AUDIOCLINIC (May 1987)

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High Impedance and Power Output

Q. Test reports and company brochures describing the performance of amplifiers provide specifications for the equipment when it is connected to an 8-ohm loudspeaker system. There are instances where specs are provided when loads of 4 or 2 ohms are present. This information does not tell me what I need to know: What hap pens to power output, distortion, and frequency response when the loud speakers are 16 ohms and 500 ohms?

-J. R. Wakefield, Barberton, Ohio

A. The higher the load impedance, the less output power can be expected. This is true because solid-state equipment has a low output impedance; no reasonable load will likely match that impedance (enabling maxi mum power to be transferred to the load). Thus, the higher the load impedance, the farther the load will be from a true impedance match. Frequency response and distortion will be substantially unaffected in most instances.

A 500-ohm load will represent a very serious impedance mismatch. There fore, the power available to such a load will be so drastically below nominal ratings that you will likely not have sufficient acoustical volume from your loudspeakers.

Most loudspeaker systems do not, in themselves, have 500-ohm voice-coils.

If you have speaker systems rated at 500 ohms, I believe there must be matching transformers within them which yield the 500-ohm impedance. If at all possible, I recommend that you remove the transformers and connect the loudspeakers directly to the amplifier. Use the terminals from which you have removed the secondary winding of the matching transformers.

If you don't wish to do that, use a matching transformer of 500:8 ohms (or a 62.5-to-1 ratio), capable of handling the power expected to appear at the speaker terminals. Such transformers should be available from dealers specializing in public-address (sound-reinforcement) equipment.

As for 16-ohm loads, the power pro vided by your equipment will likely be sufficient for your needs, though it will be somewhat reduced. The exact maximum power available will depend upon the design of the particular power amplifier. I believe it will be within 3 dB of the equipment's 8-ohm rating.

Guitar Versus Hi-Fi Speakers

Q. What are the physical differences among PM, bass guitar, and hi-fi loudspeakers? Can I use PM or bass guitar speakers in a high-fidelity sys tem?

-Name withheld

A. Regardless of any other classification, most loudspeakers are PM, which stands for "permanent magnet." Years ago, it was not possible to pro duce permanent magnets of sufficient strength to operate loudspeakers efficiently. Therefore, electromagnets were used, deriving their power from the devices to which they were connected. When the device was off, there was no magnet in the speaker.

Loudspeakers used for bass guitars are often designed with rather high, resonant frequencies, such as 100 Hz.

Thus, large amounts of acoustical power can be generated in that portion of the spectrum with relatively little amplifier power. Because of their non-uniform frequency response, they tend to sound "boomy" if used in high-fidelity systems. Also, all too often, bass guitar speakers are made to sell at low prices, so corners are cut in terms of suspension quality and magnet strength. This will result in a lack of good sound when such a speaker is installed into a high-fidelity system.

Bass guitar drivers are big (usually 12 or 15 inches). I have seen some which would doubtless work very well as subwoofers, units made with large-diameter voice-coils that are long enough to maintain a constant amount of wire in the gap during long cone excursions.

Loudspeakers for electric guitars are often small in diameter and have stiff suspensions. These cannot reproduce low frequencies below perhaps 60 Hz, and often possess serious peaks and dips in response. This is fine where these speakers are intended to color the music, but it is obviously not useful when accurate musical reproduction is desired.

Unauthorized Dealers

Q. I have had great difficulty in obtaining warranty service for a CD player which I recently purchased from a local audio dealer. As I understand it, the reason is that the store from which I purchased the player is not an authorized dealer for it (i.e., the store doesn't have a franchise agreement with the player's manufacturer). The retailer obtained the player from a shop which is an authorized dealer. Thus, my dealer was able neither to do the service needed nor to return the player to its maker. How important is it to purchase a piece of equipment from an authorized dealer? Is it unusual to have difficulty in obtaining warranty service when a piece of gear has been purchased from an unauthorized dealer?

-Yoji Shimizu, Madison, Wisc.

A. You did not make it clear whether your problem is simply that the retailer refused to provide service directly, that the manufacturer refused to honor his own warranty, or that no U.S. warranty was provided.

An unauthorized dealer will have trouble honoring warranties because he will usually be unable to obtain the replacement parts necessary to do his own service work. Further, as you've said, he cannot even return a defective product to its maker.

Many dealers sell equipment at great discounts, which means their profit margin on each sale is low. Such dealers often provide very little service, since all customer service costs time and money. Thus, discounters may not want to handle warranty problems even if they are authorized dealers and many discounters are not. In my experience, such dealers may tell customers that warranty problems are handled directly by the makers of the goods.

To obtain service from the manufacturer, you will usually have to write or call to find out how the company will handle the problem. Then you will have to either take or ship your equipment to the manufacturer's service department or nearest factory-authorized shop.

When you purchase equipment made overseas, however, you may get "gray market" goods whose warranty is not valid in the U.S. These goods have been imported directly by the retailer from foreign dealers or foreign distributors, and sometimes even direct from the manufacturers, for less than they would have cost if purchased through the manufacturer's authorized U.S. distributor. The authorized U.S. distributor is responsible for such expenses as stocking spare parts, selecting and training service people, handling warranty repairs, and advertising the product. Having spent this money helping to make the product more worth selling, he then passes on these costs, plus a profit, to his authorized dealers. He cannot be expected to stand behind products which he didn't sell, and so he usually won't honor warranties on gray-market goods.

For this reason, some localities require that dealers who sell gray-market products say so, and tell whether those products carry a U.S. warranty or not.

If you know beforehand that you are buying products without a U.S. warranty, make sure you know a service technician competent to repair it. Other wise, you'd better hope it doesn't break down.

(Editor's Note: While it is usually not possible to get warranty service on gray-market goods, manufacturers may help those who buy, through un authorized dealers, goods imported by the authorized agent or goods made in the U.S. When in doubt, ask the manufacturer or official importer. -1.8.)

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CODA


--Richard C. Heyser--

Richard C. Heyser, long-time Senior Editor of Audio, mentor to every one who had the temerity to measure a loudspeaker, and perhaps the most brilliant man in this field, died on Saturday, March 14, 1987. Since our copy deadline is more than two weeks past, and the details of the accomplishments of this hugely modest man difficult to assemble in a single morning, we will publish a more fitting tribute next month. -E.P.

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(Source: Audio magazine, May 1987, JOSEPH GIOVANELLI)

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