Amp/Preamp Asylum: RE: Moderate use of negative feedback by morricab Looking for a new Amp or Preamp? If you're after tubes, post over here.

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Posted by morricab (S ) on July 3, 2012 at 01:01:53

In Reply to: Moderate use of negative feedback posted by Feanor on July 1, 2012 at 05:45:40:

What you have to remember is that this very old plot, which originally comes from Baxandall, doesn't take into account orders higher than the 6th nor IM distortion. Why is that important? It is those two things that are the most detrimental to the sound. It is relatively easy to find FFT plots today of amps with a 1Khz tone (by far the simplest example) that have harmonics all the way to 20kHz!! And that is with just one frequency. Now, factor in all the IM distortion as well and you begin to get a nasty picture like in Nelson Pass's white paper with large concetrated peaks of distortion.

It is also interesting to look at an amp that has a very clean FFT plot but then to notice that the noise floor is 10 to 100x higher than another amp that has more discrete peaks. This is your high feedback amp most likely that has suppressed all of the lower order harmonics but has created a signal modulated "noise" floor that is really not noise(see the work of Norman Crowhearst for this one). This artificial "noise" floor is signal correlated and this means that it is not true random noise and will obscure low level signals, which has the effect of truncating instrument and room decay and soundstage. This is also partly why a good no feedback amp does the whole sense of space/soundstage thing better than most feedback amps.

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