Design parameters for SE OPTs' - diyAudio
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Old 2nd September 2007, 11:34 PM   #1
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Default Design parameters for SE OPTs'

OK...here it is I'm assembling my 2A3 SE (Acquiring parts) , & I have a local outfit here Kaizenelectronica, who will custom wind all my transformers for me.....thing is I don't seem to recall the balancing act that is winding/designing SE transformers.
I recall that two(Or more) specs worked against each other as in the Henrie value needs to be low so as not to roll off the high end too early, but the windings need to have alot of numerical turns.....which works against a low H.
I don't know where I read the Primer on SE designing....& unfortunately it didn't stick.
I ask this as I'm sure these folks will be scratching there heads for the OPT if I don't give them some direction.
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Old 2nd September 2007, 11:51 PM   #2
Tweeker is offline Tweeker  United States
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Intentional Henries dont roll off the high end. Leakage inductance and interwinding capacitance do. More turns mean more of these undesirables.

An SE xfmr needs an air gap to maintain inductance at a given current. This itself means more leakage, and it drops H per turn, meaning more turns and still more HF bandwidth limiting strays. So yes, there is a trade between HF and LF bandwidth. A lack of inductance will lead to poor base and an elliptical loadline as you approach bass.

How much DC current it will be supporting should determine the air gaps size.
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Old 3rd September 2007, 01:20 AM   #3
kevinkr is offline kevinkr  United States
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Winding output transformers is more than a bit of an art, expect to go through quite a few iterations before you get something acceptable with a power transformer vendor. You might just be better off ordering some from Electra-Print, James or similar.

For example - what is the optimum number of interleaves for a given transformer primary impedance?

How do you determine the number of interleaves to use to minimize leakage inductance without creating excessive shunt capacitance?

How do you choose the wire such that it will fit the core window or bobbin, and yet not have excessive dcr? What is excessive dcr?

Can you be sure that the wire is insulation void free or close to it? How good is their vacuum impregnation, and what is the dielectric constant of the varnish used?

What materials will you use to insulate the various interleaves, how thick can they be, what dielectric constant is acceptable, and how much voltage can they stand off?

How do you size the core for a given inductance with a gap that assures core saturation will not happen with your standing dc current + signal current.

Do they have suitable core materials available for output transformers? M6 would be the minimum I would consider acceptable, and there are much better ones. (See posts by thomas for more on this.)

I don't have the answers to many of these questions, but I do know a number of people who have successfully attempted to wind their own output transformers - in all cases a minimum of three full attempts were required to get something with reasonable performance. The difference here is they had the time and motivation to get it right, you will unfortunately have to pay for the time this vendor will need to get enough experience to make those outputs right, and they will not get it right the first or even second time around most likely.

Sorry to rain on your parade, but you would be better off paying someone who already knows what they are doing just once, and then paying the shipping required to get the output transformers to you - probably this will prove much cheaper and more satisfactory in the long run.

Making good SE output transformers is true art, and good PP transformers aren't a whole lot easier either.
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