Non-feedback amplification topology and frequency
Atmasphere, as Mapman has indicated, you should be prepared to be criticized if you state opinions in absolutes. What you state are facts are contested by others. See below. http://sound.westhost.com/articles/distortion+fb.htm |
Atmasphere, is it so that a non negative feedback design does not have any odd order harmonics? If that is not true where does that leave us? I highly respect your dialog on this forum and I don't want to stop it so my question is a sincere one. I wonder if we are making too big of a deal with NFB in high quality amplifiers. If the overall design of the amp needs minimal NFB how does that in real conditions compare to a non NFB design. Isn't that a competent design then no matter what emitter device is used? Then I don't think we are talking about bright and harsh as noticeable differences are we? Arn't we really talking about competent designs and non competent designs then? Thanks again Atmasphere for your dialog. |
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... |
