I bought used Logitech Z-10 computer speakers. They can act as a USB audio device or accept line input, and they have a headphone output jack. Some music didn't sound right, so I played a frequency sweep and heard other frequencies and beat frequencies between them and the sweep. I uploaded a spectrogram of a frequency sweep. From the very start, some but not all odd harmonics are visible, indicating symmetric distortion. Then at higher frequencies there are descending images of those harmonics, indicating non-linearity and constant higher frequency noise mixing with the harmonics. The image of...
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