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Old 29th August 2009, 05:08 PM   #11
BudP is offline BudP  United States
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jlsem,

You are correct, the field never collapses. However the AC current and DC current are not a combined event. The DC current does not "swamp" the AC signal current.

A SE OPT does not suffer from the zero crossing distortion of a PP, it does suffer from lack of wave form tracking, on the settling side of a wave form. Core polarization occurs at the gap, with AC saturation peaks and the gap does add a time to null point distortion. It is neither as pervasive as that from poorly designed PP core transformers, nor of the same audible type. Rather it is the "fattness", most especially to lower frequency signals, that is common to typical E/I SE OPT's.

Amorphous core SE OPT's have almost none of this problem. They do have a disruption to the planar magnetic fields arising within the window, that support the 1K to 100k increase in the transform of the antenna signal from primary to secondary. This is audible at the leading edge of the signals and is often mistaken for detail. This is a minor fault in most cases.

Distributed gap E/I transformers also have gap induced distortion, but it does not show up until relatively high levels of power are induced. Again, a minor fault in most cases.

In a recent ad hoc experiment, in audible characteristics, between SE and PP OPT's, both with distributed gaps and in prototypes from Wright sound, the only real difference between SE and PP 300 B amplifiers was the resolution of what can only be called musical beauty, from the SE amplifiers. They had noticeably more odd order distortion than the PP amplifiers, but they simply provide an emotional benefit missing from the deep class A PP 300 B units. Load lines were 3.3 kZ for the SE and 6.6 kZ from the PP.

Bud
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Old 29th August 2009, 07:59 PM   #12
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Originally Posted by BudP View Post
Not entirely no. B/H curve saturation occurs even in SE OPT's, though not to the degree that it occurs in PP, due to the typical gap.

Core gap in E/I transformers is also the problem with an E/I SE OPT, the larger the gap the longer it will take for core polarity to catch up with the back wave of the driving signal. C core gaps are driven on both sides of the gap thus eliminating most of this latency, their drawback being the nonlinear flux created in the window, by having a gap in the center of the winding

For the most part, core is your enemy in signal transformers, above 400 Hz, even with amorphous core. If you construct an E/I core with the same distributed gap that works so well in PP E/I core, the core will actually help by passively demagnetizing on the back wave, after a saturation peak has polarized the core. This method actually works so well that it is dangerous to use it without a gap, in a SE OPT.

The same sonic values hold true, as in PP, between amorphous core and M3 E/I core, though i don't find them as noticeable.

Probably the two best providers of custom designed amorphous core SE OPT's are Intact Audio and Tribute Audio. O-Netics, the company I work for, is the only provider of partial self demagnetizing E/I core structures that I know of. I am sure there will be others in time, we do tend to pirate from one another as occasion permits.

Bud
Can you please explain what is meant by "back wave"? In the case of a sine
wave would it be the part from positive crest to negative crest? For a square
wave it's the falling edge?

Thanks
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Old 30th August 2009, 12:58 AM   #13
BudP is offline BudP  United States
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Yes, exactly those portions, the ones beset with "settling time" issues of storage and resonance. I am after the same change in laggardly performance with the EnABL speaker treatment process and the Electron Pools. As far as am concerned, the front wave / rise time performance of just about everything is just fine.

Bud
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Old 30th August 2009, 04:36 AM   #14
jlsem is offline jlsem  United States
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However the AC current and DC current are not a combined event.
I find this statement a little puzzling. We're still talking single-ended OPTs, aren't we? An oscillogram of a steady state AC signal on an OPT without a DC component bears no resemblance to that on an OPT with.

John
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Old 30th August 2009, 05:29 AM   #15
BudP is offline BudP  United States
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All tube OPT's have DC current flowing in their primary wires. The AC current flowing is the only one that causes change. And the only current that flows in the secondary is AC, unless we refer to an auto former

As far as a SE transformer is concerned, DC current offset is not important, other than it holds the permeability of the core, hopefully, at a point just below 2K gauss or so.

Push Pull transformers have DC current flowing also, but it is not important unless it is holding the core at some level of permeability other than zero.

You were not specific about which condition you were referring to. I will point out that the only signal that is of importance is that found on the secondary and a well designed PP OPT and SE OPT will be very close to one another at this point.

It is quite likely you have never dealt with a PP OPT that does not have crossover distortion and, as might I, accept this condition as necessary to the breed. It is not. Just as the saw that SE OPT's are always weak in high frequencies and sloppy in bass is not truth either. Both of these attributes are easily remedied in SE OPT's and are just a function of treating a signal device the same way a power device is treated. The same is true with PP OPT's.

Not picking an argument here. We just come from different places, with differing experiences.

Bud
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Old 30th August 2009, 10:03 AM   #16
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Hello Bud

nice explanations and I like tom read it, thanks!

Please let me have a question from the other side... you mentioned that all the problems of amorphous core OPT is described by the audiophiles with "detailed sound".
So why are the most reputated amps out there build with amorphous or permalloy core and beloved from the crow? Do they listen all so bad? Maybe we are talking on the order of parts per thousand.

Anyway, let's follow the theory discussions. Can you explain your view of a "perfect" transformer for single ended amps?

Joao
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Old 30th August 2009, 11:27 PM   #17
BudP is offline BudP  United States
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Not all of the well reputed SE amps have amorphous core, just most of them. Wright Sound's excellent Model 7 mono blocks have an E/I core SE OPT and are very well thought of. Unfortunately it looks like they will disappear with the owners death. They are also in need of two changes, a replacement of the interstage photo flash capacitor with a multicap/reliacap PPFX tin and poly cap and the bridge resistor between the CCS and driver halves of the 6sn7 with an extremely good non inductive foil resistor, like the Rn60D Dale/Vishay brand. These small alterations remove the last vestiges of "sharpness" in these amps.

They are not considered to be in the "class" of the huge, plate choke fed behemoths, but this is more a status thing than any lack of performance.

Other than Wright Sound, O/Netics has not been aggressive about getting involved with the Audio manufacturing community, due more to the unstable financial environment than anything else. And, we do think that Intact and Tribute deserve to be in business and so are not interested in a more direct fight using amorphous core, which we have experience with. There are a number of satisfied DIY customers with our E/I products however, and they also have amorphous core units from other manufacturers.

A negative example comes to mind with Romy the Cat. His SE OPT's all have amorphous core. He has had mine and does not like them. Not because they lack detail or musical characteristics, but due to their reduced angle of acceleration on the leading edge of transients. His amps also have power supplies with 30K mf of high voltage capacitance, or more. This just might have something to do with his choices.

The false detail comes from leading edge overshoot. With an amorphous core you must be very careful with your coil construction. All dielectric materials need to be of a fairly high dielectric constant so that the release into B Field, from E Field activity, does not excite the core into half wave, uninhibited overshoot. Lundal does this very successfully with mylar, as layer to layer insulation, and separation of primary and secondary coils from each other. Another way to provide some "ballasting" is to deliberately increase wire size, by utilizing Amorphous core permeability, and thus add distributed capacitance.

However, as you say, these are really minute differences and without actually hearing a direct A/B comparison, the owner of either basic core format will not know there might be something they are missing.

So, choice of type of core for the OPT might be best done first and then the rest of the system built to suit that choice.

Bud

Last edited by BudP; 30th August 2009 at 11:29 PM.
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Old 30th August 2009, 11:33 PM   #18
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Thank you Bud for a thoughtful and educational post. I think that you have given a very unbiased opinion on the differences. I still think that the amorphous does add something that makes my pant legs shake a little though, in the right circuits
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Old 31st August 2009, 02:15 AM   #19
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"It is quite likely you have never dealt with a PP OPT that does not have crossover distortion and, as might I, accept this condition as necessary to the breed. It is not."

Hello Bud,
How does a PP OPT avoid crossover distortion? Increased air gap or bifilar winding? Pinstriping? HF bias? Or something new?

Don
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Last edited by smoking-amp; 31st August 2009 at 02:23 AM.
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Old 31st August 2009, 02:38 AM   #20
jlsem is offline jlsem  United States
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Quote:
The false detail comes from leading edge overshoot. With an amorphous core you must be very careful with your coil construction. All dielectric materials need to be of a fairly high dielectric constant so that the release into B Field, from E Field activity, does not excite the core into half wave, uninhibited overshoot. Lundal does this very successfully with mylar, as layer to layer insulation, and separation of primary and secondary coils from each other. Another way to provide some "ballasting" is to deliberately increase wire size, by utilizing Amorphous core permeability, and thus add distributed capacitance.
Bud, what kind of test protocol do you use at O-Netics that allows you to observe these phenomena? I'm interested in learning more about the latest procedures for measuring and predicting the behavior of transformer cores.

John
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