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Old 13th October 2009, 10:01 AM   #21
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So I understand how distortion cancellation with triode like devices work. But: How could it work with pentode like ones? The transfer curve of a pentode looks different to me - or is there an exponential slope in the tail that works for it???

Dirk
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Old 13th October 2009, 11:57 AM   #22
Vix is offline Vix  Yugoslavia
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Quote:
Originally Posted by ttan98 View Post
... no one teach this now.
Fortunately we have our Great Papa Guru here, so he is teaching skills that were once lost and forgotten...

Thank you Nelson!!
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Old 13th October 2009, 01:42 PM   #23
Magura is offline Magura  Denmark
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Thanks Nelson.

This file went directly in the "good read, will recommend to others" archive.



Magura
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Old 13th October 2009, 03:12 PM   #24
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Quote:
Originally Posted by HVfanatic View Post
So I understand how distortion cancellation with triode like devices work. But:
How could it work with pentode like ones? The transfer
curve of a pentode looks different to me - or is there an
exponential slope in the tail that works for it??
It doesn't require an exponential slope - just a slope that
rises with Plate voltage. You will notice the the Pentode
has a more linear characteristic to the slope than a triode,
and this is true of many of the solid state devices. Having
a degenerating resistor on the Cathode, Source or Plate
will help to get a better cancellation on this. For that
matter, if you have degeneration on a triode, it might be
helpful to use some Plate resistance.

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Old 13th October 2009, 03:14 PM   #25
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Quote:
Originally Posted by ttan98 View Post
If I am not mistaken reading the sweet spot article, as mentioned by Nelson, the valve/tube designers knew about this in the earlier 1930's by applying the Load Line Technique to obtain the minimum distortion and obtain the maximum output swing from the anode for a common cathode tube configuration.
And they seem to have forgotten about it when transistors
arrived.

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Old 13th October 2009, 04:49 PM   #26
CBRworm is offline CBRworm  United States
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Interesting stuff, Thanks.

I have learned more in this (Pass) forum than I did in engineering school. . . at least more that I can apply to my toys.

Of course I didn't pay this much attention in school.
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Old 13th October 2009, 05:19 PM   #27
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Nelson, degeneration on a triode means: The slope is not so fast rising as an exponential - correct?

But what I don't understand: If one slope is exponential, it requires another exponential to cancel, otherwiese the fourier transform would give differing terms that could not cancel... So pentodes give no perfect cancelation, only some forier terms are extinquished.

Wouldn't it be better to use a non resistive load for a pentode like element that provides fourierer terms in its behavior to cancel pentode terms???

BTW: Very instructive article. I always thougt, only 2 stages, one after the other could provide distortion cancellation...
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Old 13th October 2009, 05:28 PM   #28
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Degeneration on a triode means that you would be adding
a linear term to the curve, and it would not rise so fast.

There is no such thing as perfect cancellation - you take
advantage of what you are given. As long as the slopes
are in the same direction, you can get some cancellation,
it only remains to tweak the parameters to maximize this
(if you want to).

You can always load a gain device with another gain device
to try for distortion reduction. A mu follower would be a possible
example.

Lastly, you can indeed try for a sweet spot between successive
stages.

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Old 13th October 2009, 05:38 PM   #29
slam is offline slam  United States
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Just saw the Design & Construction of a "Beginner Amp" presentation listed in BA 09. This one two punch approach (presentation + the paper) is certainly a nice surprise. Papa always deliver in abandance for our asks.

Thanks Nelson for everything!

Stanley
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Old 13th October 2009, 05:46 PM   #30
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After reading the J2 thread and "sweet spot", I ask myself, if the 2 resistors in the J2 (where output signal is drawn) are choosen for lowest distortion. So in the end it's optimised for a special drive impedance - if we look for perfects results, we should flatten the loadspeakers impedance or add reactiove elements additionally to the resistive ones in the output stage to allow low distortion all over the frequency band...

Is impedance flattening of the speaker a "MUST" for the J2 output stage???

(Sorry for my silly questions, it's so damn interesting...)
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