Loop feedback in any form is supposed to reduce distortion. It is questionable whether it increases bandwidth, and in some models (see the link I provided) it reduces 'output impedance'. You'll see why I use the quotes if you look at the link.
The *big* problem is that loop feedback, in the process of doing all this stuff, exacts a penalty. This comes from the fact that any circuit that can amplify is doing so at speeds that are easily measured on rather pedestrian test equipment. This time is called Propagation Delay- the time it takes for the signal to propagate from input to output.
Now feedback is created by taking the some of the signal from the output, and applying it to an earlier portion of the circuit, which has a propagation delay. So you can see that the feedback signal is arriving ever so slightly too late to do its job right. The fact that it is too late causes the amplifier to become less stable as frequency increases, and there can be inharmonic noise created at the point where the feedback is returned.
This causes feedback to inject a low level harmonic distortion noise floor composed of harmonics up as high as the 81st harmonic into the output of the amp, and it has two audible artifacts.
The ear uses naturally-occurring odd-ordered harmonics to figure out how loud the sounds are. They are the 5th, 7th and 9th harmonics and they get enhanced (distorted) by feedback by a small amount. However, because these are loudness cues to the human ear this small amount **is easily audible** and audiophiles use the terms 'hard' 'harsh', 'bright', 'brittle', 'chalky', 'clinical' and so on to describe this distortion. Keep in mind that this is the case when the distortion of these harmonics may only be 100ths of a percent!! This is why two amps can measure the same frequency response on the bench but one will be bright and the other is not.
The 2nd problem is that the harmonic noise floor, through another hearing principle called 'masking', will block the ear's natural ability to hear into the noise floor of the playback system (the ear can hear 20 db into a natural noise floor like tape hiss or the wind blowing). Any information below the noise floor is not heard by the ear or not detected as easily. Since ambient soundstage information exists at low level, one of the more obvious effects of feedback is to foreshorten the soundstage depth and width.
Amplifiers in particular that use no feedback tend to have a different voltage response in dealing with the loudspeaker and the designer of the speaker has to accommodate this behavior. IMO, a speaker that requires an amplifier with feedback, due to the issues above, will never sound like real music. Speakers that *are* friendly to zero feedback amps at least have a chance.
see
http://www.atma-sphere.com/papers/paradigm_paper2.html for more information
The *big* problem is that loop feedback, in the process of doing all this stuff, exacts a penalty. This comes from the fact that any circuit that can amplify is doing so at speeds that are easily measured on rather pedestrian test equipment. This time is called Propagation Delay- the time it takes for the signal to propagate from input to output.
Now feedback is created by taking the some of the signal from the output, and applying it to an earlier portion of the circuit, which has a propagation delay. So you can see that the feedback signal is arriving ever so slightly too late to do its job right. The fact that it is too late causes the amplifier to become less stable as frequency increases, and there can be inharmonic noise created at the point where the feedback is returned.
This causes feedback to inject a low level harmonic distortion noise floor composed of harmonics up as high as the 81st harmonic into the output of the amp, and it has two audible artifacts.
The ear uses naturally-occurring odd-ordered harmonics to figure out how loud the sounds are. They are the 5th, 7th and 9th harmonics and they get enhanced (distorted) by feedback by a small amount. However, because these are loudness cues to the human ear this small amount **is easily audible** and audiophiles use the terms 'hard' 'harsh', 'bright', 'brittle', 'chalky', 'clinical' and so on to describe this distortion. Keep in mind that this is the case when the distortion of these harmonics may only be 100ths of a percent!! This is why two amps can measure the same frequency response on the bench but one will be bright and the other is not.
The 2nd problem is that the harmonic noise floor, through another hearing principle called 'masking', will block the ear's natural ability to hear into the noise floor of the playback system (the ear can hear 20 db into a natural noise floor like tape hiss or the wind blowing). Any information below the noise floor is not heard by the ear or not detected as easily. Since ambient soundstage information exists at low level, one of the more obvious effects of feedback is to foreshorten the soundstage depth and width.
Amplifiers in particular that use no feedback tend to have a different voltage response in dealing with the loudspeaker and the designer of the speaker has to accommodate this behavior. IMO, a speaker that requires an amplifier with feedback, due to the issues above, will never sound like real music. Speakers that *are* friendly to zero feedback amps at least have a chance.
see
http://www.atma-sphere.com/papers/paradigm_paper2.html for more information