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Technical and scientific discussion of amps, cables and other topics.

RE: A Challenge To Measurements & DBT Proponents

Dear Tube Guy;

Only in audio would anyone think to ask that someone prove the "validity" of double-blind testing, or entertain the bizarre notion that hiding the identity of a source somehow deafens the listener. In every other field of subjective endeavor, from drug testing to wine tasting, if you can't tell what's what with your eyes closed, you're not an expert. (Well, I guess this doesn't apply to verifying the authenticity of a painting, but I digress.)

Still, I wrote an article for Stereo Review (now Sound & Vision) in 1991 that did just what you suggest. I had found a system where changing from a tube amp (which the owner preferred) to a solid-state amp seemed to make an obviously audible difference. I set up my ABX comparator with the special power-amp-switching module, and my supposition turned out to be true. The tests revealed that the owner and I could both hear differences between the amps on both music and test signals. On choral music, which is a nice broadband source, I got 12/12 correct just listening to the unknowns and writing down whether they were A or B; I didn't even have to make any comparisons.

There was a simple reason for this: The loudspeaker presented such a weird load of varying impedance to the amps that its response was very different with the tube amp -- up more than 1 dB everywhere below 100 Hz and down about 2 dB at 6k. The response with the transistor amp was essentially flat. Since this speaker was voiced using a tube amp (perhaps the very Audio Research model we were listening to), it sounded right with the inaccurate response, and too thin and harsh with an amp producing flat response. It needed the tube amp's high output impedance, in effect a tone control, to make things right. And sure enough, with a flat-response amp had just the kind of sound that you would wrongly attribute to the output devices. (Putting a 1-ohm resistor in series with the SS amp did almost the same thing, in case anyone wants to try that trick sometime.)

I provided response graphs of the amps in the article and discussed what was going on, which was not at all mysterious. If anyone can't find this piece (in the June, 1991 issue) and wants to read it, let me know. -- E. Brad Meyer


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