DESCRIPTION:
I bought this hybrid tube amplifier and while it sounded good for about a year it started to deteriorate really fast. I did some modifications to it and detailed them here: http://projectrepair.blogspot.sg/2013/08/project-mistral-mm-1-hybrid-tube.html

The video shows the difference in sound before and after the modification. Take note that I recorded it with an external mic on the speakers and the sound quality in the video is worse than it actually is but you will still be able to hear the difference. Be sure to watch in the highest definition for the best audio quality.

You will hear that the fuzzy buzzing distortion that was present in the 'ooohs' are no longer present.

The song used for the test is "Untitled 3" by Alt-J. The relatively quiet passages in the song caused what I suspect is very bad crossover distortion in the output op amps or transistors. Changing the op amps fixed this issue, suggesting that the op amps were defective and not supplying the transistors down the signal line the appropriate voltage.

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*EDIT*: unfortunately I realize that youtube isn't exactly the best place to compare audio quality. the buzzy distorted vocals before the modification sound way worse in the .wav file i saved but I think youtube downsizes the audio quality to 128kbps or so, thus smoothing over the buzzy vocals and making the distortion not that obvious..but take my word for it that switching the op amps in this situation really helped rectify the problem.