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BFoelsch
 
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A question for the cognoscenti autoformersI am not familiar with the "Zechendorff" circuit, but I am quite familiar with autotransformers.

The basic selling point of the autotransformer is that not all of the energy that passes through the transformer need pass through the magnetic circuit. Theoretically, this could result in a smaller magnetic circuit, less weight, etc. However, these advantages are only realized if the transformation ratio is small. An autotransformer wound to achieve a 10:1 impedance transformation will only save about 1/3 of the needed magnetic circuit as compared to a two-winding transformer. Then, of course you also give up the isolation achieved with a two-winding transformer.

An autotransformer would be good to match, for example, a 6 ohm amplifier to an 8 or 4 ohm speaker, and, as a point of fact, McIntosh used to do this with their "better" line of SS power amps. I don't think you would get a great advantage at the transformation ratios typically used in tube circuits, however.
"Jon Yaeger" wrote in message ...
Assuming that a tube amplifier can be built with a balanced output (e.g. the Zechendorff circuit), what are the pros and cons of using an autoformer to match output impedance vs. a standard transformer?

It would seem offhand that an autoformer could have sonic advantages (e.g lower losses, no coupling degradation, etc.). Would it cost less to manufacture? And finally, would a toroid be a suitable implementation (I am aware of toroid problems, especially with saturation, when used for standard output coupling).

TIA,

Jon
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Jon Yaeger
 
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Default A question for the cognoscenti autoformers

Assuming that a tube amplifier can be built with a balanced output (e.g. the
Zechendorff circuit), what are the pros and cons of using an autoformer to
match output impedance vs. a standard transformer?

It would seem offhand that an autoformer could have sonic advantages (e.g
lower losses, no coupling degradation, etc.). Would it cost less to
manufacture? And finally, would a toroid be a suitable implementation (I
am aware of toroid problems, especially with saturation, when used for
standard output coupling).

TIA,

Jon

  #3   Report Post  
Tim Williams
 
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"Jon Yaeger" wrote in message
...
It would seem offhand that an autoformer could have sonic advantages (e.g
lower losses, no coupling degradation, etc.).


Until you touch your speaker wires and suddenly realize that they are
floating around 400VDC. ;o)

An autotransformer is more turns-efficient than a transformer, true, but
the need for isolation and high turns ratio makes this useless. (Of
course the latter doesn't make any difference, it's just more of a 'why
bother' case.)

Tim

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


  #4   Report Post  
Ian Iveson
 
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In addition to sensible advice so far, and linked to the turns ratio
thing...a transformer allows for thick, high-current output winding,
and thin (therefore compact, low-leakage etc.) primary. An
autotransformer could do the same but only by compromising the
advantages you state.

For high-quality hi-fi amplifiers, the harder saturation
characteristics of a torroid should not be a problem because you
won't get near saturation anyway. The best torroids are better than
the best EI, period.

cheers, Ian

"Jon Yaeger" wrote in message
...
Assuming that a tube amplifier can be built with a balanced output

(e.g. the
Zechendorff circuit), what are the pros and cons of using an

autoformer to
match output impedance vs. a standard transformer?

It would seem offhand that an autoformer could have sonic

advantages (e.g
lower losses, no coupling degradation, etc.). Would it cost less

to
manufacture? And finally, would a toroid be a suitable

implementation (I
am aware of toroid problems, especially with saturation, when used

for
standard output coupling).

TIA,

Jon



  #5   Report Post  
Phil Allison
 
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"Ian Iveson" wrote in message
...

In addition to sensible advice so far, and linked to the turns ratio
thing...a transformer allows for thick, high-current output winding,
and thin (therefore compact, low-leakage etc.) primary. An
autotransformer could do the same but only by compromising the
advantages you state.




** Autos have less copper losses and much better bandwidth than a
isolation type.

Wanna compare figures on a 4:1 impedance step down toroidal with a
160 VA size core ?





............ Phil










  #6   Report Post  
Patrick Turner
 
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Jon Yaeger wrote:

Assuming that a tube amplifier can be built with a balanced output
(e.g. the Zechendorff circuit), what are the pros and cons of using an
autoformer to match output impedance vs. a standard transformer?

It would seem offhand that an autoformer could have sonic advantages
(e.g lower losses, no coupling degradation, etc.). Would it cost less
to manufacture? And finally, would a toroid be a suitable
implementation (I am aware of toroid problems, especially with
saturation, when used for standard output coupling).

TIA,

Jon


An auto transformer is useful to match awkward speaker
impedances to impedance sensitive amps.
To get a good match of a 2 ohm speaker, to say an amp which runs best
with 8 ohms, a 2:1 step down ratio is required.
The impedance ratio is 4:1, so you get 8 ohms to 2 ohms.

To achive this transformation, with a typical low power tube amp,
which puts out only say 20 watts into 8 ohms, one could use a
toroidal power tranny, of around 300VA, with 2 x 40 volt windings
in series, and leave the mains primary unterminated.
Such a tranny will have a B of perhaps 1.2 Tesla at 40v at 50 Hz, for
one 40v winding,
but at 20 watts into 8 ohms the voltage is only 12.6 vrms, so
with this voltage applied to two "40" volt windings,
the B at 50 Hz is only 0.189 Tesla, and at 12.5 Hz its still
only 0.756 Hz, and so still well away from saturation.
The BW is usually 100 kHz so the toroidal isn't a bad device to
couple speakers with, to get an impedance match at low losses,
and wide BW.

Various taps down the one winding can be placed to get
a wide range of impedance matches, so driving a woofer, midrange, or a
tweeter
at any selected voltage level can be done can be done without using
series
resistances, to achieve well equalised levels,
and the output impedance at the taps is always lower than the amplifier
impedance,
unless one tries to step up the output voltage, which imho, is
pointless.

SO, those who do have 4 or lower Z speakers could easily
make a match to tube amps best suited to 16 ohms, such as Quad II,
even when set on the amp for 8 ohms.

There are plenty of tube amps which have the poor method
of impedance matches done by simply having a single secondary OPT
winding
for 16 ohms, with tap at 0.7 times the total turns for 8 ohms, and a CT
for 4 ohms, which means that half the sec is not carrying speaker
current
when 4 ohms is used, and HF response is poorer.

The auto tranny at the speaker allows the amp to enjoy the 16 ohms it
prefers,
and less OPT winding losses. The winding losses in the toroidal
auto tranny are minor.
The turns per volt required for a mains 300 VA tranny could be about
2 turns per volt, so 80 turns are required fore 40v at mains F.
160 total winding turns would be fine for a 20 watt amp.
Its easy to place 160 turns of wire by hand around a toroidal core,
using a shuttle made from a piece of old plastic tube, or broom handle
with notches.
Use about 1.3 mm dia wire, and wind the right length for say 1/2 the
winding on the shuttle, then wind it off the shuttle onto the toroid
and set the wires straight, tightly, and true, and neatly.
Tape insulation is easy to apply in lengths.
I have re-wound complete toroidal power
transformers like this, and of course it took ages to do the
thin wire high voltage turns, since there are so many turns.
The low voltage heater windings were dead easy to do, once I
made a sort of clamp to hold the partially wound tranny as I placed on
the turns.

Using auto trannies to have the primary in series with a speaker
secondary
doesn't convey a huge advantage.
But in a PP amp, one could deliberately
split the primary into cathode and plate windings, a'la Quad acoustical,

and have each half cathode winding in series with a grounded and centre
tapped
speaker secondary, perhaps with other symetically placed taps to allow
impedance matchings. The feedback effect is enhance this way.
The Quad OPTs don't allow for such elegant use of the speaker sec,
since it isn't a centre tapped winding.

The text books, including RDH4, will explain
the winding losses with two winding isolation trannies,
and with auto trannies, which are more efficient,
especially with a 2:1 step down transformer.


Where there is a will, there is a way.

Patrick Turner.




  #7   Report Post  
Ian Iveson
 
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Phil Allison" wrote

** Autos have less copper losses and much better bandwidth than

a
isolation type.


Sometimes. Is that what ** means?

Wanna compare figures on a 4:1 impedance step down toroidal

with a
160 VA size core ?


Read the other posts that I agreed with. 4:1 is easy. 400:1 is
nearer what most people would use here. Neither would 160VA be
appropriate in most cases. Care to cite a reasonable comparison?
Care to back up your statement about "much" wider bandwidth, in the
context of an OPT for a domestic valve amp? Can you think of any
other important parameters other than copper losses and bandwidth?

Ever wondered why no-one uses, or supplies, auto-transformers for
valve amp OPTs?

cheers, Ian




  #8   Report Post  
Phil Allison
 
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"Ian Iveson" wrote in message
...

Phil Allison"


** Better leave this para in since this is the one I referred to.

In addition to sensible advice so far, and linked to the turns ratio
thing...a transformer allows for thick, high-current output winding,
and thin (therefore compact, low-leakage etc.) primary. An
autotransformer could do the same but only by compromising the
advantages you state.


** Autos have less copper losses and much better bandwidth than

an isolation type.


Sometimes. Is that what ** means?


** Nope.


** Wanna compare figures on a 4:1 impedance step down toroidal with a 160

VA size core ?


Read the other posts that I agreed with. 4:1 is easy. 400:1 is nearer

what most people would use here.


** Even then it would not need to be inferior to a isolation type.


Neither would 160VA be appropriate in most cases.



** The same comment applies to a wide range of VAs .


Care to cite a reasonable comparison?


** Your face and a monkey's behind ?



Care to back up your statement about "much" wider bandwidth, in the
context of an OPT for a domestic valve amp?


** My remark sits quite well in the original context.


Can you think of any other important parameters other than copper losses

and bandwidth?

** None that relate to my comment.


Ever wondered why no-one uses, or supplies, auto-transformers for valve

amp OPTs?

** Never wondered at all.

The reason is so bleeding obvious.





............. Phil



  #9   Report Post  
Patrick Turner
 
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Ian Iveson wrote:

Phil Allison" wrote

** Autos have less copper losses and much better bandwidth than

a
isolation type.


Sometimes. Is that what ** means?

Wanna compare figures on a 4:1 impedance step down toroidal

with a
160 VA size core ?


Read the other posts that I agreed with. 4:1 is easy. 400:1 is
nearer what most people would use here. Neither would 160VA be
appropriate in most cases. Care to cite a reasonable comparison?
Care to back up your statement about "much" wider bandwidth, in the
context of an OPT for a domestic valve amp? Can you think of any
other important parameters other than copper losses and bandwidth?

Ever wondered why no-one uses, or supplies, auto-transformers for
valve amp OPTs?


In my last post on the subject, where I described the use of a
symetrically balanced
OPT with a centre tapped secondary, with plate, ( or cathode currents )
flowing through the speaker winding is not done because
there really isn't great benefits to transformer efficiencies,
and the cost isn't seen as worth it.

But the other benefit of the fully isolated traditional OPT is that
you have a far safer amp, since lethal plate voltages are
unable to leap out and kill a child, adult, or a dog.

160 to 500 VA toroidals will have naturally low leakage inductances,
since the sections of windings can be wound over each other in close
proximity,
and equivalent to a wide traverse width in a plain bobbin wind up.
Such a plain bobbin wound tranny could be used for a low voltage
audio tranny, and the layers could be simply
interleaved to tightly couple the windings to extend BW
to 300 kHz if desired.
I have wound UL OPTs for 300 watt applications
using 1.1k to 5 ohms ratio, with 6S and 5P sections, to
get 20Hz to 300 kHz at full power.
If my trannies were used as balanced auto OPT,
the BW might extend slightly further.

In the case of a 300 watt amp the speaker secondaries
could have a CT, and be grounded, and the 38 vrms created to make 300
watts into 5 ohms
is an ideal amount of voltage which can be +/-19v either side of the CT,

and then fed to the cathodes of multiple OPV to provide enough cathode
FB for an acoustical connection to then be effective to achieve the
power of the tetrode,
but the low distortion and Ro of the triode.
So there thus need to be only a primary, and a secondary,
and the tube current circuit and speaker current circuit
"share" the same path, but in actual fact the signal current in the
shared
portion, ie, the speaker winding is equal to to the speaker current
MINUS the cathode signal current.
Winding losses will then be slightly lower than the two
separate windings with full isolation.

But the above idea is only of benefit with tubes like KT88/6550/EL34etc,

when you have perhaps a dozen involved; when just two OPV are used,
the speaker voltage isn't high enough to provide much useful
CFB to the OPV cathodes.

But 4:1 impedance ratio for a speaker matching application
is a neat way to achieve matching, and maintain the damping factor,
or improove it.

This idea makes sense if you have say 4 x "8 ohm" woofers
in parallel, and where 200 watts is required for a decent ceiling.
Many amps, including SS might baulk at 2 ohm or lower loads
trying to make 200 watts, but the matching transformer
allows the amp to think it has 8 ohms connected, and it may be able to
drive that easily,
but at the speakers, the voltage is halved, and current doubled, so you
get
200 watts into the speakers, so its a win-win situtation.

I have yet to see this application of auto trannies for speaker use
used commercially, because of beancounter concerns.

The other benefit with an auto tranny when using an SS amp
is that if the amp blows up, and a supply rail becomes directly
connected to the auto tranny, the rail fuse WILL then blow, because the
DC from the SS amp has an easy shunt path, instead of burning out the
voice coils
of the woofers.

So the VA of the auto tranny core and wind up details for speaker use
should be
carefully worked out to ensure the Bmax is less than 0.3 Tesla at 50 Hz,

to ensure Fsat is low. This applies regardless of the U of the iron.
During normal use, such a low B, especially with GOSS toroidals cores,
or other GOSS laminations, will give excellent low distortion
and low losses.

Patrick Turner.


cheers, Ian


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Ian Iveson
 
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"Phil Allison" wrote

Sometimes. Is that what ** means?


** Nope.


Is that sometimes no?

cheers, Ian




  #11   Report Post  
Ian Iveson
 
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"Patrick Turner" wrote

...see below


Absolutely. Autotransformers are good for low ratios where isolation
is not important.

The cathode feedback, or more properly named distributed load in the
example you give, is another kettle of fish, of course. Dunno what
it's got to do with autotransformers though, except that you say
speaker windings in series...that then makes an autotransformer
between cathodes and speaker.

So then the speaker winding is *part* of the cathode winding..."in
series" doesn't quite capture the meaning IMO.

Interesting all the same.

So, with reference to my original point about wire thickness, how
would you deal with this in practise? Let's take an example. I use a
cathode windings of 10%, Zaa 5k and speaker 6ohms. Roughly
speaking, that gives me 3% speaker winding, a ratio of 3 to 1
compared to the cathode. So I could tap about a third of the
cathode windings to drive the speaker.

However, I require the speaker winding to carry several amps,
whereas the cathodes only require around 100mA. If I accomodate both
with the same wire necessary for the speaker, then there is more
leakage in the cathode circuit than would otherwise need to be the
case. If I use two windings of different wire gauge in series, part
of the cathode winding is thicker than need be, giving some penalty
in terms of leakage in the anode/cathode circuit.

My tendency has been in the opposite direction: to minimise a-to-k
leakage, at the expense of added capacitance, which I want anyway.
So my cathode windings are bifilar with the anodes'. I don't have a
bandwidth problem (about 16 to 45k without overall nfb), so the
autotransformer would not be worthwhile in my case, and defeat the
objective of the bifilar winding.

There is another issue he precisely where is best for the nfb to
come from. There are arguments for and against using the speaker
winding in the feedback loop. I prefer the simplicity of not doing
so, and suffer the slightly higher (but I believe less malevolent)
distortion as a result.

cheers, Ian

An auto transformer is useful to match awkward speaker
impedances to impedance sensitive amps.
To get a good match of a 2 ohm speaker, to say an amp which runs

best
with 8 ohms, a 2:1 step down ratio is required.
The impedance ratio is 4:1, so you get 8 ohms to 2 ohms.

To achive this transformation, with a typical low power tube amp,
which puts out only say 20 watts into 8 ohms, one could use a
toroidal power tranny, of around 300VA, with 2 x 40 volt windings
in series, and leave the mains primary unterminated.
Such a tranny will have a B of perhaps 1.2 Tesla at 40v at 50 Hz,

for
one 40v winding,
but at 20 watts into 8 ohms the voltage is only 12.6 vrms, so
with this voltage applied to two "40" volt windings,
the B at 50 Hz is only 0.189 Tesla, and at 12.5 Hz its still
only 0.756 Hz, and so still well away from saturation.
The BW is usually 100 kHz so the toroidal isn't a bad device to
couple speakers with, to get an impedance match at low losses,
and wide BW.

Various taps down the one winding can be placed to get
a wide range of impedance matches, so driving a woofer, midrange,

or a
tweeter
at any selected voltage level can be done can be done without

using
series
resistances, to achieve well equalised levels,
and the output impedance at the taps is always lower than the

amplifier
impedance,
unless one tries to step up the output voltage, which imho, is
pointless.

SO, those who do have 4 or lower Z speakers could easily
make a match to tube amps best suited to 16 ohms, such as Quad II,
even when set on the amp for 8 ohms.

There are plenty of tube amps which have the poor method
of impedance matches done by simply having a single secondary OPT
winding
for 16 ohms, with tap at 0.7 times the total turns for 8 ohms, and

a CT
for 4 ohms, which means that half the sec is not carrying speaker
current
when 4 ohms is used, and HF response is poorer.

The auto tranny at the speaker allows the amp to enjoy the 16 ohms

it
prefers,
and less OPT winding losses. The winding losses in the toroidal
auto tranny are minor.
The turns per volt required for a mains 300 VA tranny could be

about
2 turns per volt, so 80 turns are required fore 40v at mains F.
160 total winding turns would be fine for a 20 watt amp.
Its easy to place 160 turns of wire by hand around a toroidal

core,
using a shuttle made from a piece of old plastic tube, or broom

handle
with notches.
Use about 1.3 mm dia wire, and wind the right length for say 1/2

the
winding on the shuttle, then wind it off the shuttle onto the

toroid
and set the wires straight, tightly, and true, and neatly.
Tape insulation is easy to apply in lengths.
I have re-wound complete toroidal power
transformers like this, and of course it took ages to do the
thin wire high voltage turns, since there are so many turns.
The low voltage heater windings were dead easy to do, once I
made a sort of clamp to hold the partially wound tranny as I

placed on
the turns.

Using auto trannies to have the primary in series with a speaker
secondary
doesn't convey a huge advantage.
But in a PP amp, one could deliberately
split the primary into cathode and plate windings, a'la Quad

acoustical,

and have each half cathode winding in series with a grounded and

centre
tapped
speaker secondary, perhaps with other symetically placed taps to

allow
impedance matchings. The feedback effect is enhance this way.
The Quad OPTs don't allow for such elegant use of the speaker sec,
since it isn't a centre tapped winding.

The text books, including RDH4, will explain
the winding losses with two winding isolation trannies,
and with auto trannies, which are more efficient,
especially with a 2:1 step down transformer.


Where there is a will, there is a way.

Patrick Turner.








  #12   Report Post  
Greg Pierce
 
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On Mon, 29 Sep 2003 16:41:03 +0100, the highly esteemed Ian Iveson
enlightened us with these pearls of wisdom:


"Phil Allison" wrote

Sometimes. Is that what ** means?


** Nope.


Is that sometimes no?

cheers, Ian


C'mon Ian, killfile that retard already, would ya?!

--
Greg

--The software said it requires Win2000 or better, so I installed Linux.

  #13   Report Post  
Phil Allison
 
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"Greg Pierce" wrote in message
news
On Mon, 29 Sep 2003 16:41:03 +0100, the highly esteemed Ian Iveson
enlightened us with these pearls of wisdom:


"Phil Allison" wrote

Sometimes. Is that what ** means?

** Nope.


Is that sometimes no?

cheers, Ian


C'mon Ian, killfile that retard already, would ya?!




** Two more self obsessed ****wits identified beyond all doubt.





............ Phil








  #14   Report Post  
Patrick Turner
 
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Ian Iveson wrote:

"Patrick Turner" wrote

...see below


Absolutely. Autotransformers are good for low ratios where isolation
is not important.

The cathode feedback, or more properly named distributed load in the
example you give, is another kettle of fish, of course. Dunno what
it's got to do with autotransformers though, except that you say
speaker windings in series...that then makes an autotransformer
between cathodes and speaker.

So then the speaker winding is *part* of the cathode winding..."in
series" doesn't quite capture the meaning IMO.


Indeed, but it doesn't matter in principle where the
amplifier device is placed, it can be at the top of an auto tranny,
or at the bottom, or in between seriesed windings.


Interesting all the same.

So, with reference to my original point about wire thickness, how
would you deal with this in practise? Let's take an example. I use a
cathode windings of 10%, Zaa 5k and speaker 6ohms. Roughly
speaking, that gives me 3% speaker winding, a ratio of 3 to 1
compared to the cathode. So I could tap about a third of the
cathode windings to drive the speaker.


Indeed.

I did a similar thing once with the SE 6BQ5 amps in
an old sterogram, I found there was room to place more secondary turns
on the transformers, which were quite small, and the speaker winding had
been wound on first,
then the plate coil, and I was able to add another section of one layer
without pulling the tranny apart. Then there were two section of cathode

coils, and one was used for the speaker.
The BW was then much improved, and the sound was far better.



However, I require the speaker winding to carry several amps,
whereas the cathodes only require around 100mA. If I accomodate both
with the same wire necessary for the speaker, then there is more
leakage in the cathode circuit than would otherwise need to be the
case. If I use two windings of different wire gauge in series, part
of the cathode winding is thicker than need be, giving some penalty
in terms of leakage in the anode/cathode circuit.


The current in the shared section of the auto tranny which would be your
speaker winding
is the speaker current of say 3 amps, minus the 100 mA of tube current.
So in the case of a tapped cathode winding, sure, the speaker winding
still needs to be heavy wire, but all the rest can be fine guage.
The speaker secondary can be set up as several paralleled sections,
just like a normal interleaved transformer, with
these speaker windings in close proximity with the section
of cathode windings, so the bandwidth is high, and the speaker errors
all appear at the cathode of the tube, to cause feedback corrective
action.



My tendency has been in the opposite direction: to minimise a-to-k
leakage, at the expense of added capacitance, which I want anyway.
So my cathode windings are bifilar with the anodes'.


Perhaps you have a lot of capacitance that way.
I DON'T see a need for bifilar windings, ever, in OPTs.


I don't have a
bandwidth problem (about 16 to 45k without overall nfb), so the
autotransformer would not be worthwhile in my case, and defeat the
objective of the bifilar winding.


You could have seriesed windings which are still well interleaved,
or bifilar wound, depending on layout.
In my most recent designs for SE acoustical amps, there
is a layer of fine wire sandwitched between the pairs of layers of
speaker
windings, so coupling between speaker and cathode FB turns is extremely
tight,
ie, LL there is very low.
I am not using auto connection with speaker windings in series, because
I don't want the 0.3 amps of idle current in the tubes to create a DC
offset
across the speaker secondaries.

But using say 4 two layer speaker secs, with a layer of cathode turns
between each
is the best way to arrange the speaker and cathode feedback turns.
The plate winding would be 5 sections, interleaved as normal.

If all the turns are used for the tube current in a PP set up,
and the speaker winding has a CT, then equal DC offset appears
at each of the two balanced speaker terminals, so there is
no DC offset across the speaker.



There is another issue he precisely where is best for the nfb to
come from. There are arguments for and against using the speaker
winding in the feedback loop. I prefer the simplicity of not doing
so, and suffer the slightly higher (but I believe less malevolent)
distortion as a result.


It depends how well you design the OPT as to how "malevolent",
ie, unstable your amp will be with CFB + speaker winding included.

I like the acoustical connection, its far better than plain UL.
You have the advantages of reducing the thd spectral content
to that of triodes, and a low Eg2 can be used compared to Ea,
further lowering thd.
Screen supply is simple, and need not be well regulated for music use.
Far less overall FB need be used, so stability can be better than if
plain tetrode/pentode is used, with an equal total amount of NFB used.

For best results, interleave the OPT quite a lot,
and it'll be stable.

With say KT88, with lower Eg2, the G1 bias voltage can be much lower.
With say 400 v for Ea, 300v can be used for Eg2,
and Eg2 reduced from say -50 to -35, and if cathode bias is used,
the Rk is lower value, so less heat is wasted at the Rk.
Usually, Rk can be halved again, and have say only 18v of cathode bias,
and apply a fixed -17v to all output tubes.
The cathode Rk will still regulate the cathode current,
and hence the bias condition, but you have a much more DC stable
cathode voltage, with less variation in the AB operation.

Patrick Turner.



cheers, Ian

An auto transformer is useful to match awkward speaker
impedances to impedance sensitive amps.
To get a good match of a 2 ohm speaker, to say an amp which runs

best
with 8 ohms, a 2:1 step down ratio is required.
The impedance ratio is 4:1, so you get 8 ohms to 2 ohms.

To achive this transformation, with a typical low power tube amp,
which puts out only say 20 watts into 8 ohms, one could use a
toroidal power tranny, of around 300VA, with 2 x 40 volt windings
in series, and leave the mains primary unterminated.
Such a tranny will have a B of perhaps 1.2 Tesla at 40v at 50 Hz,

for
one 40v winding,
but at 20 watts into 8 ohms the voltage is only 12.6 vrms, so
with this voltage applied to two "40" volt windings,
the B at 50 Hz is only 0.189 Tesla, and at 12.5 Hz its still
only 0.756 Hz, and so still well away from saturation.
The BW is usually 100 kHz so the toroidal isn't a bad device to
couple speakers with, to get an impedance match at low losses,
and wide BW.

Various taps down the one winding can be placed to get
a wide range of impedance matches, so driving a woofer, midrange,

or a
tweeter
at any selected voltage level can be done can be done without

using
series
resistances, to achieve well equalised levels,
and the output impedance at the taps is always lower than the

amplifier
impedance,
unless one tries to step up the output voltage, which imho, is
pointless.

SO, those who do have 4 or lower Z speakers could easily
make a match to tube amps best suited to 16 ohms, such as Quad II,
even when set on the amp for 8 ohms.

There are plenty of tube amps which have the poor method
of impedance matches done by simply having a single secondary OPT
winding
for 16 ohms, with tap at 0.7 times the total turns for 8 ohms, and

a CT
for 4 ohms, which means that half the sec is not carrying speaker
current
when 4 ohms is used, and HF response is poorer.

The auto tranny at the speaker allows the amp to enjoy the 16 ohms

it
prefers,
and less OPT winding losses. The winding losses in the toroidal
auto tranny are minor.
The turns per volt required for a mains 300 VA tranny could be

about
2 turns per volt, so 80 turns are required fore 40v at mains F.
160 total winding turns would be fine for a 20 watt amp.
Its easy to place 160 turns of wire by hand around a toroidal

core,
using a shuttle made from a piece of old plastic tube, or broom

handle
with notches.
Use about 1.3 mm dia wire, and wind the right length for say 1/2

the
winding on the shuttle, then wind it off the shuttle onto the

toroid
and set the wires straight, tightly, and true, and neatly.
Tape insulation is easy to apply in lengths.
I have re-wound complete toroidal power
transformers like this, and of course it took ages to do the
thin wire high voltage turns, since there are so many turns.
The low voltage heater windings were dead easy to do, once I
made a sort of clamp to hold the partially wound tranny as I

placed on
the turns.

Using auto trannies to have the primary in series with a speaker
secondary
doesn't convey a huge advantage.
But in a PP amp, one could deliberately
split the primary into cathode and plate windings, a'la Quad

acoustical,

and have each half cathode winding in series with a grounded and

centre
tapped
speaker secondary, perhaps with other symetically placed taps to

allow
impedance matchings. The feedback effect is enhance this way.
The Quad OPTs don't allow for such elegant use of the speaker sec,
since it isn't a centre tapped winding.

The text books, including RDH4, will explain
the winding losses with two winding isolation trannies,
and with auto trannies, which are more efficient,
especially with a 2:1 step down transformer.


Where there is a will, there is a way.

Patrick Turner.





  #15   Report Post  
Ian Iveson
 
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Sorry. Done

cheers, Ian

"Greg Pierce" wrote in message
news
On Mon, 29 Sep 2003 16:41:03 +0100, the highly esteemed Ian Iveson
enlightened us with these pearls of wisdom:


"Phil Allison" wrote

Sometimes. Is that what ** means?

** Nope.


Is that sometimes no?

cheers, Ian


C'mon Ian, killfile that retard already, would ya?!

--
Greg

--The software said it requires Win2000 or better, so I

installed Linux.





  #16   Report Post  
Phil Allison
 
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"Ian Iveson" wrote in message
...
Sorry. Done

cheers, Ian



** Wimp and well as ******.



........... Phil






  #17   Report Post  
Greg Pierce
 
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On Tue, 30 Sep 2003 10:52:07 +0100, the highly esteemed Ian Iveson
enlightened us with these pearls of wisdom:

Sorry. Done

cheers, Ian

Ahh, the lovely killfile - how sweet the sound :-)

--
Greg

--The software said it requires Win2000 or better, so I installed Linux.

  #18   Report Post  
Phil Allison
 
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"Greg Pierce" wrote in message
news
On Tue, 30 Sep 2003 10:52:07 +0100, the highly esteemed Ian Iveson
enlightened us with these pearls of wisdom:

Sorry. Done

cheers, Ian

Ahh, the lovely killfile - how sweet the sound :-)


** You two are a couple of bum boys - right ??




............ Phil


  #19   Report Post  
Choky
 
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prick
"Phil Allison" wrote in message
u...


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Choky
 
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prick
"Phil Allison" wrote in message
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  #21   Report Post  
Choky
 
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prick
"Phil Allison" wrote in message
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  #22   Report Post  
Choky
 
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prick
"Phil Allison" wrote in message
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