Non-feedback amplification topology and frequency

Jmacinnis, The issue here is how the amp will drive the speaker if it has no feedback. This article will explain what is going on:
http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php

IMO/IME amplifiers that operate without feedback are more musical as they make less odd ordered harmonic distortion. Since our ears use odd ordered harmonics to figure out how loud a sound is, this is arguably one of the more important human hearing rules! The use of feedback violates this rule.

However the audio industry does not like to talk about this, and is something that separates the high end audio community from everything else; we talk about it occasionally instead of never :)

Ultimately you have to be more careful of your speaker choice; this is doubly important because a speaker that requires the amp to have feedback will never be able to sound like real music- it will at best sound like a good hifi.

It really is that black and white. Sort of like being pregnant.

What is poorly understood is our ears are so sensitive to odd orders that we can hear things that cannot be measured. So it might look 'OK' on paper and our ears say something else.

And there is nothing we can do about it. Our ears are the product of millions of years of evolution. Its not going to change just to suit our inadequacies of design.

More often than not, I've found tube amps that use feedback sound more realistic and are more compatible with wider range of speakers than amps that use no feedback. I've looked into why audiophiles think no feedback is sonically superior, but I can't find any good reason to support this contention. Yes, NFB does increase odd order harmonics but the levels are low, and I doubt if anyone can actually hear them reliably. I've found most tube amps with no feedback to have less controlled bass, less dynamics, and subtle softening of the highs.

I did not state an opinion, but fact. Nor is it something I made up- this has been with us for a long time. You might want to read the article that I linked.

It is quite true that there are more speakers designed for amps with feedback than those without. And its also true that if you use an amp with no feedback (Power Paradigm) on a speaker that needs it (Voltage Paradigm) you will get a tonal coloration of the types you describe.

You are incorrect in stating

the NFB does increase odd order harmonics but the levels are low, and I doubt if anyone can actually hear them reliably

The fact of the matter is that slight increase in odd-orders are described as 'bright' harsh' 'hard' 'clinical' etc. by audiophiles, yet we are talking about distortion levels that may only be increased by a few 100ths of a percent! This is because the ear/brain system uses odd ordered harmonics to sort out how loud a sound is.

This is why two amps can measure flat on the bench, but one might sound bright and the other might not.

IOW the ear is considerably more sensitive to these harmonics than it is to human vocal frequencies.

General Electric proved this back in the 1960s- its not anything new.

Again I recommend you read the article and most of this will be made clear.

Onhwy61, No. Its simply the way it is. One example (if you read the article) is using transistors on ESLs. Its difficult for them to play bass, and often too bright.

Mapman, Correct- never is a long time. The problem is, we are not likely to change the way our ears work. We can do a lot of things in the medical world, but changing the system that our ears employ is not one of them.

Dracule1, I have no worries about criticism. If I had made the stuff up, that might be different. But I can point you to examples easily enough that will show that my article is not something new, but in fact something old:

Here is a link to a Google search: http://www.google.com/search?client=ubuntu&channel=fs&q=fisher+A-80+amplifier&ie=utf-8&oe=utf-8

The very first hit is a YouTube video image of a Fisher amplifier from the 1950s. The control is inscribed "Constant Voltage" at fully counterclockwise, "Constant Power" at noon and "Constant Current" fully clockwise. The control is a feedback control and balances voltage feedback against current feedback. At noon they cancel each other out (zero feedback). So to disprove me you have to disprove history. All my article does is collect the facts in a single place.

Marqmike, unfortunately all amplifiers make odd ordered harmonics. It is the 5th, 7th and 9th that we are concerned about (the 3rd is considered musical to the human ear). When you add negative feedback to any amplifier design, the result will always be an enhancement of odd orders, even if the feedback is successful in removing most of the open loop distortions ('open loop' means the amp operating without feedback).

The reason this is so has to do with the fixed amount of time it takes a signal to move through a circuit. This time is called 'propagation delay'. The issue is that a low frequencies feedback works pretty good, but as frequency is increased, the propagation delay means that the feedback signal is arriving later and later to correct the circuit. In fact a designer has to be careful with feedback because this effect means that at some frequency the feedback as actually positive rather than negative. This is entirely due to the propagation delay in the amp.

Further complicating things is the fact that the negative feedback analysis performed on amplifiers is done with steady-state signals like sine waves. Audiophile OTOH listen to chaotic, constantly changing waveforms arising from real music. If you think that the amp will behave the same with both signals you are incorrect- the amp will be more distorted with constantly changing waveforms than it will be with a sine wave!

If you analyze and amplifier using Chaos Theory, what we see is that the amp with feedback is considered a chaotic system. In fact the formula for feedback in an amplifier is the same as it is for a classic Chaotic System. However, an amp without feedback, thus analyzed does not behave according to the same formulae! This is a major difference between the two.

Interestingly enough, most of this has been known since the 1950s. Anyone interested should read Norman Crowhurst. He wrote a very interesting booklet on amplifiers and negative feedback. It is available as a free download off of the Pete Millet website. His writings apply as much now as they did 50 years ago.

The bottom line here is that amps without feedback, if otherwise designed to be linear, will have less odd-ordered harmonic content if also operated within their linear region. However, this may not mean what most people think so I will give a classic example:

We all know that SETs have a reputation for 'inner detail' and also that they are amazingly dynamic for their small power output. The reason this is so has to do with the way the SET amplifier makes distortion. At low power it essentially makes no distortion at all- this is the source of that 'inner detail'. But at powers above about 1/4 full power, the odd orders begin to make their appearance (masked by the 2nd, 3rd and 4th harmonics). The ear/brain system uses these harmonics to figure out how loud a sound is and since music has a lot of transients, the odd orders start to show up on the transient first. This means the loudness cues are on the transients of the music- presto! the amp *sounds* 'dynamic'.

The fact of the matter is that as a result, in about 90% of all audiophile conversation, the word 'dynamics' can be safely replaced by the word 'distortion' without changing the meaning of the conversation.

Sorry for the long post...

Mapman, they do, but even in high end audio we have to deal with facts occasionally :)

The reason I made that comment is that an amplifier that violates one of the most fundamental rules of human hearing will never sound like real music. Its not so much the failing of the speaker as it is the amp.

Mapman, I'm pretty sure that part of the reason this is so has to do with that article I link all the time:

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

If you happen to compare the two amps on a speaker that is in one camp, you are going to come away with a mistaken impression unless you are aware of the different design paradigms.

This is something the industry won't talk about. Its too inconvenient! And its why we have that darn objectivist/subjective debate. I am of the opinion that if the industry was transparent about this stuff it would be a hell of a lot easier to set up a system and a lot less money would go down the loo. But, OTOH the industry thrives on that, so maybe I'm trying to kill the goose that lays the golden egg huh :)