Re: NFB101 part 3

Patrick Turner wrote:

Ian Thompson-Bell wrote:
Part 3 of NFB101 is now available for download as a pdf here:

http://www.ianbell.ukfsn.org/nfb101/nfb101.pdf

Comments welcome.

Cheers

Ian

After a very quick read of your work I concluded you are on a good
track,
but have some comments/questions.

How about making all your terms the same as in RDH4?

Models for ß x Vo, the fraction fed back and how it is derived looks
different to samples
used in 99% of cases in amps made now, ie,
the FB voltage is from a simple resistance divider, so there isn't any
reason
why the basic block drawing of FB application doesn't show the divider.


Except I would argue that in 99% of cases it ends up NOT being just a simple resistor divider. Most power amps have a compensation capacitor across the feedback resistor and every RIAA stage that uses feedback is not a simple resistive divider.

Where you have a block diagram with amp stages, why not have all stages
shown
as a triangle with +/- input and always assumed + output as in most
textbooks?


You may be right, but I thought real circuits with tubes in them would be more familiar to tube heads than op-amp like triangles.

With regard to tube amps and Rout after NFB is applied, the reduction of of Rout due to series voltage NFB is not just the same
amount as the gain reduction.

I am not clear what you mean here - series derived feedback would be current feedback. Voltage derived feedback would be shunt. Please clarify.

The figure for gain used in Rout calculations after Global NFB FB is
applied is the ( open loop gain of input and driver stages of an amp ) all multiplied x ( µ of the output tubes ) all divided by
the turn ratio of the OPT.

The figure used for gain for Rout calculations should be the open loop gain. I don't know enough about PP tube stages to know if that is the same as the formula you gave above.


The Rout without NFB is taken as the Rout at the secondary of the OPT,
which equals the total reflected P and S winding resistance of the OPT
plus the ( Ra-a ) / ( ZR of the OPT ).
Ra-a is twice the Ra of one tube of a PP pair, and the Ra must be determined for the idle Ea and Ia condition of the output tubes.


Agreed.

Basically, what I am saying is that Rout after GNFB is reduced by MORE
than the closed loop gain / open loop gain.


Is it? Why?

Cheers

Ian
Perhaps you could include a simplified explanation which conveys the concept of just how NFB works rather than just describe math.

See http://www.turneraudio.com.au/tube-operation3.html

Best Regards,

Patrick Turner.
.