Why do audiophiles shun feedback in amplifiers?

You might say that the use of global negative feedback causes the amplifier to violate one of the fundamental rules of human hearing/perception: how we determine how loud a sound is. We do that through analyzing the odd ordered harmonics rather than processing fundamental tones.

So if the amplifier has trace amounts of the 5th, 7th and 9th harmonics added, it will not only sound louder than real music of the same volume, but bright/less relaxed as well.

This is one reason why two amps can measure perfectly flat with the same bandwidth on the bench, but one will sound bright and the other won't.

As Kijanki pointed out, global feedback reduces 'output impedance' (I put the term in quotes because it is a definition that is only used that way in audio!). So amps without feedback will have a higher output impedance. If coupled with a speaker that demands a lower output impedance of the amp, tonal aberrations may result.

So you can see that a more ideal combination might be such an amplifier with speakers designed to work with higher output impedances. Then you get proper tonality coupled with no violation of human hearing rules.

For more information see: http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php

The amp/speaker match is why I dropped this link earlier:

http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php

Mapman, if you have speakers that your class D amps are really happy with, there is the chance that those speakers may not be compatible with tubes.

Dracule1, 'tightness' in the bass is not a function of real music but is a function of an over-damped speaker. IOW its an audio system artifact.

Kijanki, I usually do not include degenerative feedback when I make my comments about the negative effects of loop feedback. This is because degenerative feedback occurs simultaneously with the signal i.e. there are no propagation delay issues.

I think it should be pointed out that the use of negative feedback to reduce IM distortion is a bad move. We get very low IM figures without using feedback- IM has a lot to do with power supply design, grounding and parts quality. I am of the opinion that is much better to design the amp to have low IM operating open loop, since IM will occur at the feedback nodes of the amplifier, resulting in a harmonic noise floor (rather than a natural hiss noise floor caused by component noise). The human ear can hear about 20 db into a natural hiss noise floor, but if there is a harmonic noise floor caused by IM (with harmonics and in-harmonc distortions extending to the 81st!), the ear will not hear below that point at all due to the way the masking principle works.

This is one way to get more detail out of an amplifier, since a harmonic noise floor will block the ear's ability to hear detail below that point.

Dracule1, I have played string bass since 7th grade. 'Tight' is one thing is isn't. Energetic- yes, detail- sure- its a real instrument. But you won't ever find one sounding 'tight'. We may have a semantic problem here; for me 'tight' is punch but little else. Sure, I want the impact, but I want the detail too, and that is something that lots of feedback robs from the bass frequencies- things 'stop' too quickly. IMO/IME its the bass detail and ambiance that goes away first as things go wrong in a stereo.

Kijanki, I would agree with you regarding what NFB is **supposed** to do, but if you also have propagation delay in the amplifier there is no way that the NFB is not mixing with a different frequency- thus the IM. You might want to read Norman Crowhurst- he mentioned this very issue in some depth about 55 years ago.

In a nutshell, low IM is a function of linearity in the various circuits of the amp. If there are non-linearities and NFB is applied, its not reasonable to expect that there will be no IM afterwards. Instead, while the IM will appear to go down, you will find that the energy of the distortion is spread out over the spectrum- that is to say it is by no means eliminated.

Chaos Theory does apply here. If you analyze an amplifier operating with NFB it basically is a chaotic system, complete with bifurcation (which we audiophiles call distortion) and a strange attractor (which interestingly, Norman Crowhurst graphed before Choas Theory was a recognized science!). The formula for NFB and a classic Choatic system are strikingly similar, if not identical.

When you use Choas Theory to analyze an amplifier, then it is easy to see how IM and NFB interact. Imagine a balloon on the floor with air in it, and then a weight placed on top of it; the balloon will squish out to hold the air before it bursts. An amplifier with NFB is similar- when you look at the open loop spectra, using NFB is like adding the weight to the balloon. The spectra expands across many harmonics, with inharmonic information added due to intermodulations at the feedback node (I am nearly quoting Crowhurst verbatim here but that is the succinct way of putting it). IOW the energy of the distortion does not go away nearly so much as we have been led to imagine in many school classes!

I listen to all sorts of music and even play electronic keyboards in my band.

'Tight' to me in its simplest definition is an artificial coloration imparted by overdamped speakers. You get a fast attack, but not so much body behind the initial thump. Depending on the amount of overdamped issues (no speaker is made that needs more than 20:1 BTW) this amount of body is variable. What I find is the low frequency ambient signature of the room is the first fatality to this problem.

Any tube amp can be made to have a 20:1 damping factor or more with enough feedback. Most transistor amps have considerably more. What I am talking about here is not really saying that the amp can't play bass right, but if there is no speaker that is not overdamped with that amp then its a moot point.

The head engineer of EV wrote a 2-part article about this back in the late 50s. You might think that somehow the physics that he was writing about went away in that time, but they didn't... about the only thing that is really different is that there are 4-ohm speakers now. If we are talking about a 4 ohm speaker, then the damping factor of the amp can be up to 40, as damping factor relates to 8 ohms only during measurement, whereas any speaker can be overdamped if its impedance is more than 20X that of the amp.

Learsfool, when you are not talking about orchestra musicians, the idea of long scale and short scale basses is well-known. A short-scale bass (electric, BTW) will have a neck about the same length as a guitar. A long-scale bass will have a longer neck, so the translation between the scale for a string bass player to a long-scale bass is about the same.

Long scale and short scale concepts are not used with classical instruments. But there *are* different sizes, at least with basses; I played a half-sized bass in jr. high, but my personal bass was a 3/4 size. As I understand it, full-size basses are rather rare- I don't think I have ever seen one. Most of the basses you see in orchestras are 3/4 size.