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A flat response
Editor: In John Atkinson's report from the London AES Convention in Vol.10 No.4, he reported that CBS's infamous Copycode notch removes the note A-flat from the top of the piano's register. Surely 3840Hz, the center frequency of the proposed notch, lies between B-flat and B?
Bo Fleming, Santa Fe, NM
Mr Fleming is correct; the CBS device is a B flat remover I must have been feeling a little flat from jet-lag when I looked up the note corresponding to the notch frequency. -JA A sailor's best friend?
Editor: Congratulations on the best magazine for the audio enthusiast to ever hit the presses. I'm currently serving this great country in the US Navy, and I must say that every time I have to leave my family, Stereophile becomes my best friend. The reviews are always excellent; so good, in fact, that in my mind I can often project myself into the same listening room as AHC or JGH. A lot of people complain that Stereophile focuses only on the expensive stuff. All I can say is, "can they help it if the best costs more?" To be totally honest, I would buy a component on the basis of one of your reviews alone, I trust your opinion that much.
Unfortunately, Uncle Sam doesn't pay me enough to afford what I would like. But one of these days .. . Randy Dirilo USS Independence C., peanut-butter, & A/B tests
Editor: Martin C.'s articles are "peanut-butter sandwiches": they provide nourishment, and can be read next month, next year. They are 50% of the value of your magazine to me.
I can feel the difficulty your reviewers have shifting gears from describing "My favorite fantasy" to "This is where I live" in your review of your reviewers ("A matter of taste," Vol.10 Nos.1, 2 & 3). Thanks to reviewers and editor for the extra hard effort.
A/B testing, etc: My 9-year-old claims he knows his multiplication tables perfectly - except when I ask "What's 9 x 7?" I could be insulted and angry if he launched into a half hour explanation of "left brain, right brain" just because he didn't know 9 x 7.
I would say, "You're OK, keep trying."
Charles W. Fowlkes Bozeman, MT
Why bicker with Bob?
Editor: Why don't you guys quit bickering with Bob Carver and just give him an editor's job? It may work out to both your advantages. But insist on paying him with "rolled pennies via parcel post"!
N. mal, Balboa, CA
Enough already?
Editor: Enough already! Bob Carver is simply not worth the kind of effort--or ink--you've devoted to him in the past year. As an almost legendary member of the "I don't care what you say about me as long as you spell my name right" brigade, his interests are being served with every word you write. And the irony is that you have never given this kind of coverage to Bill Johnson, Lew Johnson, Dan D'Agostino, Nelson Pass, or Tim de Paravicini--truly original designers all, whose minor efforts over shadow the best work Bob Carver has done in the best year he ever lived.
Let's look at the facts:
• Carver's company was funded with the proceeds of his sale of Phase Linear to Pioneer. . . a transaction that remains a mystery to this day, based on what Pioneer did with the name and the products after purchase. But the new company allowed him to correct the one egregious error from Phase Linear days: he could now look out his office window and see CARVER writ large on the side of the building.
• His work at Phase Linear probably came closest to being serious or original; certainly nothing he has done since could be considered a contribution to the high end, consisting, as it does, of tricks and effects, supported by specmanship that would make the Japanese blush.
• Presently he builds products to a price point. There's nothing wrong with that. Solid reputations have been built in this industry by people who made and marketed very good products to a price point.
But he's not satisfied with this. He wants the sales figures and profits of a price-point manufacturer and he wants the reputation of the designer giants. He wants to build products that must, by definition, be compromised by price considerations, and he wants these pro ducts to be taken as seriously as the products of designers who make no compromises. Like Rodney, he don't get no respect. And he's not happy about it. Rich is nice, but ego gratification is nice, too.
• Now we get his latest approach. Bill John son, Lew Johnson, or whoever, designs the "reference amp," builds a monster product, and Carver responds, "I can do that." And not only can he "do that" but he can do it at his price point. Sure, Bob. Pontiac builds decent cars to a price point. But Pontiac does not waste its time trying to convince people that a car built to an $8500 price point will be as good as my $25,000 Porsche 944, specs or no specs. And by the way, meeting a spec for a moment or "sounding the same" for a week or a month does not mean that the Pontiac- or the Carver-is as good as the high-priced spreads. There's more to a Krell or an ARC or a Porsche or any product that aspires to be "The Best" than good short-term performance; things like residual value, long-term performance, pride of ownership. (Check the "Audiomart" pages and see how a Carver pro duct holds its value in the marketplace.)
• And now, deja vu. When you guys find things that keep his amplifier from being the equal of the reference amplifier, Carver says you guys are wrong. And he isn't the only one who thinks so. Who else? Why, The Inner Ear Report (established September 1986), one of the leading voices of that bastion of the high end, Canada. Of course, the review hadn't been printed yet when he quoted it so copiously in his ads, but we all know that that's never stopped Bob Carver. He's one of the few people who ever saw the last issue of The Audio Critic, and he kept quoting the review of his amp that allegedly appeared in that issue-"sounds just like a Mark Levinson and Pete Aczel thinks so, too" -for years in spite of the fact that the ultimate TAC was never distributed to subscribers.
Did Bob blush about being party to a high end scandal? Certainly not. Business is business. (By the way, Bob, when you see Peter, would you ask him where my $30 refund is?) So back to my original plea: enough already.
I'd rather hear how Bill Johnson designed the SP-11 or the M300, or how Nelson Pass balances handling design projects for a number of client companies while still delivering top rank designs for his own firm, or what Dan D'Agostino had for breakfast, than read one more word from or about the Whining Engineer. Remember, you could accuse him of murder in big block letters on the cover of your next issue just as long as you got one thing right: that's B-O-B C-A-R-V-E-R.
John C. O'Hara Oak Brook, IL
Carver & self- parody
Editor: Your article in Vol.10 No.3 evaluating the Carver M-1.0t amplifier, and Bob Carver's reactions to this evaluation in "Manufacturers' Comments," provided some of the most entertaining reading that I have recently encountered. All parties involved exhibited a devotion to the cause of excellence in sound reproduction, and an intensity in pursuit of this goal, that took them virtually to the limits of self parody. While I admire the zeal exhibited in the articles, I feel that a more balanced perspective can be presented.
Clearly a difference between the reference amplifier and the Carver M-1.0t amplifier was exhibited. Bob Carver blamed this difference on old tubes, and J. Gordon Holt worried that 17
the difference was not as dramatic in blind testing as it was when he knew the identity of the amplifier in use. This leads me to conclude that the difference is slight, and I will have to do much soul and pocketbook searching to decide whether to shell out an additional
$4500 to obtain this degree of difference when I decide to buy a power amplifier. I consider this much of the debate to be very useful to me, and to be candidly and carefully presented by the writers.
But in addition, Stereophile mounted an assault on blind testing in general, claiming that it allows only the grossest differences to be distinguished. Differences between the left and right sides of the brain were proposed as one possible explanation for this phenomenon. While your condescension for Stereo Review is clear throughout Stereophile, you have provided their cartoonists with ample material for the next several issues: I look for ward to seeing drawings featuring "left-brain amplifiers" and "right-brain amplifiers," as well as to statements like "Show me the power amplifier, so I can hear how bad it really is!" I do not think that the problem is with blind testing, although I agree with most of the problems with this method stated by J. Gordon Holt. I think that the problem is that when the identity of the power amplifier is known, one has a tendency to focus on its imperfections and differences. This is somewhat similar to the way one observes a well-known friend: his hair is a little thinner, he's pined a few pounds, or the wart on his chin is darker than it used to be. But when the identity of the power amplifier is in doubt, one hears the sound produced as a totality, at least more so than before To be sure, the imperfections and differences are still there, and can still be noticed, but they are no longer as dramatic because one is not focusing on them to the same degree.
Now, if this is correct (and it seems to me that this was what J. Gordon Holt was really saying), then it follows that blind testing is a very useful form of testing, arguably the most useful form of testing, and should become part of the test procedure for every component.
With the identity of the component known, one could do early tests to establish the differences from a reference component, then do a blind test to establish a degree of significance for these differences in the component's over all sound. In addition, one cannot lose with this method of testing, because those readers who respect blind tests will rejoice that you have finally seen the light. Meanwhile, you can bask in the assurance that as the left side of your brain catches up with the right in audio analysis acumen, the blind test will no longer give you embarrassing moments of doubt.
Stereophile will blaze new trails toward the ambidextrous audio brain-clearly a higher form of listening than any heretofore known.
Robert L. Bernardt Greenville, NC
Carver & human perception
Editor: When Stereophile Vol.10 No.3 arrived, the first thing I read was the article about the Carver challenge. Whet% I had also read JGH's editorial and the readers' letters on the AB and Hafler null tests, I felt very grateful: Stereophile has advanced the discussion of perception of amplifier sound.
I'm not only interested in, but also familiar with, differences between amplifiers and the problems that arise when trying to objectify these lb make dear where I stand: I love tubes more and more but I certainly don't reject transistors. It seems consistently difficult, though, to make a beautiful transistor amp, and equally hard to make a tube amp sound bad.
I must say I consider Bob Carver's manufacturer's letter unfair. It is outrageous and blunt.
He had no reason for that, as your review, although not without shortcomings, had been very serious and your language friendly. You could arrogantly have laughed away the M-1.0t amplifier, but you didn't. Even after Carver's insane letter, you were restrained: "we've already had our say." It would have been better for everyone, however, if the listening process had been clearly divided into three parts. One, did the M-1.0t sound exactly like the original hand tuned version? And, if all three reviewers agreed on that, part two: did the M-1.0t sound exactly like the reference tube amp? If it didn't, part two would have been a triangular listening test between the three amplifiers. Part three could, at Carver's request, have been an objective (statistical) test of one or more reviewers' ability in hearing such differences. The way it went looked more like a mixture of bits of everything. The conclusion, however, remained clear: Carver lost, and he knows it.
In the course of the review, you hit on the amazing but repeatable phenomenon that amplifier differences seem to shrink to almost irrelevant subtleties if one is put under stress.
JA's remark (mentioned in the editorial) on left and right brain functions is very intriguing, but could he please indicate where to find this in the literature? It would be interesting to con front this with the right/left brain differences in musical perception between Europeans and East-Asiatics, on which Jean Hiraga reported in audiophile (#12). You will probably also have experienced that amplifier differences equally shrink in an ABX test when switching forth and back during the music (or when changing plugs quickly), even when this is done in a very relaxed manner, at home and alone.
So what's happening? Are "subjective" re viewers all over the world merely idiots who by some wondrous chance hear the same things in this or that amplifier? Or is our knowledge of how we hear largely insufficient? If so, even Hafler's null test won't help (there is good evidence it doesn't!), because if we don't know how we hear, who are we to say what exactly an amplifier should or should not do? The letters from Babrauskas and Nel son in Vol.10 No.3 put a very clever finger on this weak spot in Hafler's test.
I have read quite a lot on human aural (and visual) perception. Not only is there a poor correlation between "our" technical standards regarding frequency response, distortion, etc., and the way the ear and brain seem to analyze sound; there are also quite a number of contradictions between hypotheses that cover one aspect of aural perception. There isn't such a thing as one integrated theory explaining hearing. Even when taking these contradictions for granted, you don't get very far when trying to understand how we perceive our beloved stereo image. I have the feeling that we know only 25% of what we should know, in order to have a workable idea of what is going on beyond our tympani.
I know from my own experience that it takes time to evaluate an amplifier. It is as if your brain adapts slowly to a certain situation, the growing adaptation enabling greater and greater resolution in hearing. You can even be aware of such an adaptation. Differences in loudspeakers are less subtle and more "natural" than those in amplifiers. If you listen to one pair of speakers, then disconnect them, put them away install a pair of different speakers and then listen, it's okay. If you use an A/B box, the moment you switch, your first reaction is one of awe, you may even unconsciously move your head backward to protect yourself; after 30 seconds or so you feel yourself adapting to the new sound and can start listening again. Differences between amplifiers are "un natural" and are more difficult to grasp. Use an A/B switch a couple of times and you're lost.
I think this has to do with the data reduction applied by both our aural and visual systems.
This is easier to grasp in visual perception: if you look out of the window, you seem to see everything there is outside. In fact, however, that is not true. In reality you directly see only structures and some details, the rest being "made up" by the brain, using memory, logic, and expectation. Take those double "find-the eight-differences" cartoons in the newspaper: they look identical, even on second and third inspection. Then, slowly, you become aware of one difference, then a second, etc Once you know where they are, you can see them all at once and you wonder, was it that easy?" Then again, these cartoons are easy from a data point of view. Imagine two Rembrandts with eight differences! If you always literally see all the visual information that reaches your eyes, you would have gone instantly crazy the instant you opened your eyes after you were born.
Back to hearing. When you start listening to the sound in a stereo image, you may think you are hearing everything, but in fact you mainly hear what your memory leads you to expect to hear. Then, slowly, helped by the tonal structure of musical instruments (de pending on your familiarity with them) and by the structure of the music itself, you gradually become aware of anomalies. This is where the Guilbert letter on "Music and A/B-testing" in the same issue comes in. You simply can't hear everything at once. Otherwise, why would people ever play a recording twice? I would be thrilled to see people from perceptual sciences and people from the hi fi scene cooperate on that mysterious sound-object we call a stereo image.
Peter van Willenswaard
Rotterdam, Holland
The difference in how the two sides of the brain handle information was first pointed out to me by the British audio writer Adrian Orlowski, following which I found a complete report in the British magazine New Scientist.
Unfortunately, I cannot now find the reference, but a thorough discussion is included in Gary Zukav's classic study of modern physics, The Dancing Wu Li Masters, published in 1979 by William Morrow & Co. Inc., New York. It was also briefly mentioned in the June 13-19 issue of The Economist. -JA Carver & common sense
Editor: I feel compelled to throw in my two cents' worth to the Stereophile/Carver M-1.0t controversy in Vol.10 No.3.
Facts: I have continually upgraded my hi-fi component system for the past 22 years. I act as a consultant to most of my friends about their audio enquiries.
My American Express Platinum Card allows me the flexibility of choice, but not insanity.
I own an M-1.0t, serial number 8.
I don't subscribe to Stereophile but greatly enjoy fiction. (One part fact plus seven parts fantasy equals fiction.) I trust my ears and common sense to allow me to make sound decisions.
A question: If an average audiophile has an annual income of under $100,000, then where does he get the money to buy $5000 power amplifiers, or find the time to listen for days on end, or get the technical training to become a self-proclaimed expert? I would personally like to thank Bob Carver for his innovations and down-to-earth approach to high fidelity. DW Tomamichel Redmond, WA PS: No, I haven't missed the point. Stereophile is in the entertainment business, and apparently controversy and self-proclaimed opinions are the kind of things on which religions can be founded.
Subject vs object
Editor: In reviewing (subjectively) the Stereophile Carver controversy in Vol.10 No.3, I noted that both sides seemed to consider the performance of the ad hoc-modified prototype only as a standard to be met by regular production models.
But 18 months ago, it was a proxy for the sound of your reference amplifier(s). At that time JGH and LA had arrived, by exhaustive listening, at the perception that their reference amplifier and the hand-tweaked unit sounded identical. By excluding the prototype M-1.0t from the latest listening sessions, you seem to have removed a valid opportunity for judging the effect, if any, of speaker substitution and tube aging, as well as the level of manufacturing ability and quality control at Carver Corporation.
The Spanish philosopher Jose Ortega y Gasset has pointed out how the conception of "subject" entails and must be completed by that of "object," if we are not to go astray in a world of hard realities. The mutual dissatisfaction resulting from your re-challenge would indicate that subjectivity is as much problem as solution. Evidently the design and production of an electronic object by a skilled engineer contains just as many subjective traps as the review of its performance by equally skilled listeners.
But thanks for accepting the original challenge, and for your excellent reportage of its sequel. This is the stuff that makes Stereophile the most adventurous and readable publication in the audio field. H. Kenneth Hayes River Forest, IL I appreciate the concern shown by readers who feel that we missed a trick by not reviewing the production M-1. 0t amplifier in the context of the performance of the original band-tweaked prototype. In matter of fact, as pointed out by LA in his response to Bob Carver's "Manufacturer's Comment" in Vol. 10 No.3, although the prototype was in Santa Fe for a short period of time it was not avail able either to JGH or myself when we carried out the review, Bob Carver having insisted that it be sent back to Lynnwood. He could have brought it with him when be visited Santa Fe for the listening tests, but he didn't. In addition, we only have Bob's word for it that the prototype was identical to the amplifier auditioned by JGH and LA two years ago With all best wishes concerning Bob's integrity, that is hardly something I would like to take on trust.
Frankly, I regard this prototype business as a smokescreen thrown up by Mr Carver to obscure the fact that two experienced listeners-pace Mr Abmamichel-proved to him that they could distinguish a production M-1.01 from the amplifier to which it was widely and loudly proclaimed to be sonically identical. In essence, the Carver Corporation bas been claiming publicly that its M-1. 0t amplifier effectively offered $5000 worth of sound for $500--if $5000 A = one-off B, and B = 500 C, then A = C-and that was the point examined in the Stereophile review If we bad been unable to distinguish it, we would have said so; indeed, that would have been news! As it was, however; Bob Carver failed; that, no hint of malice, is what we reported.
Something we omitted from the review which, with hindsight, should have been included, was that our observations on the M-1.0t were not only made with the review sample sent to us by Carver They were also based on auditioning and measuring a representative sample M-1.0t borrowed from a dealer This latter produced a worse null against the reference amplifier than did the "official" review sample.
As far as Stereophile is concerned, the matter of the Carver Challenge is now closed.
Moreover; in response to Carver pressure with respect to the editorial content of Stereophile, we have suspended all business dealings with Carver Corporation, including the acceptance of advertising.
Who stole the bass?
Editor: In response to AHC's ridiculous comment regarding small monitor speakers in Vol.10 No.3: "virtually all of today's small 'monitor' speakers are reasonably incapable of high fidelity reproduction, and have no place in a decent audio system," I say that AHC is measurably incapable of listening to and reviewing high-fidelity reproduction and has no place writing for a decent audio magazine. I there fore suggest that Mr. Cordesman stick to re viewing for Audio magazine, where everything is "decent." Howard Butler Audio Advisors of NY Pure subjective garbage
Editor: I take strong exception to JGH's remarks that he was the first to invent subjective testing.
What he meant to say must have been that he was probably the first to publish an under ground magazine using subjective testing to mislead the consumer based on his personal opinions. I am not saying this was intentional, it has just developed that way.
Having had 40 years' experience of hi-fi, both as hobby and as a business, I can confidently say that you, Pearson, and our own Hi-Fi Screws and Rubbish Reviews magazine are continuing to mislead the consumer. Virtually everything Stereophile prints is pure subjective garbage, personal opinion, and has no real substance in scientific principles.
I can see from Sam Tellig's review of the Quad 306 in Vol.10 No.3 that Americans-not all, of course-do not understand what is meant by an amplifier having an output that is a larger facsimile of the input. You prefer bass-heavy amplifiers, and colored ones to boot-the Adcom 545, for example. However, I agree that the Quad 606 drives the "World's Best Loudspeaker for Music Lovers in the Home--the Quad ESL-63-slightly better than the 306.
I am surprised that JA can test any amplifier with the Celestion SL600s: they are very room-dependent, and although not box-like, are still a poor speaker. The American reviewer who gets it right is Len Feldman of Audio.
I am going to continue to expose you for what you are: a bunch of charlatans, as I say in my ads. The only "High" in high-end audio is the price. Readers, believe absolutely nothing written in any hi-fi magazine-you must listen for yourself.
Gerald Bearman Mayware Ltd Edgware, England A bad experience
Editor: I have had a bad experience with one of Stereophile's advertisers:
Sound Vision by Marc, 265 E. Eau Gallic Causeway, Melbourne, Florida 32937.
I ordered a pair of Quad speakers from this store on March 19, 1987. They were to deliver the speakers freight collect, and charged the price to my MasterCard. I called a week later, and they told me it had been delivered by special carrier. I called them again a week later and got a busy signal five times during different times of the day. I got the same response four days in a row. The telephone company in Florida could not give me the status of their phone. On the sixth day of trying, I got a message from the phone company that the telephone had been discontinued and was not in service.
At the present time I am writing to Master Card, trying to get credit for the charge Sound Vision by Marc so promptly made.
Please cancel their advertisement in Stereophile; it would be a service to your readers. I believe they also advertise in other magazines such as The Absolute Sound.
Russ C. Tongco
College Station, TX
Sound Vision by Marc has been cancelled by us as an advertiser though it was for non payment of advertising invoices-since that happened before we received your letter There bas been no answer at their phone for some time. We regret any inconvenience or loss of money occasioned our readers by response to this ad, and will keep you informed if we are able to get hold of the deadbeat(s) responsible
-LA Hafler on the Hafler test
Editor: Since J. Gordon Holt's description of the SWDT (straight wire differential test) in Vol.10 Nal, there have been many comments and inquiries directed both to Stereophile and directly to me concerning this test. I will try to answer most of these in this catchall commentary.
The starting point is that it is very difficult to compare amplifier A with amplifier B to make a quality assessment. The conventional A/B test, or any comparison, may indicate that the amplifiers are different, but how can one determine which is "best"? "Best" is a matter of judgment, and even though many golden eared experts think their ears are infallible, they disagree among themselves as to amplifier quality What is needed is an accurate reference against which an amplifier can be judged- a reference which does not involve the entire listening chain, from source to loudspeaker, but which is the equivalent of a perfect amplifier. That reference is the straight wire.
If readers recall the Carver "challenge," they read where Bob Carver put two amplifiers into a bridge configuration and adjusted one to match the other on a differential listening test.
In analogous fashion, the SWDT matches an amplifier to a straight wire rather than to another amplifier. This is precisely a com parison of the input of an amplifier with its output. If the input and output are the same, the amplifier is accurate. If they differ, the amplifier is inaccurate. This has nothing to do with the subjective reaction of whether the amplifier has "pleasant" or "natural" sound.
It has to do with correct sound.
To carry out the SWDT, the input signal is applied to one terminal of a headphone or monitor loudspeaker; through a variable attenuator, the same signal is fed to the input of the amplifier under test. The output of the test amplifier is connected to the other terminal of the monitor transducer. The normal loud speaker is also connected to the output so that its load becomes part of the test, but it must be moved to a separate area so that its sound will not interfere with the sound through the monitor transducer. The test signal for this arrangement must come from a low-impedance source: this can most simply be one half of the stereo amplifier. The attenuator is adjusted for minimum sound output in the monitor speaker while the input signal is set to normal listening levels in the load loudspeaker. The input and output conditions are precisely those which prevail in normal listening operation.
When the attenuator is adjusted for mini mum sound, the best "null," the signal in the monitor represents the difference between in put and output. If input and output are the same, there is no voltage potential across the monitor, and consequently no sound in the monitor. When the null is inaudible, it means that all of the distortions in the amplifier are inaudible; at that point, there cannot be improvement in the audible performance of the amplifier-until, perhaps, some new program sources come along which will be more difficult to handle.
The logic is irrefutable: if the input and the output of the amplifier are the same, the audible performance of the amplifier cannot be faulted.
Now let's consider the questions which have been raised about the SWDT.
Mr. Babrauskas convoluted arguments in Von() No.3 against low-source impedance amplifiers take me back some 40 years. He has discovered interface intermodulation distortion at a time when it is no longer of consequence. All present loudspeaker manufacturers use amplifiers which are essentially constant voltage devices to design and test their loud speakers, and modem amplifiers are all designed to give uniform voltage output into speaker systems in which the impedance varies with frequency With properly designed amplifiers with very low impedance output, the back EMF from the loudspeaker has no detrimental effect and does not affect the SWDT. An ac curate amplifier can still achieve a deep "null" despite Mr. Babrauskas' contentions.
Reg Williamson, whose opinions I very much respect, says that what we have done is not new but is the reinvention of the wheel.
I agree that differential testing is not new- I used it myself back in the 1960s to test components. However, has anyone else applied the SWDT to amplifier design to make an amplifier which has no audible aberrations? I have not seen any signs of that. I am aware, of course, of the excellent work done by Peter Walker of Quad in testing amplifiers with a differential bridge. Quad's approach was a laboratory approach, not readily applicable by the hi-fi hobbyist. The SWDT can be set up and checked with a minimum of equipment.' Quad also introduced phase and amplitude compensation in the straight-wire path to improve the null by making the amplifier and the wire more alike. This alteration-"bending," if you will-of the straight wire was done on the assumption that phase and amplitude errors are inaudible I prefer to make no assumptions as to audibility, thus making the test more stringent.
Mr. Williamson incorrectly faulted our arrangement in claiming that it was not operating under "normal working conditions of source signal and load." If he examines the arrangement carefully, I believe he will see that his conclusion was not correct. He also criticizes our tweaking of the amplifier to make it have a better match with the straight wire. I feel that it is standard good engineering practice to trim an amplifier for minimum distortion under actual working conditions. Matching the straight wire is exactly the same as trimming for minimum distortion.
--- 1. The David Hafler Company will supply a schematic and instructions for any reader who sends a stamped. self-addressed envelope with their request to: 5910 Creicent Boulevard, Pennsauken. NJ 08109
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There were several readers who have questioned the use of a driving amplifier to supply signal for the SWDT. They mistakenly believed that the characteristic of the driving amplifier is to provide a low-impedance signal at a level equal to the level desired in the test amplifier.
As long as the drive amplifier supplies a wide band, fast-risetime signal, its own distortion characteristics do not matter. It could be a noise generator or a spark gap, and the SWDT would still be valid. If the drive amplifier has distortion, then the test amplifier is being tested as to its capability to handle a distorted signal without adding further distortion.
Several people mis-analyzed the circuit as having positive feedback and assumed that this would affect the results of the SWDT. The connection of the monitor speaker from input to output of the test amplifier superficially appears to be a positive feedback connection.
However, if there were positive feedback, it would be reduced to insignificance by the low output impedance of the drive amplifier. Further, with an accurate amplifier, there is little or no current in the monitor speaker; it is, effectively an open circuit, not making a feed back connection.
The question of phase shift and time delay from input to output has been raised by several readers. That difference between input and output will prevent a good "null" and will pro duce residual sound. Obviously, a constant time delay (uniform across the band) will not produce an adverse audible effect. However, the residual from phase shift might mask other distortion components in the SWDT. I prefer to see, therefore, an amplifier design which is sufficiently broadband to minimize time delay in the audio band. This can be accomplished with some phase compensation.
While many people do not believe that phase shift is audible there is one place where it definitely has an effect, and that is between the two stereo channels. Unless the two sides track phase precisely, there will be problems of stereo imaging. The most practical way to have the left and right channels track is for each of them to have little or no phase shift.
The design efforts which make the Hafler XL-280 amplifier perform well on the SWDT give a side-to-side differential null of about 70dB. This preserves the stereo imaging better than on any other units we have examined.
The question has been raised as to why some subjective reviews have not always been consistent with the rankings of the SWDT. Preferences do not necessarily have to correlate with objective tests. A deep wideband "null" may not be necessary for euphonious sound, but it is sufficient to establish that the amplifier has no audible aberrations. Although there is high correlation between a deep "null" and good (accurate) sound, it is conceivable that there can be a poor "null" with pleasant sound. This can happen if the distortions are of a non-offensive nature, such as a small high frequency roll-off. It can also come from a listener's inaccurate mental reference standard as to what is accurate sound.
There is a philosophical question here.
Should an amplifier be pleasant sounding, or should it be accurate even if accuracy is not as pleasant? The SWDT reveals accuracy or the lack of it. It is the only way to assess accuracy directly and objectively with the amplifier operating under normal signal and load conditions. It does this in real time, with a dynamic signal source. It encompasses all forms of distortion. It requires no special instrumentation or skill. If the residual "null" is inaudible, the conclusion that the amplifier is accurate is unequivocal.
Can anyone suggest a more meaningful or practical test of amplifier accuracy?
David Hafler
The David Hafler Company, Pennsauken, NJ
What it's all about
Editor: I jotted down a few lines to express an appreciation for my husband's most cherished belongings. I wanted him to know that I did not view the left side of our living room as a messy menace anymore. Would you please print this? It may help some of your reader's partners to surrender and possibly enjoy the losing battle.
"Oh please stop soldering in the living room! I can't stand that smell. We're having company over and I'll scream if you don't hide that stuff along the side of our walls. Yes, that umbilical cord, moose cable, or whatever you call it. Honey, what are those brackets over the dining room table for? The rear channels?! To be honest, that black thing doesn't look beautiful to me, and why must we have speakers long enough to seat five comfortably? If we turn them sideways we'll have an extra couch. Oh, I'm sorry, I didn't mean to hurt your feelings.
Your system is perfect now? That's nice. Oh, well, I'd love to listen, just let me put these groceries away. Please don't look at me like that. I'll listen now. What's wrong with where I'm sitting? OK. This really makes a difference, huh? Yes, I'm ready.
"Close my eyes, are you serious? OK, OK.
Uh . . . It's like I am being held by this glorious music. It's as though I am perceiving this harmony both physically and emotionally. I can feel myself willingly surrendering every imperfect thought, allowing this perfect sound to take its place It's so warm and soothing. My mind is taking hold of the notes and wrapping my whole body in a melodious blanket. I don't really want to cry but her voice is so beautiful. Something deep inside of me has wakened; I feel as though some insensible feelings are being challenged to surface.
"This is like the best dessert I've ever tasted.
It's a kind of food that has not been provided before. I've caught what he has and now I'll want to consume it every day. I love it. Oh God, it seems right to speak to you now. No wonder they play music in church, it can sound so wonderful.
"Thank you for letting me enjoy it, and thank you for making me understand why he is so occupied with this mania. This is like an escape or maybe not so drastic. No, it's more like taking a vacation without actually leaving home. Oh, it's over.
"It sounded pretty good, Dear. Help me with these bags, will ya? As a matter of fact, hearing that made me decide not to pester you so much about the cost and the mess and all.
Just one question . . . How long would it take to put a system together for upstairs? I wouldn't mind if you wanted to hang a pair of minis in the hallway. What about some, uh, bookshelf speakers for the kitchen? Mind if I invite a few more friends over?... wait till Gilbert hears Evita!"
Debbie Romeyn; Novato, CA
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We regret that time does not permit us to reply individually to letters, particularly those requesting advice about particular equipment purchases. Were we to do this, a significant service charge would have to be assessed-and we don't have time to do it anyway! Although all are read and noted, only those of general interest are selected for publication.
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[based on a Aug-1987 Stereophile review article]
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