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Difference bewteen ultralinear & triode mode?
Difference bewteen ultralinear & triode mode?
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Old 20th December 2012, 05:51 AM   #71
artosalo is online now artosalo  Finland
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Whatever your measurements, simulations or whatever say, it remains a fact that re-entrant distortion exists. The maths is clear.
I have not seen this clear "math" you refer and I have not seen any test results done with tube circuits that would prove the theory is correct.
Can you provide something more that supports your opinion ?

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Whether re-entrant distortion is significant is a separate issue.
To me this is the main issue. I am considering this subject at the practical situation only.

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I said people who worry about re-entrant distortion should avoid low amounts of NFB.
Why should they avoid using low NBF ? To just feel comfortable ?
Even low NFB seems reduce the level of higher order harmonics, not to increase. If there were new harmonics created the level of these would be at most negligible level, assuming 120...140 dB below fundamental signal.

I think people who worry about re-entrant distortion should open their eyes and see the reality, which surely is much better than they have supposed.

Below is a graph that mostly will be referred when this subject arises.
We all can now see that the reality is far from what the graph shows.

Click the image to open in full size.
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Old 20th December 2012, 08:39 AM   #72
artosalo is online now artosalo  Finland
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It seems that this subject was thoroughly discussed here some one and half years ago.
I was wondering why it felt so familiar:

Distortion spectrum vs feedback
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Old 20th December 2012, 09:29 AM   #73
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by artosalo View Post
Quote:
I said people who worry about re-entrant distortion should avoid low amounts of NFB.
Why should they avoid using low NBF? To just feel comfortable ?
*Facepalm*
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Old 20th December 2012, 10:14 AM   #74
artosalo is online now artosalo  Finland
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Facepalm ??

Not even Google translator could enlighten me about your comment.
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Old 20th December 2012, 10:27 AM   #75
DF96 is offline DF96  England
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Quote:
Originally Posted by artosalo
I have not seen this clear "math" you refer and I have not seen any test results done with tube circuits that would prove the theory is correct.
Can you provide something more that supports your opinion ?
Take a polynomial. Square it. Notice that it now has higher-order terms which were not there in the original polynomial. That is the maths (actually, this is a simplification - reality is even worse). If you understand how feedback works and how distortion is generated then that will convince you. If you don't understand these things then you first need to learn them before you can be convinced. It is not a matter of opinion, just fact.

Many real circuits are likely to have enough intrinsic higher order distortion that the effect will be hidden, as I keep saying. Not absent, just hidden.

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I think people who worry about re-entrant distortion should open their eyes and see the reality, which surely is much better than they have supposed.
I agree. I have admitted that at one time I did not agree. The point I was trying to make was that people who worry about re-entrant distortion caused by feedback should not suppose that they can have less of it by having less feedback. To get less of it they either need no feedback at all (to avoid re-entrant distortion), or lots of feedback (to suppress all distortion). People who understand distortion and feedback will not worry, but simply use the appropriate amount of feedback.
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Old 20th December 2012, 10:56 AM   #76
artosalo is online now artosalo  Finland
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I may be possible that doing the mathematical operations you mentioned the result will be as you said, but I am looking for the connection between the mathematics and real life.
So I look for any test results or any reliable reference that proves that this is also the case with vacuum tube amplifiers.

I found this:
http://www.its.caltech.edu/~musiclab...er-acrobat.pdf

Please study the pages 17...23.
The results achieved here conform with my earlier test results and later simulations.
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Old 20th December 2012, 11:30 AM   #77
DF96 is offline DF96  England
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The results in that paper are compared for equal input levels (see p22), not equal output levels. Therefore not comparing like with like.

You still seem to be asking for proof that feedback in valve circuits obeys the normal laws of mathematics. I have no answer which will satisfy someone asking such a question, because by asking the question they are denying the basis of the correct answer.
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Old 20th December 2012, 01:58 PM   #78
artosalo is online now artosalo  Finland
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Yes, I am still looking for information that will connect the math with actual tube amplifying circuits. Even one reference would be fine to see.
But I am not denying anything. It may be possible that low NFB can create high order harmonics, but I am wondering why I can not find those and why nobody else has not found them, as it seems to be.
It would be unusual if the behavior you claim to exist could not be found in any research or publication, if it really existed.

Unless you or somebody else have any new information to show, I am ready to finish this subject.
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Old 20th December 2012, 04:10 PM   #79
DF96 is offline DF96  England
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No new information, but I will have one last final try at getting you to correctly interpret the information you already have seen.

To keep things simple, assume an amplifier or device which is fairly linear but also has smooth non-linearity. This could mean a triode or BJT or a complete amplifier. All I am excluding is nasty stuff like crossover distortion or peak clipping. Put a signal through the amplifier. Coming out at the other end you will see the original signal plus distortion (plus some noise, but we will also ignore that). The distortion will go up to all orders, but we can typically only measure down to around the noise level. As the non-linearity is smooth the higher order products will be smaller than the lower order products. These all arise from the input signal being multiplied by itself by the terms in the in-out function of the device/amplifier. There is not a direct one-to-one correspondence: for example, a third-order term causes both third-order and first-order outputs, a 5th-order term gives 5th, 3rd and 1st-order outputs.

Now add some feedback. Even with low amounts of feedback the situation gets more complicated. The output now gets not just products from the amplifier distortion, but the amplifier distorting its own distortion. So at every level above 2nd, the output now consists of both intrinsic distortion and re-entrant distortion - but in both cases also reduced by the feedback. The re-entrant distortion can be the same phase as the intrinsic distortion and so add, or the opposite phase and so subtract and partially cancel. Then the result is reduced by feedback. Generally, the worse the amplifier was before feedback the greater will be the contribution from re-entrant distortion. Anyway, the net effect depends the balance between new distortion being created and all distortion then being reduced by feedback. An FFT of the output cannot distinguish between intrinsic distortion and re-entrant distortion; you just see the total at each frequency. That is why you 'cannot see it' - it looks exactly the same as intrinsic distortion. To see it clearly you need to start with a device with large low-order terms and very small high-order terms, so the effect is not hidden by intrinsic distortion.

Note that nothing I have said is device-specific. It applies to all amplifers and all devices. That may be why you have never seen a publication showing the effect for a particular device: there would be little point. The paper you referenced in your post 76 probably would show the effect if they had used equal outputs instead of equal inputs. As a rough estimate, take their results (e.g. on p22) and multiply each distortion product by 2^(N-1), where N is the order (or add N*6dB to each). I think you may then see the effect. They were using 6dB of feedback, so the input voltage seen by the device was halved. Doubling that, to get the same output, means that distortion products are potentially increased by 2^(N-1) (apart from complications created by changes of sign causing partial cancellation). Can't guarantee anything, as the device might have too much intrinsic distortion.
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Old 20th December 2012, 04:41 PM   #80
jan.didden is offline jan.didden  Europe
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I think Bruno would support that Doug.
Page 13 and following in particular.

BTW Nice article in AX.

jan
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