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INSIGHTS INTO SE VERSUS PP TRANSFORMER DESIGN
Excerpt from Rec.Audio.Tubes newsgroup
By Mike LaFevre
 

A SE transformer has to do two things in life... carry the unbalanced DC and
have enough magnetic oomph to do the AC duties well.  PP outputs (largely so)
are relieved of the DC duties.  As a very general approximation the SE transformer will be four times as large as the PP transformer for equivalent AC power handling capability.

Designing a SE output transformer is the imposition of the transformer's gods'
fury.... look at the formula's for AC flux density vis-a-vis DC flux density.  What we find is that if we want to keep the AC flux density low then (all other things equal) we would want a large number of primary turns.  But, conversely, if we want the DC flux to be low (all other things being equal) we would want to
decrease the number of primary turns.

In part of the design process for SE... you must calculate what your AC flux
Consumption will be (at full power and lowest frequency of interest) and then subtract this amount from the maximum flux capacity for the specific core material that you are using. The amount left over tells you what the maximum DC flux could be.... if you wish to stay below the knee.  And this, provided only as a one approach to design, assumes that your willing (or feel it smart) to use all of the flux available.  And there are good arguments against this.

Another approach as explained in detail in Rueben Lee's book on magnetics is
that to maintain a high degree of inductive consistency... then your DC flux should be(I think it was... short of looking up the reference) 3 to 4 times the
magnitude of your AC flux. And remember in a magnetic component with both an AC and a DC flux component the effective perm is a function of the absolute values of each as well as the relative (to each other) ratio of fluxes.  As just one example... on one of our part's (NLA in the DIY community btw) the DC flux was approximately 9850 gauss while the AC flux was approximately 3000 gauss.  Rueben Lee demonstrates that if these conditions hold then the deviance in primary induction will be approximately 20% or so.
 

 Push-pull coils can (and perhaps should be) mirror-imaged around a centerline (both geometric and electrical)... so that each half has the same leakage inductance and each half has the same capacity. And given the "equal but opposite" (at least in theory) nature of the two halves of a PP transformer the coil design and it's geometry will be (or do I mean should be?) different than a SE output transformer design would be.

The SE output has one end of it's winding at AC ground potential and the other
end at "AC HIGH".... for push-pull we have two ends at "AC HIGH" and (let us say
informally) the center (the B+) is at AC ground.  This is a fundamental difference.... if I am making this clear enough in my writing..... so if we follow the AC voltage gradients in a PP transformer we will find that they are "equal but opposite"  in a mirror imaged way between the two halves of the coil.

SE outputs don't offer us this "degree of symmetry" (or at least not on first
blush).... here one end of the winding is at AC ground and the other end at AC HIGH.  So that as we traverse the coil, through each primary winding turn, the AC voltage potential is changing... at our interstices (junctions between primary and secondary windings) we then have certain voltage gradients... the gradients will be different across each interstice... and the capacities different.  So if you have a PP output that was optimized for a different voltage gradient and a different voltage potential at the interstices... then using it as a SE device the unit will not achieve the intended optimization that the transformer designer had wished to achieve.

There is a way to do a SE output transformer as a quasi-symmetrical design....
wherein you design it as a PP coil geometry but then reverse wind one half of the primary and put it in parallel with the other half. Using this technique a design can achieve the same level of "symmetry" in terms of the capacities being
equal about a coil geometric centerline....
 

POST 2

>  Mike, please excuse my technical misunderstandings in advance. In your
>post, considering symmetric winding geometries for SE transformers, you
>described a technique of taking two primary windings, as used in a PP,
>reversing one winding, and connecting them in parallel. Question: are
>the two windings not bucking each other, if paralleled out of phase?
>If an ordinary PP transformer was reconfigured this way, would DC
>currents in the paralleled windings be nulled out?

Several of the responses to my post thought that the example used a bilfilar
wound
primary winding.  Although perhaps it could, the illustration does not rest on
it being
bifilar wound.  And the language I used was imprecise enough to lead to this
confusion.

In the example.... what is happening is that you wind the whole primary
twice... using three
guages smaller than normal.   for instance suppose the primary needed 2,000
turns of #30.
then what you would do it wind 2.000 turns of #33 in P1 and P3 and another 2000
turns in
P2 and P4.  The example I had in mind derives from the Acrosound patent... and
is a modification
of that patent in that P2 and P4 are reverse wound and then placed in parallel
with P1 and P3.

Barry concerns about phasing... remember, and I always have to think this
through and on some
coils with tons more interleaving it becomes a real puzzle to keep the reverses
straight.... when
you reverse a winding the physical start becomes the electrical finish.  And
the reverse physical
finish becomes an electrical start.  so if you go through these reverses....
and their placement
in parallel with the non reversed windings (say P1 an P3) then the electrical
polarities
of the reversed windings... you'll see (I hope) that they are not bucking....

>Question: Can an SE
>winding symmetrically configured give true symmetrical cancellation of
>stray reactances, as reflected to the secondary? Its not clear to me how
>symmetrical, out of phase components could be set up in an SE winding,
>in order to effect a true cancellation. I hope there's a cogent question
>in here.

I used the term quasi-symmetrical moreso as an analogy to describe the physical
configuration of the windings and their electrical polarities in response to a
question
about the capacitances within a coil and etc.  In a single ended output...your
right....
no differential phases (the push and the pull) are present...

So the strategy in illustrating what I called a quasi-symmetrical coil geometry
in a SE output
is to achieve a minimum number of different voltage gradients across the
interstices of
primary to secondary and to achieve a "sameness" of voltage potentials across
these
interstices.  Wish you (and everyone who is interested in this topic) had a
copy of the Acrosound
patent and perhaps a diagram showing the winding sequences and etc.  Then it
would be easier to
explain...

Just another note.... the illustrations of coil geometries should not be
construed (on the basis of
my choosing it as an illustration)  as the method that
I use on our own products.  I actually use several different strategies for
interleaving in our own
products... but to fend off any misconceptions or speculations... I just wanted
to put this disclaimer in here....

POST 3

I have been threatening Joe Roberts to do some more
"Core Issues" in Sound Practices for quite some time
and this is one of the articles that I would like to do.

Anyone have access to the old SP where I did an article
on "how to pick a power transformer"?  In it there was
a discussion of current capability and how to evaluate
or, minimally, establish a context for assessing qualitatively
the numbers (the current ratings) which mfgr's spec out.

Part of the problem in power trans is that the "current" number
must be provided  in some sort of context if it is to have
meaning as a useful basis of discriminating btwn products.
My article tried to show that to compare "raw numbers" from
one company against "raw numbers" from another was often
like comparing "apples to oranges" as opposed to "apples vs.
apples".  My sense is that the same difficulty applies to especially
the comparison of raw DC current numbers which manufacturers
use for SE output transformers.  But.... least I get a head of myself...

Steve, I don't mind putting up some of my ideas and research on
"AC versus DC characteristics in SE and PP" as you stated above....
but I want to figure out how to structure the response so that it
goes in some logical order.  And I would be grateful for constructive
comments and criticisms since I want to turn some of this stuff (that
I have done over the past week or so) into an article for Sound Practices
or Positive Feedback.

so give me a day or two or three or four or..... yep.... probably never get it
done... so let me throw out a teaser and see if any of you RAT's can
"tell" me about this formula and what it means and what the variables
are and what is unique about some of the variables... it is one of the building
blocks of designing a SE output trans.... a focus and understanding of it....
leads, IMO, to clearer sight on some of the other issues that come after it.
 

energy factor equals  L times (Isubdc) squared divided by core volume

core volume equals L times (Isubdc) squared divided by energy factor
 

L is primary inductance--- unit measure is henries

Isub dc is the magnitude of dc current (and it is squared)---
unit of measure is ADC

core volume is the volume of the magnetic core being used--- unit of measure is
cubic centimeters
 
 

Mike LaFevre