"Jim Candela" wrote in message
If
this is not practical is it feasible to just add a additional winding to
bias up the core (with DC current) to in essence buck the field from the
class A SE plate current? Has anyone looked
into this, or am I smoking from the wrong pipe?

Jim

Can put a second winding on the xfmr and drive with a current source
to buck the dc field. Has been done before, rather inefficient, about
25% max. I think. Some people try to convert P-P ampls. to SE this way
by removing inverted signal from one output tube. If you drive the
second winding with an inverted signal, you have the usual P-P output
stage of course.
Another way to get SE effect with a P-P xfmr. is to just use a
standard P-P amplifier design (70% max. eff.) but replace its usual
resistive global feedback network with a triode operated in reverse,
ie. ampl. output connects to triode plate (thru a cap. for isolation
from a HV plate current source), grid gets grounded, cathode provides
negative feedback signal back to ampl. input stage. Have seen this
done on some Japanese designs. The feedback loop essentially requires
that the ampl. output signal is the same as the triode would produce
SE from the input signal. (Keep in mind that grid to cathode voltage
is 1/Mu times plate to cathode voltage for constant plate current)
Neat trick.

Don

Jim Candela wrote:

I have long wondered if you can take a single ended output transformer with
"E" and "I" laminations, and bias the core with a high strength permanent
magnet? If so where would the poles of the magnet have to be located? If
this is not practical is it feasible to just add a additional winding to
bias up the core (with DC current) to in essence buck the field from the
class A SE plate current? I ask this because if this is doable, the SE
output transformer could get much smaller (same VA), and the "E" and "I"
laminations could be interleaved. Interleaving should raise the inductance,
and the frequency response would benefit. With no net DC field, you would
not need to gap the core to provide saturation control. Has anyone looked
into this, or am I smoking from the wrong pipe?

Some may say that hooch you been smokin is mighty powerful stuff,
but seriously, one could have permanent magnets included in the
magnetic core loops. There are some disc type of magnets which are
extremely powerful these days.
Don't ask me how long they'd stay magnetised,
or how effective they'd be at opposing the DC coil magnetism.
Try building it to find out.

Patrick Turner.



Jim

smoking-amp wrote:

"Jim Candela" wrote in message
If
this is not practical is it feasible to just add a additional winding to
bias up the core (with DC current) to in essence buck the field from the
class A SE plate current? Has anyone looked
into this, or am I smoking from the wrong pipe?

Jim

Can put a second winding on the xfmr and drive with a current source
to buck the dc field. Has been done before, rather inefficient, about
25% max. I think. Some people try to convert P-P ampls. to SE this way
by removing inverted signal from one output tube. If you drive the
second winding with an inverted signal, you have the usual P-P output
stage of course.
Another way to get SE effect with a P-P xfmr. is to just use a
standard P-P amplifier design (70% max. eff.) but replace its usual
resistive global feedback network with a triode operated in reverse,
ie. ampl. output connects to triode plate (thru a cap. for isolation
from a HV plate current source), grid gets grounded, cathode provides
negative feedback signal back to ampl. input stage. Have seen this
done on some Japanese designs. The feedback loop essentially requires
that the ampl. output signal is the same as the triode would produce
SE from the input signal. (Keep in mind that grid to cathode voltage
is 1/Mu times plate to cathode voltage for constant plate current)
Neat trick.

Don

This triode use for a feedback divider network IS interesting,
since the FB network has the inverse linearities of the SE output tube,
so I assume its VOLTAGE cancellation of the 2H distortion,
rather than the CURRENT cancellation that is used in PP amps to
get thd low, but which is then all oddorder thd.

The odd order thd in SE amps is low, and very low where
we want to listen, during the first few watts, so any attempt to
use voltage cancellation of 2H could result in less thd than a PP amp.

I am completing 45 watt SE amps which have 4 x 6AC7,
but use CFB in the output stage, ( 'acoustical' ) and the
thd is 2% without global FB at 45 watts, and mainly 2H.
The drive voltage is about 50vrms, so if the driver, a 12AU7,
makes 2% of 2H, then there is nearly no 2 H in the output.
A small amount of global FB is then required
to reduce Ro to a usefully low value, and not to reduce thd,
which has already been achieved, although the residual thd
after the cancelling technique has been applied is also recuced
further to astonishingly low levels.

I am yet to confirm how it all sounds, but I expect no ugly surprises.

Patrick Turner.

I have never seen a good answer to the permanent magnet
proposal.

The additional bias winding needs a little thought on your part.
The key point is that in order to maintain a constant DC current,
you will need a constant current source able to handle the voltage
variation that will appear across the winding, induced by the signal
on the primary winding.

Let's say you have an output stage passing 100mA and swinging
300Vrms. If you use the same number of turns on your DC winding as
the primary, then you will need the same current, and to keep it
constant you will need to swing the same number of volts. With half
the turns you would only swing 150V, but you would require twice the
current.

Whatever circuit you used to do this would be playing such a large
role that you would have to use a valve, or call it a hybrid amp if
you used mosfets, for example. It would be questionable also
whether you could properly call the stage "single ended".

I suspect there is a name for such a stage. A pentode CCS and a SE
triode would be an obvious choice. But the pentode would not be a
perfect source, and would add some of its own characteristic
distortion that might defeat the object.

Back to the permanent magnet. Seems to me a tricky one. The magnet
would have to occupy a complete slice of the core. If it didn't
then you would have a local flux circuit around the magnet, and the
rest of the core would be unaffected. Someone should correct me if
I am wrong. That would amount to a gap, considering that a
permanent magnet would by nature be relatively unaffected by the
induced field.

cheers, Ian

"Jim Candela" wrote in message
. ..

I have long wondered if you can take a single ended output
transformer with
"E" and "I" laminations, and bias the core with a high strength
permanent
magnet? If so where would the poles of the magnet have to be
located? If
this is not practical is it feasible to just add a additional
winding to
bias up the core (with DC current) to in essence buck the field
from the
class A SE plate current? I ask this because if this is doable,
the SE
output transformer could get much smaller (same VA), and the "E"
and "I"
laminations could be interleaved. Interleaving should raise the
inductance,
and the frequency response would benefit. With no net DC field,
you would
not need to gap the core to provide saturation control. Has anyone
looked
into this, or am I smoking from the wrong pipe?

Jim

(smoking-amp) wrote in message . com...
Another way to get SE effect with a P-P xfmr. is to just use a
standard P-P amplifier design (70% max. eff.) but replace its usual
resistive global feedback network with a triode operated in reverse,
ie. ampl. output connects to triode plate (thru a cap. for isolation
from a HV plate current source), grid gets grounded, cathode provides
negative feedback signal back to ampl. input stage. ....

Oops!, constant current source goes in the feedback triode's cathode
circuit, not the plate circuit. Constant current out of the cathode
wouldn't be too useful as feedback the other way. Cathode voltage then
has to connect to a high impedance NFB point of the amplifier (ie.
LTP/diff. amp. input grid) Probably can just use a high value cathode
resistor to neg. supply for effective current source.

Since we have to use a LTP/diff. amp. input stage anyway, a little
more late night smoke-filled amplifier thinking might lead one to try
just connecting the neg. feedback (with isolation cap.) to the plate
of the inverting LTP tube and ground its grid. However, the LTP will
no longer be providing both inverted phases now, so will need another
inverter stage off the other LTP plate to get that for P-P I think.
One could tube-roll different feedback triodes in either scheme for
different sound. Trying triode feedback in an existing SE amplifier
should have some interesting possibilities too as Patrick suggests,
can trade off harmonic cancelation effects.

Forgot to mention parafeed as another more common way to use a P-P
xfmr. for SE, but that just moves the DC current problem over to the
choke.

You know, I have never understood this claim. Let us look for a second at
RDH4, p230, Fig 5-16.

Using this chart, who can explain this "crossover distortion" theory to me?

Any valiant souls willing to try? No, I don't have a copy of Morgan Jones,
but surely this effect is explained in more than one reference!

Thanks in advance.

"David R Brooks" wrote in message
...
As I understand it, one of the reasons SE sounds so good is that the
core is biassed away from the zero-flux point. Essentially, cores are
subject to a kind of "crossover distortion" just like Class B PP
outputs. The DC component pushes the operating point out on to the
"linear" part of the B-H curve.
See Morgan Jones, Fig. 5.3


Engineer wrote:

:Jim Candela wrote:
:
: I have long wondered if you can take a single ended output transformer
with
: "E" and "I" laminations, and bias the core with a high strength
permanent
: magnet? If so where would the poles of the magnet have to be located?
:
:I'll pass on that one... interesting idea, though. Hmmm...
:might use a N-S disc ceramic (poles on face) on a cut back
:centre leg of the EI laminations, or two on the outer legs.
:Think I'll still pass...
:
: If
: this is not practical is it feasible to just add a additional winding
to
: bias up the core (with DC current) to in essence buck the field from
the
: class A SE plate current?
:
:Let's see... Perhaps use the same number of turns and a
:spare same type tube for this so that we can set the same
:current easily for the same DC MMF. OK, so far. Wait a
:minute... if we added a phase-inverter and drove this spare
:tube with it we'd have a PP output stage! Result, more
ower and less distortion. Add lots of NFB with gain and
hase margin correction and we'd be well on our way to a
:"straight wire with gain" out to over 25 kHz and a higher
:damping factor on the speaker.
:
:But, perhaps I've missed the point. There are people out
:there who actually like SE amplifiers. Not me, I'll
:admit!
:
: I ask this because if this is doable, the SE
: output transformer could get much smaller (same VA), and the "E" and
"I"
: laminations could be interleaved. Interleaving should raise the
inductance,
: and the frequency response would benefit. With no net DC field, you
would
: not need to gap the core to provide saturation control. Has anyone
looked
: into this, or am I smoking from the wrong pipe?
:
:No, just one tube short of a power stage. g
:
:Cheers,
:
:Roger
:
:
: Jim

"BFoelsch" wrote in message
...
You know, I have never understood this claim. Let us look for a second at
RDH4, p230, Fig 5-16.
Using this chart, who can explain this "crossover distortion" theory to
me?

As it crosses through zero, its slight tendancy (depending on material)
to remain magnetized consumes energy (hysteresis loss) and causes a
slight bias you could say, as it crosses through zero. Thus on falling
back from the positive peak, as H crosses through zero, B is still a
bit positive. The opposite will happen after the negative peak.
H is the input, and is determined from the current of the primary winding;
if at a constant frequency, then current can be canceled out of the
equation and we can concentrate on the voltage across the winding instead,
if it matters.
B is the output, the strength of the magnetic field as it runs in the core.
Divided by the number of secondary turns, it represents the output voltage.

Standard disclaimer: I'm tired so I'm probably wrong. Someone proof-read
for me.

Tim

--
In the immortal words of Ned Flanders: "No foot longs!"
Website @ http://webpages.charter.net/dawill/tmoranwms

"BFoelsch" wrote in message ...
You know, I have never understood this claim. Let us look for a second at
RDH4, p230, Fig 5-16.

Using this chart, who can explain this "crossover distortion" theory to me?

Any valiant souls willing to try? No, I don't have a copy of Morgan Jones,
but surely this effect is explained in more than one reference!

Thanks in advance.

"David R Brooks" wrote in message
...
As I understand it, one of the reasons SE sounds so good is that the
core is biassed away from the zero-flux point. Essentially, cores are
subject to a kind of "crossover distortion" just like Class B PP
outputs. The DC component pushes the operating point out on to the
"linear" part of the B-H curve.
See Morgan Jones, Fig. 5.3

I have both books here so I will try to connect info in the diagrams.
Fig. 5.3 in M. Jones is similar to diagram 5.15 (not 5.16) in the RDH
but with only the dark curvy line for flux density B vs. H drawn, and
symmetricized to both polarities of magnetizing force H (take dark B-H
curve and rotate around zero point 180 degrees, or just negate B and H
to get other half, like what's done for composite P-P tube
characteristics)
I suspect that this claim for SE advantage got started from the idea
that the flat part of the B-H curve where permeability U is greatest
(dotted curve in 5.15 RDH) could be exclusively operated on by DC
biasing in SE operation. And it's TRUE that P-P operation has a
problem with low permeability with low level audio signals (of course
no one gives any suitable data on manufactured audio xfmrs, usually
the peak inductance is quoted, if at all, one really needs the minimum
initial figure, easy to measure with an LC meter though, 5.9H versus
300H peak for a Hammond 1650T). (Ferrite or NiFe/permalloy core
material are the usual fixes for low "initial" permeability)
But it is NOT true that SE biasing fixes this. In fact, it causes
the transformer to operate with uniformly lower permeability than the
low U point in P-P due to the air gap required to prevent saturation
in SE. Even without an air gap, it turns out that magnetic materials
just have another low permeability spot at the average operating
point, so one PAYS DOUBLY in SE. (Thats why SE xfmrs have to be so
big) The reason is, that to flip magnetic domains in the material
requires a certain minimum "force" ie H or current to get them to flip
due to coercive "friction" in the material. So the AC part of a DC
biased SE signal still has to exceed a certain minimum to get flux
change going just like in P-P operation around the zero H point. In
other words, it doesn't matter how many domains are tipped one way or
the other at the average operating point, it still takes a threshold
magnetization force to get any more to change and then they almost
avalanche above this level causing the high permeabilty peak. So, one
should take the flat spot at zero H in the M. Jones curve and just
move the WHOLE CURVE over to the new DC bias point. One ALWAYS
operates out of the crummy initial permeability (H=0) spot under
steady state averaged conditions, P-P OR SE.
(regarding fig. 5-16 in RDH: the curved line with arrow is the initial
magnetization path, its starting slope is less than the max slope of
the steady state hysteresis loop shown, accounting for the lower
"initial" permeability at small signal levels. When DC biased, this
type of curve is re-established around the new operation point, called
"minor hysteresis" loops in magnetics books. See fig. 7.1 in
"Transformers for Electronic Circuits" by Grossner, 2nd ed.)
There ARE a couple of ways around this problem though. One elegant
solution is the David Berning output stage reviewed in Vol. 12 No. 1
yr 2000 Glass Audio. The high frequency employed in its switching
scheme effectively acts like the HF bias field in tape recorders to
get rid of hysteresis effects. The high frequency signal exceeds the
magnetic material coercive force (or friction) threshold so any audio
signal component gets to operate in the "easy" high U region. A small
technicality here is if the audio signal is tiny, so is the HF signal,
but the use of ferrite material here with its high initial
permiability solves this nicely.
The other way is to use two complete transformer core sections. The
audio signal windings use both sections as if they were one core. An
additional HF winding is placed on each of the two cores and connected
in series, but with one core polarity reversed with respect to the
other, so the HF cancels out in the audio windings. This is similar to
how some magnetic amplifier cores are wound. The HF windings get a HF
current from a power oscillator of sufficient magnitude to overcome
the coercive forces, once again acting like the HF bias used in tape
recording. I have tried this technique on some standard laminated E-I
cores and it works poorly, since the HF signal has to be large, above
audio frequencies, and hence causes huge losses in the cores. But on a
ferrite setup it should work pretty good. Only problem is that
ferrites have only about a quarter of the saturation flux of xfmr
steels so takes an enormous core to handle the audio signal component
(try 4 or 6 six inch diameter cores stacked up from Fair-Rite
Products,$50 ea.), so not very practical. The Berning design gets
around this problem since the audio is modulated on the HF then
demodulated again, so no actual audio frequencies appear in the cores,
all high frequencies. Xfmr can be small.
Have to catch my breadth, sorry about so long. Don

"smoking-amp" wrote
...[loads of seemingly sensible stuff about B and H]...

Thanks, Don. I almost posted my own essay on domains, but yours
probably makes more sense. I got to the paragraph where I was
explaining what we both call a kind of "avalanche" effect. I was
having difficulty there because it is a stable process, not like a
real avalanche that sustains itself with no further excitement.
Then I got to the bit where the curve levels off towards saturation
and couldn't remember why. Still can't. Similar to a population
curve...these sigmoids arise from some fundamental contradiction but
dunno which one in this case. Glad you got me out of that one.

I remember an experiment in which the secondary of a transformer was
monitored with sufficient resolution to see the individual flips.
Rather like a sloping line on an old low-resolution computer screen,
it is fairly clean at 45 degrees, but very lumpy at shallow angles.

But I don't get your argument that the effect centres itself around
any bias point. I can see that there will be hysterisis about any
established point, but the curve is steeper so it should be less,
surely? Flipping a domain is easier or harder depending on what
proportion is already flipped.

Van der Veen says don't worry about it...nearly all the effects
cancel out with a decent transformer.

cheers, Ian

Tim Williams wrote:

"BFoelsch" wrote in message
...
You know, I have never understood this claim. Let us look for a second at
RDH4, p230, Fig 5-16.
Using this chart, who can explain this "crossover distortion" theory to
me?

As it crosses through zero, its slight tendancy (depending on material)
to remain magnetized consumes energy (hysteresis loss) and causes a
slight bias you could say, as it crosses through zero. Thus on falling
back from the positive peak, as H crosses through zero, B is still a
bit positive. The opposite will happen after the negative peak.
H is the input, and is determined from the current of the primary winding;
if at a constant frequency, then current can be canceled out of the
equation and we can concentrate on the voltage across the winding instead,
if it matters.
B is the output, the strength of the magnetic field as it runs in the core.
Divided by the number of secondary turns, it represents the output voltage.

Standard disclaimer: I'm tired so I'm probably wrong. Someone proof-read
for me.

Tim

Tim, don't be lazy, do your own proofing.

Anyway the "hysterical" nature of the iron means it laughs and cries
uncontrollably when excited by a magnetic signal from a coil
of wire with a wave form in it. You wave and I'll cry, or maybe I'll laugh.

But seriouosly, the iron does not change its level of magnetism
linearly with applied voltage to the coil.

If you didn't know what was inside a black box containing the primary
of a tranny, you'd swear blind that what was inside the box was an inductor,
but one with non linear impedance, whose value changed
with applied voltage, as well as with applied F, which would indicate
it was a coil of wire, ie, an inductance.
The value of the inductance changes with voltage.
At real low voltage, L is quite low, a typical good OPT
has 100 Henrys at 5 volts rms and 50 Hz across the primary.
But at 500vrms across the coil, the L = 600H.
During the voltage cycle, the inductance value changes.
So, we have a load connected to the tubes of the amp, and the
inductance of the OPT is connected in parallel with the load.
At 50 Hz, the impedance of the L varies from say 31k to say 186k
( say 100H to 600H ) so if the R part of load was 5k the actual total load
varies
from 4.3k to 4.87k, and the result is 3H distortion if the driving impedance,
ie, the dynamic plate resistance, Ra, is high.
Pentode amps are worst in this regard, since Ra-a is so highm maybe 60k ohms,
but with triodes, the Ra = maybe 3 k a-a, so the inductance is also shunted
by the low Ra, so the L's dynamic change of value has the least effect,
and one which is cancelled by the 3H of the PP output circuit.

The 3H of the tranny will show the peaks of the 3H making the
fundemental more peaked, wheras the the 3H distortion in the tubes
when ideally biased for classAB with lots of A is that the
3H phase is opposite, and tends to make the wave form flattened
slightly, or compressed.
If the tubes are biased for nearly class B operation, like a lot of
guitar amps, the 3H becomes like that of the transformer,
and adds to the amplitude peaks of the wave.
The amount of 3H harmonic product we see as a result of the iron is
usually a lot lower than the tube 3H especially in the case of
triodes.
The type of thd seen as a total tube plus iron combination is like crossover
distortion.
There are low amounts of other odd order distortions present, 5H,7H, and 9H.

In interstage transformers, it isn't unusual to see perhaps 0.1%
of 3H, and this amount is highest at low voltages, right in the listening
region
of operation.
That's because of two factors, one being the high impedance
drive to such a transformer, perhaps 10k from a 6SN7 plate circuit,
and the fact that the inductance change between 0.0v across the coil
and say 5v, the inductance change, and the coil impedance
varies more than from above 5v to say 50v!
This thd is harder to avoid, so somewhat special core materials
and using a gapped core, even in a PP tranny, along with using
a lower Ra driving tube, perhaps a cathode follower, will suppress the
variations in L and the distortions.
A graph of the iron caused distortion with applied voltage shows
a rapid rise in distortion to perhaps 0.1%, then a slow fall as voltage is
increased,
until a rise again when saturation begins to cause distortion.
This is what happens at mid frequencies, even at 1 kHz.
At low F, the effects are similar, but the saturation distortions
start earlier.

The inductors used with audio do have to be designed carefully.
using too few turns around a core taken from some old crummy power
tranny is a recipe for high thd, and it spoils what might otherwise be a nice
outcome.
The thd of interstage iron is not able to be removed by NFB, since
it rarely is included in a NFB loop.
The OPT is included in a loop of NFB, so its thd, and that of all the tubes,
is reduced by the NFB.

Its for the reasons mentioned, that Williamson designed his famous
amp with R&C couplings, and not with interstage transformers,
which are simply too riddled with response and distortion problems
and phase shift and cost to bother with.
He wanted a high amount of NFB, 20 dB, which was high for 1947,
and seems high for today, and its impossible with an IST included.

Having said all that, many recordings were made on equipment
with transformers used often and they sound OK,
but there is a line one has to draw for what's permissable distortion.
0.1 % is where I draw that line, and if whatever your listening to has
less than that much thd, your'e lucky.
Transistor amp systems today measure better but often sound worse than
many tube sets, and crossover thd in the output stage isn't
the only reason for the bad SS performance.
The similar "crossover" distortions from the iron
seem to have less effect than the SS type of distortions,
but there is no thd that improves the sound!

Patrick Turner.



--
In the immortal words of Ned Flanders: "No foot longs!"
Website @ http://webpages.charter.net/dawill/tmoranwms

We found that if we interleaved grain oriented EI lamination
2 x 2 or even 3 x3 we improved the low frequency performance
of the PP output transformer. We measured the L at 0 DC and then at
5% of tube DC current and drew the line to see what the presumed
loss of L was as opposed to the "NoDC unbalance" current. The
hi-fi folks like HK, Fisher, Marantz seemed to like our concept
so we continued making them with 3x3 interleaving. I have the
curves somewhere in my files. What is the plate current tolerance
on output tubes today. I always presumed a 5% unbalance, that
is what got me on the kick of looser interleaving..

Gerald

On Sun, 24 Aug 2003 13:40:50 -0500, "Tim Williams"
wrote:

"BFoelsch" wrote in message
...
You know, I have never understood this claim. Let us look for a second at
RDH4, p230, Fig 5-16.
Using this chart, who can explain this "crossover distortion" theory to
me?

As it crosses through zero, its slight tendancy (depending on material)
to remain magnetized consumes energy (hysteresis loss) and causes a
slight bias you could say, as it crosses through zero. Thus on falling
back from the positive peak, as H crosses through zero, B is still a
bit positive. The opposite will happen after the negative peak.
H is the input, and is determined from the current of the primary winding;
if at a constant frequency, then current can be canceled out of the
equation and we can concentrate on the voltage across the winding instead,
if it matters.
B is the output, the strength of the magnetic field as it runs in the core.
Divided by the number of secondary turns, it represents the output voltage.

Standard disclaimer: I'm tired so I'm probably wrong. Someone proof-read
for me.

Tim



-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
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"Ian Iveson" wrote in message
...
Then I got to the bit where the curve levels off towards saturation
and couldn't remember why. Still can't.

As H continues to rise, the domains get more aligned; as they approach
maximum alignment, mu drops in proportion to the number of domains that
are 100% pointing in the direction of the magnetic field. As a result,
B-H becomes nonlinear and flattened.

Tim

--
In the immortal words of Ned Flanders: "No foot longs!"
Website @ http://webpages.charter.net/dawill/tmoranwms

Gerald Stombaugh wrote:

We found that if we interleaved grain oriented EI lamination
2 x 2 or even 3 x3 we improved the low frequency performance
of the PP output transformer. We measured the L at 0 DC and then at
5% of tube DC current and drew the line to see what the presumed
loss of L was as opposed to the "NoDC unbalance" current. The
hi-fi folks like HK, Fisher, Marantz seemed to like our concept
so we continued making them with 3x3 interleaving. I have the
curves somewhere in my files. What is the plate current tolerance
on output tubes today. I always presumed a 5% unbalance, that
is what got me on the kick of looser interleaving..

I have seen the use of placing the core laminations in bunches
of 3 x E and 3 x I one way, then same but all the other way, I assume this is
what
you mean.
This often indicates a povety of turns on the transformer, to
try to stave off saturation to a higher F than otherwise.
It makes a marginal difference.
With GOSS material OPTs, the iron magnetises sharply with very
low voltages, and thd in the iron at 50 Hz is low even when the source
resistance
of the signal is high. It is a small fraction of what it is with non
grain oriented material, ie, just plain 3% SiFe.

The more inductance, the better, mostly, but for a typical
PP two tube amp 100H is enough.
It needs to be kept high, and not reduced by a gap if we are
to use NFB with LF stability, and the gap won't stop the sag in inductance
at real low F, the PP amp will still experience a change in primary
inductance.
Sharp and suddent hard saturation will be more limited by the gap,
so a compromise can be made, to still have adequate L after the gap has been
applied.
A full gap, as in an SE core reduces the iron U from say 7,000 to
effectively say 500, and the inductance is reduced by the same amount.

There will always be iron distortions,
but as long as its less than the tube distortions, then all is well.
In OPTs, this is easy to achieve.
The formulas are in the RDH4.
If the B is below 0.3 Tesla at 50 Hz, at full power, there isn't much
to worry about.

Patrick Turner.





Gerald

On Sun, 24 Aug 2003 13:40:50 -0500, "Tim Williams"
wrote:

"BFoelsch" wrote in message
...
You know, I have never understood this claim. Let us look for a second at
RDH4, p230, Fig 5-16.
Using this chart, who can explain this "crossover distortion" theory to
me?

As it crosses through zero, its slight tendancy (depending on material)
to remain magnetized consumes energy (hysteresis loss) and causes a
slight bias you could say, as it crosses through zero. Thus on falling
back from the positive peak, as H crosses through zero, B is still a
bit positive. The opposite will happen after the negative peak.
H is the input, and is determined from the current of the primary winding;
if at a constant frequency, then current can be canceled out of the
equation and we can concentrate on the voltage across the winding instead,
if it matters.
B is the output, the strength of the magnetic field as it runs in the core.
Divided by the number of secondary turns, it represents the output voltage.

Standard disclaimer: I'm tired so I'm probably wrong. Someone proof-read
for me.

Tim

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"Ian Iveson" wrote in message ...
"smoking-amp" wrote
...[loads of seemingly sensible stuff about B and H]...

Thanks, Don. I almost posted my own essay on domains, but yours
probably makes more sense. I got to the paragraph where I was
explaining what we both call a kind of "avalanche" effect. I was
having difficulty there because it is a stable process, not like a
real avalanche that sustains itself with no further excitement.

Yes, your right, I maybe overdid the analogy a bit to be more graphic,
it should be stable, alignment of other domains increases field to
make flipping easier.

Then I got to the bit where the curve levels off towards saturation
and couldn't remember why. Still can't. Similar to a population
curve...these sigmoids arise from some fundamental contradiction but
dunno which one in this case. Glad you got me out of that one.

I think Tim has got this, just running out of domains to flip as
saturation sets in. Hardest ones to flip go last.

I remember an experiment in which the secondary of a transformer was
monitored with sufficient resolution to see the individual flips.
Rather like a sloping line on an old low-resolution computer screen,
it is fairly clean at 45 degrees, but very lumpy at shallow angles.

I recall doing a similar experiment in high school once, we just
connected a high turns coil wound on an open U core to the input of an
audio amp and then brought up a bar magnet to the core. Could hear the
domains flipping, sort of like the sound of crunching plastic wrap.

But I don't get your argument that the effect centres itself around
any bias point. I can see that there will be hysterisis about any
established point, but the curve is steeper so it should be less,
surely? Flipping a domain is easier or harder depending on what
proportion is already flipped.

I may have jumped to conclusions here. I need to look at one of the
real magnetics books (the transformer books don't get down to this
detail) here to see what really happens in minor hysteresis loops.
Definitely there is hysteresis at any DC operating point, how the U
varies with DC and AC magnitudes here needs to be checked. This
morning when I got up I realized that I was relying on hand waving
arguments here, so I went and measured a real SE transformer. Tested
with a Hammond 1640SE. Used a Heath digital LC meter on the primary
and connected an IRF230 Mosfet as a variable DC current source to the
combined secondaries. This measurement turned out to be a lot more
difficult than I expected. Had to get the current source very quiet or
it generated effects on the meter reading. Had to use an isolation
xfmer on the meter and another on the power supplies for the current
source. Still found some interaction from connecting the current
source with no current. I now plan on purchasing one of those pocket,
battery powered, digital LC meters. Also found that hysteresis in the
SE transformer caused inconsistencies in readings versus DC current
between ascending current measurements and descending current
measurements. Finally went and got a variac to demagnetize the SE xfmr
with a slow dropoff of AC voltage. Then did two sets of ascending DC
current measurements which agreed (demagnetized before each set). Here
are my results: 0 Amp DC - 27 Henry , .05A - 27H , .1A - 27H , .2A
- 26H , .4A - 25H , .8A - 24H , 1.2A - 23H , 2.0A - 22H , 3.0A -
21H Pretty apparent that U is dropping with DC current here, even
when diluted out by an air gap. Would need to do a further test with
variable AC level in measurement of inductance to see if U picks up
and peaks at some AC level before one could conclude my earlier claim
of just moving the whole inductance curve with DC operating point.
Maybe will get to testing that yet. 3.0 Amps DC on the full secondary
here would correspond to 409 mA on the primary when the turns ratio is
taken into consideration. The 1640SE is rated for 200mA max. DC bias.
M6 steel.


Van der Veen says don't worry about it...nearly all the effects
cancel out with a decent transformer.

cheers, Ian

Yes, a low output impedence from triode output, distributed output
topology, or global feedback should fix up xfmr effects nicely unless
the xfmr is really designed badly.

Also, a note to Ronald or David, I didn't mean to sound like I was
trashing SE designs, obviously I am playing around with them too in
distributed load (Quad) topology, have Hammond 1640SE and 1628SE xfmrs
here (using secondary as cathode winding). Just takes a bigger xfmr
for SE. I want to compare a real SE design sound to the psuedo SE
design (P-P using a triode in the feedback loop) I mentioned in an
earlier post. If the pseudo SE sounds like real SE then could make
high powered ampls. for more usual low efficiency speakers. Another
more extreme variation would be to try triode feedback in a SS ampl.,
but don't want to cause a firestorm over this. Undoubtedly, hard
clipping issues will limit the quality of this.

Don