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patrick-turner patrick-turner is offline
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Default Speaker Matching transformers.

Hi all, I have a new web page for matching speakers to amplifiers to give a better match than may be available, ie, make a 2 ohm speaker look just like a 16 ohm speaker, etc.
http://www.turneraudio.com.au/output...-matching.html

Drive yourself mad by reading another of my pages,
Patrick Turner.
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Lord Valve Lord Valve is offline
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Default Speaker Matching transformers.

patrick-turner wrote:

Hi all, I have a new web page for matching speakers to amplifiers to give a better match than may be available, ie, make a 2 ohm speaker look just like a 16 ohm speaker, etc.
http://www.turneraudio.com.au/output...-matching.html

Drive yourself mad by reading another of my pages,
Patrick Turner.


http://taweber.powweb.com/store/zmatch.htm


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Phil Allison[_3_] Phil Allison[_3_] is offline
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"Lord Valve"

http://taweber.powweb.com/store/zmatch.htm



** Hmmm: from what I can find using Google, it's an E-core, multi-tapped
auto tranny in a box.

Ought to work OK with valve / tube guitar amps - but maybe not so well
with bass amps.

And a definite NO NO with solid state amps, cos the core will saturate at
low frequencies and overload the output transistors, something fierce.

One guy here is Australia found using it interfered with the reverb sound
from his Fender.

I wonder if he had the box sitting smack on top of his amp as shown in the
advertising.




..... Phil








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John Byrns John Byrns is offline
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In article ,
"Phil Allison" wrote:

"Lord Valve"

http://taweber.powweb.com/store/zmatch.htm



** Hmmm: from what I can find using Google, it's an E-core, multi-tapped
auto tranny in a box.

Ought to work OK with valve / tube guitar amps - but maybe not so well
with bass amps.

And a definite NO NO with solid state amps, cos the core will saturate at
low frequencies and overload the output transistors, something fierce.


I have a 50 Watt solid state amplifier that is rated at 75 Watts into 2.4 Ohms.
You seem to be saying that I can't use a "multi-tapped auto tranny" to deliver
75 Watts, how can I match an 8 Ohm speaker to 2.4 Ohms?

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/
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Phil Allison[_3_] Phil Allison[_3_] is offline
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"John Byrns"
"Phil Allison"
"Lord Valve"

http://taweber.powweb.com/store/zmatch.htm



** Hmmm: from what I can find using Google, it's an E-core, multi-tapped
auto tranny in a box.

Ought to work OK with valve / tube guitar amps - but maybe not so well
with bass amps.

And a definite NO NO with solid state amps, cos the core will saturate at
low frequencies and overload the output transistors, something fierce.



I have a 50 Watt solid state amplifier that is rated at 75 Watts into 2.4
Ohms.


** That's a bit of an oddball ?


You seem to be saying that I can't use a "multi-tapped auto tranny" to
deliver
75 Watts, how can I match an 8 Ohm speaker to 2.4 Ohms?



** You better not use that Weber unit unless your 75W amp has VERY good
overload and short circuit protection - ie VI limiting.

An auto tranny is not out of the question, but it would be wise to add a 50W
rated series resistor of about 3 ohms on the 2 ohm input and bypass that
with a pair of 1000uF, 50V electros wired back to back. Ideally, the core
ought to be toroidal too - for way less losses and better bandwidth.

The 8 ohms tapping needs to be rated at 50Vrms at 60Hz ( so 25Vrms at 30Hz)
for true hi-hi.

A simple centre tap should be close enough 2.4 ohms.


..... Phil








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Lord Valve Lord Valve is offline
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Default Speaker Matching transformers.

Phil Allison wrote:

"Lord Valve"

http://taweber.powweb.com/store/zmatch.htm


** Hmmm: from what I can find using Google, it's an E-core, multi-tapped
auto tranny in a box.


Yep.

Ought to work OK with valve / tube guitar amps - but maybe not so well
with bass amps.


Seems to be OK with guitar stuff.

I have one on my bench to match heads to my
bench speaker. Works great, but I don't know
how it would do at sustained high power. My
guitarist uses one occasionally, and he plays
fairly loud. No smoke. Yet. ;-)

And a definite NO NO with solid state amps, cos the core will saturate at
low frequencies and overload the output transistors, something fierce.


I only do tube amps these days, so I haven't tried it.

One guy here is Australia found using it interfered with the reverb sound
from his Fender.


??????

I wonder if he had the box sitting smack on top of his amp as shown in the
advertising.


It's a steel box, and the reverb tranny in the Fender is shielded.

I don't see a mechanism for coupling there, unless it's
through the spring tank, and that'd be pretty far away
from the unit in most Fender combos. Unless there's
interaction between the speaker leads going back
and forth to the unit and the reverb tank leads...???

LV

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Phil Allison[_3_] Phil Allison[_3_] is offline
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"Lord Valve"
Phil Allison wrote:

One guy here is Australia found using it interfered with the reverb sound
from his Fender.


??????

I wonder if he had the box sitting smack on top of his amp as shown in
the
advertising.


It's a steel box, and the reverb tranny in the Fender is shielded.


** Thin steel covers are not good magnetic shields.


I don't see a mechanism for coupling there, unless it's
through the spring tank, and that'd be pretty far away
from the unit in most Fender combos.



** Maybe the fool had the box sitting on top of the reverb bag ?

I've seen AC power step-down trannys mounted on the side panels of Fenders,
right down low almost touching the reverb unit.

Or, it was reverb and speaker leads all tangled up.



..... Phil



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patrick-turner patrick-turner is offline
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Discussion flowed thus..................

I have a 50 Watt solid state amplifier that is rated at 75 Watts into 2.4
Ohms.



** That's a bit of an oddball ?

You seem to be saying that I can't use a "multi-tapped auto tranny" to
deliver
75 Watts, how can I match an 8 Ohm speaker to 2.4 Ohms?


** You better not use that Weber unit unless your 75W amp has VERY good
overload and short circuit protection - ie VI limiting.

An auto tranny is not out of the question, but it would be wise to add a 50W
rated series resistor of about 3 ohms on the 2 ohm input and bypass that
with a pair of 1000uF, 50V electros wired back to back. Ideally, the core
ought to be toroidal too - for way less losses and better bandwidth.

The 8 ohms tapping needs to be rated at 50Vrms at 60Hz ( so 25Vrms at 30Hz)
for true hi-hi.

A simple centre tap should be close enough 2.4 ohms.

..... Phil

Unfortunately, the red boxed Weber LV referred to has no specs except that there are 50W and 100W versions, and very cheap, and probably made in Asia and probably it saturates at maximum applied voltage for power at 30Hz. There are no claims made for power bandwidth, so assume the worst. But probably it will work fine with a guitar amp in which a bit of saturation at 50Hz and HF extensing to only 6kHz might be fine, and there's no worry about winding resistance losses which could easily be 15%.

The matching tranny at my website is a hi-fi item, with very low Fsat at full PO and very wide BW from 10Hz to 200kHz. It has E&I lams, to suit DIYer attempts at making such a thing. The size of the GOSS core Afe of my photographed 100W auto-trans is 44mm x 50mm, and Np = 204t. Core is partially air gapped to make the Lp appear as an inductance rather than saturable reactance at LF. If one were to use a toroidal core, and Np was 204t, then the Afe section of toroid will need to be same area, 44 x 50 = 2,200 sq.mm, which means its going to be a big toroid, about equivalent of a toroid used for a 500VA mains trans maybe, to get the same low Fsat with 204t. But a toroid without any air gap with GOSS may have µ = 40,000, and its saturation behavior is more sudden at very low F. Partially air gapped toroids are difficult to find or to make, so E&I will do fine.

I've never seen an SS amp designed specifically for 2.4 ohms and 50W.
It means it generates 10.95Vrms into 2.4r, and maybe it makes 13V into 8r which is only 21W. But there's no reason why 4, 8, or 16 ohms can't be used with an amp that is specified to handle 2.4r, if the volume level is high enough without clipping.

Matching trannies are needed where an amp struggles with say 4r and you have a 2r speaker, or you have an amp where maximum AB PO is say 100W at 4r and you want the amp load to be 16r, so that PO may only be 30W but it is all pure class A, and with better BW, lower THD, better DF etc. So if the speaker was 2r, then you'd need a Z ratio 16r : 2r, ie ZR = 8 : 1, and TR = 2.83:1, which may be difficulr to get exactly it taps on a single winding are to be positioned at ends of layers in an E&I type tranny. But 3 : 1 is easily configured, and would be close enough.

I have used an OPT to couple mosfets to speaker.
http://www.turneraudio.com.au/solids...ono-mosfet.htm

The use of mosfets may be better than bjts with transformer coupling. Bjts may indeed need carefully arranged V/I limiting.
I'd suggested that if pp fear using an IST with SS amp then maybe 10,000 uF cap would be OK. Phil's idea of say 3r0 plus 1,000uF in parallel is probably better, ( certainly for SS guitar amp ) and XC = 3r0 at 53Hz, way above what Fsat will probably be for any audio tranny connected. For 10Hz, tranny may saturate, but XC = 15r9, so the C+R network has become mainly resistive and current spikes from core saturation are limited by value of V0 / 3r0.

For hi-fi, most audiophiles would cringe with horror at using the red boxed Weber and they expect to never have to put any damn series FILTER thing between their amp and speaker. Its bad enough having to use an auto trans. But well designed auto trans will be fine, and work just as well as say the step up transformers used in most ESL speakers, where the tranny has an input winding not unlike the secondary of a normal tube amp OPT, while the ESL secondary has many thousands of fine wire turns to get several thousand volts to be applied to ESL stators.
Some ESL step up trannies are horrible items. I've tested a few. I've seen one which was included in a poor quality kit where Fsat was 32Hz at moderate volume, and this could kill SS amps. To make the Fsat at 10Hz for the same applied voltage, turns must be trebled, or AFe trebled, or both Np and Afe increased a lot. This always means turn length must increase so wire size must be much increased, so you end up with a far heavier item for real hi-fi than the crap mostly sold. Kit maker morons like to shove crap on the public at huge price while avoiding high costs of parts supplied.

Patrick Turner.


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AUDIOCHEF AUDIOCHEF is offline
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Quote:
Originally Posted by patrick-turner View Post
Hi all, I have a new web page for matching speakers to amplifiers to give a better match than may be available, ie, make a 2 ohm speaker look just like a 16 ohm speaker, etc.
http://www.turneraudio.com.au/output...-matching.html

Drive yourself mad by reading another of my pages,
Patrick Turner.
Dear Patrick,
You were interested in self compensated output transformers, I believe.
Have you read the article DECONSTRUCTING THE SC-OPT on the May issue of AudioXpress ?
If so, what is your opinion?
All the best (Ari - AUDIOCHEF)
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AUDIOCHEF AUDIOCHEF is offline
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Quote:
Originally Posted by patrick-turner View Post
Hi all, I have a new web page for matching speakers to amplifiers to give a better match than may be available, ie, make a 2 ohm speaker look just like a 16 ohm speaker, etc.
http://www.turneraudio.com.au/output...-matching.html

Drive yourself mad by reading another of my pages,
Patrick Turner.
Hi again, Patrick.
I read some of your works of your site.
They are very interesting and comprehensive.
May I suggest that you update your knowledge with the novel single ended output transformers? I recommend the corresponding article of Pierre Touzelet in the May 2013 issue of AudioXpress.
Pierre has demonstrated with his authoritative mathematical style that the SC-OPT developed for single ended amplifiers works exactly, under certain conditions, as the conventional, gapped OPT.
This kind of transformer is suitable for any quiescent dc value, opening the way to powerful S.E. valve amplifiers .... etc.
You seem to be very interested in the construction of audio transformers and this is an opportunity to reconsider the idea that "nothing can be invented in this field". Please do not use the "ipse dixit" attitude.
I'm open for further information, if requested.
Cheers,
Ari (Audiochef).


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patrick-turner patrick-turner is offline
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Default Speaker Matching transformers.

patrick-turner;968570 Wrote:
Hi all, I have a new web page for matching speakers to amplifiers to
give a better match than may be available, ie, make a 2 ohm speaker look
just like a 16 ohm speaker, etc.
http://www.turneraudio.com.au/output...-matching.html

Drive yourself mad by reading another of my pages,
Patrick Turner.



Dear Patrick,
You were interested in self compensated output transformers, I believe.
Have you read the article DECONSTRUCTING THE SC-OPT on the May issue of
AudioXpress ?
If so, what is your opinion?
All the best (Ari - AUDIOCHEF)

I don't pay money to subscribe to AudioXpress, mainly because most of the projects do not interest me greatly, and because the standard of design and workmanship used always leaves me feeling I don't need to subscribe, and there was always so little I could learn. I had a career building amps for customers wheras AX seems to cater for hobbyists. Standards for hobbyists are far below those needed where you want to sell something worthy of buying.

I searched Google to see if I could see Pierre's idea posted up or talked about anywhere else than in the article by AX but found nothing. Do you have web address where I could read what he says? I won't be forced to pay to read AudioXpress.

AFAIAC, if I can give all my 38MB of website content to the world for free, why can't other ppl do likewise?

Usually, there are no free lunches in electronics engineering, and yet many hobbyists search high and low to avoid paying the prices for the parts they use for their amp and speaker projects. So we have some "self compensating" used to make a cheap PP OPT without an air gap behave like it has an air gap, or make it not need an air gap, by means of passing DC current through the speaker secondary from a constant current source to counter the drastic magnetization of the OPT core caused by the primary Idc to an SE tube. Its relatively easy to think how it works, and if you have a 25 : 1 turn ratio, then primary might have say 100mAdc, so secondary can have 2.5Amps dc, but with direction arranged to balance the core magnetization to give Bdc close to 0.0Tesla. Its a difficult thing to get right, and I would never both doing it. There were plenty of attempts discussed here at r.a.t. in the last 13 years and I know of no huge repositries of info on websites to optimize the technique, where for example, the much different Idc flows have a circuit which regulates the Idc to give a minimum Bdc.

The more reliable way to use a PP OPT for SE tubes is to use the more simple solution of choke feed of DC to the tubes with choke having an air gapped core and specially designed to give more inductance with DC than is needed in a good SE OPT with air gap. So the choke has to be similarly sized to the OPT, and hobbyists vomit when they read they must spend on a decent choke, and most hate winding any turns around iron. The SE tube is then coupled to OPT primary via a few parallel-series poypropylene caps of high value, OOPS, more vomit, more expense, and chassis space!. The OPT primary is connected to 0V, and may be in series with its secondary to make a kind of auto-transformer. But all this involves an extra phase shift because of the additional L&C coupling and the LF stability with NFB needs to be addressed. So the anode coupling cap values need to be higher than one might have initially calculated, so electro caps will probably be needed.

Another alternative is to use a solid state CCS feeding Idc to anode instead of using a choke, but the CCS must be fed from B+ of over twice the Ea of the tube plus its B+, so maybe +900Vdc and there is say 400Vdc across the CCS at idle. This allows +/- 400Vpeak anode signal swing. But the CCS must dissipate the same amount of heat at the tubes. OOPs, lunch costs money.

Patrick Turner.
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GRe GRe is offline
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"patrick-turner" wrote in message
...
patrick-turner;968570 Wrote:

[...]

I searched Google to see if I could see Pierre's idea posted
up or talked about anywhere else than in the article by AX
but found nothing.

[...]


Pierre's article about an amp using the SC-OPT:
http://www.polisois-audio.com/docume...plifier.01.pdf

More, relating to the subject at:
http://www.polisois-audio.com/informazioni_eng.html

Regards,
Gio Re


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Alex Pogossov Alex Pogossov is offline
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"GRe" wrote in message
l...

"patrick-turner" wrote in message
...
patrick-turner;968570 Wrote:

[...]

I searched Google to see if I could see Pierre's idea posted
up or talked about anywhere else than in the article by AX
but found nothing.

[...]


Pierre's article about an amp using the SC-OPT:
http://www.polisois-audio.com/docume...plifier.01.pdf

More, relating to the subject at:
http://www.polisois-audio.com/informazioni_eng.html

Regards,
Gio Re


Thanks for the links.

These SC-OPT looks like charlatan-craft.

The tertiary (compensation) winding is virtually short circuited at audio
frequencies by a large capacitor. It is like a transformer with short turns
in the winding. What happens in this case everyone knows -- magnetsation
inductance drops, losses increase. Virtually the magnetisation inductance
is replaced by the leakage inductance of the tertiary winding. Bass is lost.

Obviously the "inventor" of such SC-OPT does not want it. To increase the
leakage inductance of the tertiary (which is now serving as magnetisation
inductance), he introduces "loose coupling" by making the tertiary bobbin
larger than the core. This measure makes the primary inductance reach 2H
(they claim), which is still not enough.(Partick would like 20H.) So the
"inventor" goes further -- he sticks a "magnetig band", i.e. several turns
of magnetically soft steel ribbon. Well, the leakage inductance goes up a
bit more. (Does anyone see similarities with a transformer in a microwave
oven with a magnetic shunt?)

The inventor is happy. But... now at low AF not the main core, but his lousy
magnetic band will be saturating, because it has insufficient cross-section.
The difference is, its saturation will be creating more 3-rd harmonic, as
opposed to 2-nd harmonic from a conventional gapped softly saturating SE
OPT.

Not surprising that his sample SPARTAN amplifier has 60Hz low frequency
limit (Partick would like to have 18Hz).

The smart "invertor" does not tell you all that...

Besides, his SC-OPT has two large bobbins, and to fit them you need a large
core, so the overalll cost saving is quite questionable.

On the positive side, his stereo self-compensating solution is smarter. It
becomes mono at LF. To help it, it would be better to provide some RC
circuit at the front end which would mix left and right channels at the
input too at LF. Thus below 100Hz such design will work as a push-pull mono
woofer, and at mid and high frequencies -- as a stere SE amp.

Regards,
Alex


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AUDIOCHEF AUDIOCHEF is offline
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Quote:
Originally Posted by AUDIOCHEF View Post
Hi again, Patrick.
I read some of your works of your site.
They are very interesting and comprehensive.
May I suggest that you update your knowledge with the novel single ended output transformers? I recommend the corresponding article of Pierre Touzelet in the May 2013 issue of AudioXpress.
Pierre has demonstrated with his authoritative mathematical style that the SC-OPT developed for single ended amplifiers works exactly, under certain conditions, as the conventional, gapped OPT.
This kind of transformer is suitable for any quiescent dc value, opening the way to powerful S.E. valve amplifiers .... etc.
You seem to be very interested in the construction of audio transformers and this is an opportunity to reconsider the idea that "nothing can be invented in this field". Please do not use the "ipse dixit" attitude.
I'm open for further information, if requested.
Cheers,
Ari (Audiochef).
[May I suggest.....
Hello Patrick, I would like to add that you can find more information on the self-compensated OPTs in the European Triode Festival Site (Year 2008- Lectures) and, if you have access to the AES papers library, see Paper 6346 AES 118th Convention - 2005 - Barcelona, paper 6831 - AES 120th Convention - 2006 - Paris , Paper 7151 - AES 122nd Convention 2007 - Vienna ).
Should you need more information, please do not hesitate to contact me at my site (polisois-audio.com or by e-mail ) and I'll be glad to respond.
Best regards,
Ari (Audiochef)
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Mr Audio chef said....

Hello Patrick, I would like to add that you can find more information on
the self-compensated OPTs in the European Triode Festival Site (Year
2008- Lectures) and, if you have access to the AES papers library, see
Paper 6346 AES 118th Convention - 2005 - Barcelona, paper 6831 - AES
120th Convention - 2006 - Paris , Paper 7151 - AES 122nd Convention 2007
- Vienna ).
Should you need more information, please do not hesitate to contact me
at my site (polisois-audio.com or by e-mail )
and I'll be glad to respond.
Best regards,
Ari (Audiochef)

I have no access to "AES papers library, see
Paper 6346 AES 118th Convention - 2005 - Barcelona, paper 6831 - AES
120th Convention - 2006 - Paris , Paper 7151 - AES 122nd Convention 2007
- Vienna )"

Just what is a self compensated OPT? I see no reason why it could not be defined in text in less than one screenfull of non-ambiguous text.

I had a quick look at the Spartano amp at the site where a simple looking schematic seems to exist at
http://www.polisois-audio.com/docume...artan_copy.pdf

This is a very simple and not special amplifier, but it used two stacked B+ rails to allow direct coupling of driver tube anode to grid of output tube..

This idea of stacked rails was tried first in 1930s when capacitors were notoriously unreliable and prone to shorting out easily. Wasn't the idea invented by Loftin White?

I am not able to detect any reason why the Spartano schematic would perform any better than something with R&C coupling done with one B+ rail.

Then there is the site at http://www.polisois-audio.com/docume...plifier.01.pdf

There at the top they have a Noman Koreen triode model with current generator and capacitances. The maths which follows is totally incomprehensible to anyone here including me, unless proven to be otherwise. Unfortunbately, the model cannot be imagined to represent a triode easily so ppl grasp the concept without knowing the maths.

Triodes can be modelled TWO ways.
1. Voltage generator with 0V and active Vo ports, output resistance = 0.0 ohms. Input is between 0V and high Z signal input port. Vo produced = voltage signal = -µ x Vg. Anode resistance Ra is a series resistance between generator Vo and outside world RL loads, This point is the anode. The 0V of gene is the cathode, and the input port is the grid.
Sure, capacitances between the electrodes may be added to this model but needn't be for LF audio.

2. Current generator with 0V and vo ports, output resistance = infinitely high ohms. Input is to between 0V and high Z input port. Vo produced = voltage signal = gm x RL. Anode resistance Ra is a shunt resistance between Vo port and 0V. The Vo port is anode, 0V is cathode, and input is grid.

Either model can be used to draw up an equivalent model of a whole amplifier to assist the methodical calculation of all signal voltages and currents in all resistors and caps. All is very simply done, and easy once you know how, and Ohm's Law is all you need.

Pentode and beam tetrodes require slightly more understanding and I like the dual current generator model where you have one gene for input grid making current output = g1 Gm x Vg, and this is shunted by the second current gene which makes current output = screen g2 Gm x Vg2. The Ra is that of the pentode and appears between Vo and 0V.

See my pages on basic tube operation starting with this
http://www.turneraudio.com.au/basic-tube-1.html

There are 4 pages with many ideas about how tubes can be considered to work..

I'll drop you an email to establish a link between us so you can question me further. I am always open to consider something different about any design idea, but I realise its almost impossible to market a new idea using tubes because usually someone somewhere has already employed the idea between 1903 and now. Probably some old ideas can be resurected and done well using modern surrounding supports and to good effect, ie, music sounds very well. usually, it doesn't matter how anyone manages to connect up tubes, mosfets or transistors, they function can all be easily explained bit by bit at least, and whether one way of getting music from CD to ears is better than another is considered in terms of how it sounds and how it measures.
Patrick Turner.


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Mr Audio chef said....


Hello Patrick, I would like to add that you can find more information on
the self-compensated OPTs in the European Triode Festival Site (Year
2008- Lectures) and, if you have access to the AES papers library, see
Paper 6346 AES 118th Convention - 2005 - Barcelona, paper 6831 - AES
120th Convention - 2006 - Paris , Paper 7151 - AES 122nd Convention 2007
- Vienna ).
Should you need more information, please do not hesitate to contact me
at my site (polisois-audio.com or by e-mail )
and I'll be glad to respond.
Best regards,
Ari (Audiochef)

Your email address at seems to be invalid because I got failed message notice after i tried to email me so I will NOT try again.

Please be sure not to waste everyone's time here by posting vague ideas without clear supporting evidence that the ideas actually exist.
Patrick Turner


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Alex Pogossov Alex Pogossov is offline
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"patrick-turner" wrote in message
...
Mr Audio chef said....

Hello Patrick, I would like to add that you can find more information on
the self-compensated OPTs in the European Triode Festival Site (Year
2008- Lectures) and, if you have access to the AES papers library, see
Paper 6346 AES 118th Convention - 2005 - Barcelona, paper 6831 - AES
120th Convention - 2006 - Paris , Paper 7151 - AES 122nd Convention 2007
- Vienna ).
Should you need more information, please do not hesitate to contact me
at my site (polisois-audio.com or by e-mail )
and I'll be glad to respond.
Best regards,
Ari (Audiochef)

I have no access to "AES papers library, see
Paper 6346 AES 118th Convention - 2005 - Barcelona, paper 6831 - AES
120th Convention - 2006 - Paris , Paper 7151 - AES 122nd Convention 2007
- Vienna )"

Just what is a self compensated OPT? I see no reason why it could not be
defined in text in less than one screenfull of non-ambiguous text.

I had a quick look at the Spartano amp at the site where a simple looking
schematic seems to exist at
http://www.polisois-audio.com/docume...artan_copy.pdf

This is a very simple and not special amplifier, but it used two stacked B+
rails to allow direct coupling of driver tube anode to grid of output tube.

Alex:
Besides it looks like a bad design.

Firstly, DC coupling in the trube environment requires very stable DC
performance, otherwise any drift in the driver will project in a significant
drift on the grid of the output tube. In this circuit particularly -- if the
driver tube is out of socket or fails to warm up, then the output tube is
immediately cooked by running with no bias (unless Mr Fuse saves the day).

Secondly, to drive the output tube to full power instantaneous zero grid
voltage will be reached on the output tube grid. This in turn requires the
driver tube to run into cut-off. How can one talk about low distortion here?
Definitely a pull-up is needed on the output tube grid, so that AC plate
driver current is low than its DC quiescent current.

Thirdly, Rk2 || Ck circuit rolls off low frequencies and advances phase,
similar to what an interstage RC circuit would do. So this "DC coupling"
will not make life easier in case anyone decides to introduce NFB.

Overall impression is that the competence of these "designes" participating
in these "Triode Festivals" is very questionable.


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