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Patrick Turner Patrick Turner is offline
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Default Determination of Maximum Positive Feedback in Bootstrapped Driver

On Mar 19, 8:39*pm, flipper wrote:
On Fri, 18 Mar 2011 16:10:52 -0700 (PDT), Patrick Turner





wrote:
Delete unecessary argy bargy.


I remain correct about what I said,


No and I've proved it by quoting RDH4 itself.


OK, I agree to disagree.


I leave the outside world to decide who's right.


RDH4 is open to interpretation.


Delete more stuff that leaves nobody wiser.


I just ain't got time to arhue about what seems true for me.


Sorry Pat but electronics is not a Van Gogh to be freely 'interpreted'
by the observer.


I didn't mention Van Gogh. Only you did.



The confusion is coming from using PFB to enable 'something else' that
is beneficial, such as enabling GNFB in the example you cited. PFB
enables it but GNFB is what 'does the good thing'.


Youv'e got it. That's exactly what I was trying to tell you.

And RDH4 gave the figures to back up their claim about the schematic
they show.


Another example is John's boostrap, but it's 'something else' is
something else.


John's circuit has PFB with NFB. There's just enough PFB to
effectively increase the RL value seen by the 6SN7 so its THD should
reduce maybe 10dB. I say that because with a plain 27k you'd have 1%
THD at 10Vrms, but with CCS you'd have 1% THD at 50Vrms, depending on
the sample.

But the bootstrapping feeds back THD created in the OP tubes plus 6SN7
back through the R divider formed by Ra and 27k so that a fraction of
D appears at the OP grid to be re-amplified and add to D already
there.
some second order products are formed on this merrygoround. But the
THD measured with bootstrapping is the results after the merrygoround;
the artifacts don't keep on increasing infinitely.


If you look at my reply to John you'll see I simulated a 6SN7 plate
PVFB circuit where the gain was greater than mu so I went back and
measured the distortion.

Gain with no PVFB was 11.97 with 2'nd harmonic at -54.6dB

Gain with PVFB was 52.2 with 2'nd harmonic at -42.9dB

PFB increases distortion.


Sure, and you have applied far too much PFB.

But you need to measure an amp built like John's or a Mcintosh, and
vary the amount of PFB from between no bootstrapping to where gain is
say double µ. Do this at a low level of output and plot the THD for
same output voltage while adjusting input voltage to keep Vo constant.
Maybe the experiment might change your mind.
Minds were changed in the McIntosh lab well before 1950......

However, we can use the PFB to enable 'something else'; namely to make
Rl 'appear' as if it's infinite (bootstrap). If I do that, by
selecting the feedback ratio to match the expected plate swing, which
we can estimate by mu since an infinite load should make gain equal to
mu, then

Gain with selected PFB is indeed 20 and 2'nd harmonic becomes a
glorious -131dB. (an unrealistic number indicating the limits of the
spice simulation)


Huh, you jest, surely?



It's the special case (the 'something else') of Rl appearing to be
(near) *infinite* that lowers the distortion, not the PFB (which was
illustrated by the first example).


The increase in RL via bootstrapping value lowers 6SN7 THD but creates
a PFB path to make OP stage THD greater. Its about that simple.



By "not the PFB" I mean one could, in theory at least, put a 'real'
infinite resistor, with an infinite B+ to supply it, there and get
even better distortion numbers, because PFB increases distortion over
what the equivalent circuit has (and note than an infinite load is not
the same circuit as one with a 27k load), but that's not terribly
practical while PFB is.


I've done all this and have avoided bootstrapping wherever I can
because I wanted the full 5 steps forward benefit of driver linearity
without the 2 steps backward which PFB brings.
But bootstrapping can give you 3 steps forward.

One can easily arrange each side of a balanced amp to be a pair of
triodes in a µ follower config to make the DC carrying RL far larger
than 27k. Or one may use a solid state CCS. The cap coupled output
load is the biasing resistor, and its value becomes the main load for
the gain triode.

I found the use of a CT choke plus series RL at each end gave best
results.

http://www.turneraudio.com.au/300w-1...tput-jan06.htm

But people hate chokes, so they bootstrap instead !!!


We could also achieve the same result with a constant current source
on the plate (which also simulates a near 'infinite R') so it's
clearly not PFB that 'does the good thing'. It's the artificially
created (near) 'infinite load' that lowers distortion.


Indeed.



PFB enables use to do (in special cases) *'something else' (apply
GNFB, simulate an 'infinite R', etc) that would be more difficult to
achieve by other means.

In real life it won't be that good because my sim uses a 'pure' sine
for PFB and not a distorted OPT feedback.


Using any useful amount of PFB in mass market amps has been avoided
like the plague by all manufacturers.

Its just too difficult to get right.

Its SO EASY to just arrange a suitable amount of OLG and apply only
NFB.

Patrick Turner.



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