On Mon, 19 May 2008 15:31:51 GMT, Paul <paulguy@[EMAIL PROTECTED]
> wrote:
>On Sun, 18 May 2008 22:41:24 -0500, flipper <flipper@[EMAIL PROTECTED]
> wrote:
>
>.....snip!......
>>
>>I don't know how you got the idea that local NFB has no effect. Local
>>FB works just like global with the difference being how long the loop
>>is.
>>
>>Your basic gain vs distortion math is correct but it applies to any
>>FB, not just global. I.E. Apply local NFB around (or in) one stage
>>and the distortion of that stage is reduced in the same manner as
>>overall amp distortion is reduced by global NFB (or vice versa with
>>PFB). e.g. Take just a single triode. What would the gain be without
>>the Rk (using, say, fixed bias instead)? Now, what is it with the Rk?
>>That's how much NFB the Rk adds and distortion is reduced by that
>>amount. Or, conversely, that's how much increase in distortion there
>>is when you bypass Rk to get more gain on that stage.
>>
>>However, it doesn't do you any good to have a squeaky clean front end
>>and then feed it into a heavily (by comparison) distorted power stage
>>because distortion over multiple stages is the square root of the sum
>>of the individual squares so if one section is significantly higher
>>than the other it dominates the overall distortion figure (and by more
>>than if it were a linear relation****p).
>>
>>But you also have to have the gain in order to apply global NFB so
>>it's better to use some local PFB, to get the gain with some increase
>>in distortion of the squeaky clean stage and use that acquired gain to
>>apply global, thereby reducing the overall distortion.
>>
>>The fallacy some engineers fall into is observing that equal PFB and
>>NFB cancels so they conclude you gain (pun) nothing. I.E. You add
>>10dB, then subtract 10dB, and you're right back where you started. Or,
>>when not equal, why add 10dB and subtract 18dB when just subtracting
>>8dB gets the same result? And that would be true if it was the same
>>loop but what they miss is the PFB is local, over the much better
>>stage, while the NFB is global and, as mentioned, the combined
>>distortion is not linear but the square root of the sum of the
>>squares.
>>
>>Of course, there are other ways to achieve the same result but I get a
>>kick out of this one because, component wise, it's 'free' as the
>>difference (besides picking good component values) is simply
>>terminating the phase splitter into Rk rather than ground.
>
>
>Can I summarize this as follows? :
> A linear disturbance (no sqrt of sums of squares) would not benefit
>from juggling gains of local feedback paths within a global feedback.
I don't know what you mean by a "linear disturbance." If it's not
following the signal then, 'disturbance' being 'linear' or not, NFB
will work toward bringing it back in line. Otherwise, "linear
disturbance," it seems to me, would be gain (if it were 'perfect')
>This assumes that you haven't messed local gains up so badly that
>overloading or slew rate limiting occur.
> A disturbance that adds non-linearly (square root of sums of
>sqyares) WOULD benefit. That makes sense, you would want to linearize
>(more local feedback) the offending section, and boost gain (lower
>negative feedback, or POSITIVE feedback) in the other amplifier
>sections to make up the required overall global feedback or loop gain.
>
> Now here's MY problem..... it's a mathematical one. It's my belief
>that square root of sums of squares applies to uncorreleated signals
>like white or thermal noise. Distortion is correlated (it's a
>ploynomial base), and I don't think that the non-linear sqr root of
>sums of squares (RMS) applies here.
Correlated to what? What 'correlates' the distortion of a 12AX7 with a
6BQ5?
Following stages certainly amplify the distortion of prior stages but
they also add their own.
> For noise issues, yep, I have no doubt that juggling gains will
>help, but for non-linearity, it's still my belief that loop gain
>(overall) is what will reduce your distortion.
It will. That's not in question. It's that local feedback reduces (and
PFB increases) the distortion of that stage which you seem to be
disputing. That and how they add.
Can we agree that you can reduce a tube's distortion with local
feedback, as in it's Rk? Or any number of other means? (UL taps,
plate-grid feedback, and so on)
Well, if that's the case then if you reduce the distortion on each and
every stage independently there's no need for 'global feedback'. At
least from the tube distortion perspective (might want to for output
impedance, damping factor, and the fact distortion comes from more
places than just the tubes, like the OPT).
Conversely, if you can make each and every one better you can also
make them worse and both affect the overall distortion absent global
feedback.
> This is a tricky question, and it gets down to some hairy
>theoretical issues..... do you know for sure that non-linearity
>(distortion) is in fact added up in the sqrt or sums of squares? You
>can decompose nonlinearity into a fourier series, and treat the
>distortion as added sine waves, and they aren't random!
Maybe not in ONE tube but there's nothing that makes tube A's good, or
lousy, characteristics 'correlate' to tube B's behavior.
You'd have a better chance of arguing there's correlation between the
2'nd, 3'd, 4'th, and so on, harmonics in a single device (since
they're multiples of the 'same signal' and, supposedly, 'not random')
but those add as the square root of the sum of squares too. At least,
that's what the text books tell me.
> It was nice that you clearly backed up your assumptions, so I can
>argue the point without getting personal !
>
> Thanks,
>Paul
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