Some interesting reads

[Feanor]

So how does the "good" distortion mask the bad distortion? Especially given the fact that the magnitudes of distortion for line stages are typically in the 0.01-0.03% range if not lower. Unless of course this buffer design, unlike any line stage, has added deliberately high levels of distortion to achieve this effect. The theory would also suggest that this masking effect would hide the *real" distortion profile of sources upstream of the tubes. ...

But I think this masking does occur. People do report the "warmth, body, harmonic richness, depth, an 'organic' quality" resulting from the insertion of a simple, unity gain tube buffers. And I can reported based on my use of a tube preamp versus the passive preamp I used before.

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I don't find that any really good tube gear can turn a sow's ear into a silk purse.
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Of course I agree with this.