural roll-offs of each driver determine the frequency at which response crosses over from one driver to the next-that is, where each driver is producing half the output.

You should at least consider using the frequency the manufacturer recommends. Shifting the crossover point can have undesirable consequences.

Raising a midrange-woofer crossover point, for instance, may make the woofer operate at frequencies where its response is uneven, or where it is rolling off so quickly that there will be a dip in response until the midrange starts coming in. Lowering the tweeter/mid range crossover point may also expose peaks and dips in the tweeter response, or force it to handle frequencies so low as to damage it. (Tweeters tend to be delicate, because of the need for low moving mass at high frequencies.)

Hi-Fi and Stereo History

Q. When did high fidelity and stereo come into use on records?

-Lee Kihm, San Diego, Cal.

A. For "high fidelity," there is no single date, as its growth has been a continuous evolution rather than a revolution. Then, too, the sound that one era considers state-of-the-art "high fidelity" may be considered obsolete in later days. The phrase "high fidelity" dates back to at least the '30s and possibly the '20s. If you can locate some of Stokowski's recordings of the late '20s and early '30s, you will probably be surprised at hearing high frequencies out to about 8 kHz on some of them, assuming the discs are still in good condition. I have heard a few Edison cylinders whose high end ex tends out to nearly 5 kHz-remarkable, when you consider that no micro phones or electrical amplifiers were used.

It is possible to date some important milestones along the route to high fidelity, however. Electrical recording first appeared in the mid-'20s. The use of negative feedback around the cutter head was first done, I believe, by London Decca during World War II. Some of the 78s produced toward the end of the war were incredibly well recorded and certainly qualify as high fidelity.

About that time, some radio transcriptions and 78s were pressed on vinyl, which provided a much better signal to-noise ratio than shellac.

In 1948, Columbia Records introduced the LP record, using a slower turntable speed (33 1/3 rpm, a speed previously attempted by Vitaphone for synchronized movie soundtrack discs and by RCA for commercial records) and a thinner groove and stylus. Adding to this the London feedback techniques plus the use of vinyl as the base material yielded an improvement in signal-to-noise ratio and fidelity.

Stereophonic disc recording, as we know it today, was proposed back about 1931, but did not actually arrive on the market until about 1957.

(adapted from Audio magazine, May 1983; JOSEPH GIOVANELLI)

= = = =