It's pro forma for audio reviewers to mention that the amp under review has so many hours of burn in time. Very often the review proceeds to describe the sound of the amp before and after the burn in period. Why do so many people experience this phenomenon, or think they experience it? From where does the belief arise? The answer repeatedly given here: if you expect to hear a difference, you inevitably will hear a difference. People's ears are led astray by their eyes, and also by biases and expectations. The placebo effect and expectation bias are real phenomena; that can't be denied. Ethan mentions comb filtering. These are all plausible explanations, but they have not been proven to be the major causes. A few posts back, Nick Charles asked Norbert Lehmann, the manufacturer of the respected and well reviewed Black Cube Linear Headphone Amplifier, about burn in. This was Lehmann's response:
[size=11pt]Dear Dr. Charles,[/size]
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[size=11pt]thank you for contacting me in this matter. I experience sound improvements in several aspects over time, at least for some weeks. To me it does not matter where the effects come from. What I guess to be very important is the overall thermal stability of the circuit. There are, however, _no_ differences to be measured at least with my Audio Precision ATS2 which is a very nice unit for production tests but no real state of the art unit like the 2700 type. [/size]
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[size=11pt]This is a rather unacademic approach for an engineer - but it helps. Sorry that I can't supply a more satisfying answer. [/size]
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[size=11pt]With kind regards from Germany[/size]
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[size=11pt]Norbert Lehmann[/size]
What a surprising statement! This person who designs and produces amplifiers, who would have more experience with amps than probably all of us on this thread, confirms that he experiences "sound improvements in several aspects over time, at least for some weeks." The statement is extraordinary. I expect for the average person to be taken in by the sound of a given amp, but I find it more difficult to believe that the guy who has the most experience with this particular amp, confirms that he also experiences an improvement "in several aspects" over a period of weeks. As a possible explanation, Lehmann guesses the "overall thermal stability of the circuit." He could not measure these improvements with the equipment on hand, though hastens to add that the equipment wasn't necessarily the best for the purpose. Placebo, expectation bias, comb filtering -- none of these sounds convincing in this case. The accusation is then made that the manufacturer is lying. Lehmann is keeping the myth of burn-in alive because it helps him sell more amps. I thought about -- and discarded -- that latter possibility. Just based on his note, I didn't get that impression. I believe it when he says that he's experiencing sound improvements. I keep going back to his statement about the overall thermal stability of the circuit. Can this stability vary or improve over time depending on how often the amp has been turned on or used, and why does it happen for only x number of hours before it reaches a certain stability? It basically has to do with how well it can conduct the signal?