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The DBT that proved all amplifiers sound DIFFERENT

It's "common knowledge" that DBTs, (double-blind tests), of properly functioning amplifiers reveal no differences amongst them. A few famous tests are quoted to demonstrate this assertion. Since most audiophiles know that amps do indeed sound different, many dismiss DBTs as irrelevant and become incensed at the mere mention of them. However they should not, because not all amplifier DBTs have, in fact, been negative.

The editors of Sound Advice magazine conducted a very rigorous DBT as well as other testing of a group of 19 "high-powered" amplifiers. Results were published in the summer of 1975 in Volume 1 Number 1 of that publication. I still have a copy of the magazine. The publisher of Sound Advice was Edward S. Wodenjak and the Editor & Technical Director was J. Peter Moncrieff. To my knowledge, the magazine was short-lifed, publishing only a few editions.

A total of 19 amplifier models were tested. Amps tested included ARC Dual 75, Crown DC300A, Dynaco 400, Harmon Kardon Citation 16, McIntash 2300, Phase Linear 400, Sony TAN-8550 (early FET design), and Infinity Switching Amp (SWAMP) 500; in some cases multiple examples of a given model were tested. Source material was original master tapes recorded using a Ampex 351-2 recorder and Neumann U-65 microphones. Listening was done through Stax SRX and SR3 and Koss ESP-9 electrostatic headphones, and KLH 9, Ohm F, ESS Heil Tower, and Infinity Monitor II speakers, (amongst others).

I am neither a scientist nor engineer, but it seems to me based on the detailed description of the testing procedures that very great, and apparently rigorous, pains were take to remove any experimental bias. All amps were very carefully match for volume to +/- .05 dB. It must be noted that switching devices were, in fact, used that permitted rapid switching between amps.

As well as A-B'ing amps against each other, the editors employed another blind test approach that they described as "straight wire bypass". In essence a line level signal was feed from the tape player through a "straight wire" to either headphones, or a reference amp and speakers; an alternate circuit could be selected that included the test amp. Hence testers could listen for differences that could be attributed to the test amplifier. The editors admitted that did limit the test conditions for the test amp to a very benign situation, but that the procedure did remove the tester's preference in judging the accuracy of the amplifier with respect to source.

The editors also defined specific set of descriptions covering various aspects of sound in order to minimize semantic vagaries. Complete neutrality, i.e. faithfulness to the source, was the declared benchmark for all aspects. To make a very long story, (as told in detail over 19 page in Sound Advice, clear differences could be heard among all of the amplifiers and these were report in some detail for each amplifier. The editors also rated the amps overall.

To me this Sound Advice report demonstrated that (1) DBTs work, and (2) that amplifiers do indeed sound different from each other. Testing of this scope and rigor just don't happen very often. Obviously it is complex and therefore costly to carry out. It isn't unreasonable to assume that Sound Advice folded simply because it couldn't afford to keep on conducting tests of the type of this one for amplifiers.

Based on the Sound Advice report I confidently purchased my Phase Linear 400 amp, the top-rated amp, which I used as my main amp for 25 years. With respect to the Phase Linear 400, the editors admitted the then already common dislike for this amp's sound. To quote the editor, "We're only human, and we admit to having shared this prejudice against the Phase 400. In a small way, it's measure of the objectivity of our blindfold testing procedures that this prejudice had no way of creeping into the results of our tests. We were utterly flabbergasted when we discovered that in our early A-B comparisons the amplifier which sounded dramatically cleaner and more revealing turned out to be, when we removed our blindfold status, the Phase 400. We obtained a second Phase 400 ... the two Phase 400's kept coming back to the surface. ... This, however, occurred on high quality program material: master tapes and low distortion discs. On the majority of discs, the Phase 400 does sound hard and unpleasantly bright. Unless you are skilled at listening through distortion to hear the added music which such a revealing amplifier will give you, you might want to write off the Phase 400 as a purist's amplifier."

Hence this old report demonstrated something else to me: that audiophiles commonly are not looking for accurate but for euphonic sound. And what is euphonic is not always high fidelity. We should bear in mind that when another audiophile reports that a piece of equipment sounds "musical", "involving", "toe-tapping" and any of many other such highly subjective terms, that he/she isn't talking high fidelity.

Bill Bailey
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Beauty? Or Truth?

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Topic - The DBT that proved all amplifiers sound DIFFERENT - Feanor 04:26:07 12/28/05 (161)
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