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Choosing The Iron For A SE OPT?

filips

Member
2014-10-18 1:34 pm
I have just completed my first PP tube amp from scratch. For my next amp I would like to build my own OPT for a class a design. After a considerable amount of research I realized just how much I didn’t know and how difficult this task was going to be. It took a considerable amount of time just to understand the terminology and happens inside an OPT at all audio frequencies. I think I have reached a point where I understand most of what I need to know but there is one conflict I can’t seem to get past and I goes as follows.

One of the first things I have to do is chose the core material (Iron). From what I have read it just seem to make sense to choose M6 steel instead of Permalloy or any other nickel based alloy. The advantages of Permalloy would be less distortion at lower frequencies due to the fact that it has a very little hysteresis and also has very high permeability. Both very desirable HOWEVER… If I have to introduce an air gap to reduce the permeability in order to keep the DC current from saturating the core then it would seem to defeat the purpose. Permalloy also saturates sooner the M6 steel. M6 steel suffers from more hysteresis but only at low frequencies where distortion is least audible. It would seem M6 steel would be the better design over all. What am I missing?
 
About choosing ferromagnetic material.

Because the amplifier is single-ended and there will be a gap in the magnetic circuit, you don't want maximize permeability. Once you have the air gap, you have essentially lost the effects of high permeability.

High flux density is desirable because then you may have a small transformer.

Low hysteresis is desirable because that means low distortion.

Use thin laminations of magnetic steel to prevent eddy current losses at high frequencies. There should be at least oxidation on the surfaces of the laminations to prevent electric circuits between laminations.

Orthosil M-6 is quite good.
 

20to20

Member
2010-06-23 9:25 pm
Thank you for clearing this up. This falls in line with what I was thinking to begin with. What confuses me is why repeatable OPT transformer manufactures make SE OPTs with highly permeable (and very expensive) cores?

Learning what it takes to build a high quality SE OPT is an admirable goal. But at your stage, what makes you think that you could build a higher quality OPT than you could buy? What will be the value (cost in $$) of the time, materials, effort, and if you build one will you be able to test it on its own to qualify it and compare it to a known high quality OPT commercialy available? Do you have the test equipment or the knowledge to qualify what you build in order to have a basis to discuss the project once it's done?
 

filips

Member
2014-10-18 1:34 pm
Don’t get me wrong, as stated in my first post, I didn’t realize to the extent or the complexities involved in designing an OPT and I am certainly no expert

These are good questions, all of which I have already asked myself along the way. Also these are question which anyone interested in winding their own OPT transforms need to ask themselves before they consider such a task.

I will admit I am in little over my head but this thing is going to get build one way or another. I have a lot of time researching invested in this venture. I doubt I can build an OPT (or anything for that matter) the first time better than I can purchase from a repeatable dealer. This is a curiosity just as was my first tube amp build. As far as cost well… probably going to cost more but money is not issue. If I wanted to save money and time I would simply have purchased an amp already build.

Equipment I have = 4 channel O-scope (also have Two 1000 ma current shunts to go with), Two Fluke 289 Multi meters, Two Fluke 87V Multi meters, Process meter, LCR meter, and ESR Meter. 2 Variacs, BK Precision DC power supply 1760A, 500mA variable high voltage DC power supply (Built myself), 1000VA benchtop isolation transformer, Agilent Function Generator, High wattage resistor bank and Decade box, There is still some other equipment I would like to purchase but just would not use that much. O ya… almost forgot, I also have a coil winder.

Source material I have read= “Handbook for Sound Engineers” by Bill Whitlock Third edition, this is probably the best information for understanding OPTs I have read and re-read several times trying to understand and still learning. Also “Audio Transformer Design Manual by Robert G Wolpert. This is probable the best information I have found for calculating build design but it is not the end all. There are also several good web sites on design.
 
About high permeability and high flux density

Thank you for clearing this up. This falls in line with what I was thinking to begin with. What confuses me is why repeatable OPT transformer manufactures make SE OPTs with highly permeable (and very expensive) cores?

I don't know the physics of ferromagnetic materials well enough to explain why it is so, but it seems that many high permeability ferromagnetic materials coincidentally have also this most desired feature low hysteresis. Supermalloy is one good example.

I think that high permeability would be most desired in common-mode chokes and saturable reactors (in magnetic amplifiers).
 

20to20

Member
2010-06-23 9:25 pm
I will admit I am in little over my head but this thing is going to get build one way or another. I have a lot of time researching invested in this venture. I doubt I can build an OPT (or anything for that matter) the first time better than I can purchase from a repeatable dealer. This is a curiosity just as was my first tube amp build. As far as cost well… probably going to cost more but money is not issue. If I wanted to save money and time I would simply have purchased an amp already build.

I'd buy one of Edcor's HI-FI OPT's and unspool it then see if I could rewind it. The lams will be what you want. See if you can realign them and then rewind it. The real excersize then is to test it to see if what you built was worth the trouble. In the end, whatever you build will sound great.
 
Learning what it takes to build a high quality SE OPT is an admirable goal. But at your stage, what makes you think that you could build a higher quality OPT than you could buy? What will be the value (cost in $$) of the time, materials, effort, and if you build one will you be able to test it on its own to qualify it and compare it to a known high quality OPT commercialy available? Do you have the test equipment or the knowledge to qualify what you build in order to have a basis to discuss the project once it's done?

The best stuff I've seen on the internet in terms of designing your own transformers is at Turner Audio. He's retired now but still maintains his website. Essential reading if u are designing a transformer.
index
 
You've never said specifically whether your new amp is to be single-ended or push-pull. Push-pull amplifiers, with standing ("DC") magnetization nulled, are a different critter than those with single-ended (DC biased) core magnetization.

If you look closely at an iron core B/H curve you can see a tilt in the slope (which is the definition of permeability) as the curve passes though zero crossing. The (possibly only) advantage of single ended amplifiers is that signal zero crossing is biased away from this variable (and therefore non-monotonic) permeability region, which introduces a (slightly important, or not, depending on source impedance) variable loading by the primary inductance.

For push-pull amplifiers with nulled primary DC current, high perm. metals like nickle have been included in the core to extend the region where otherwise low slope matches low and normal signal levels. Peaks are allowed to change slope and higher max magnetization metals start working. PFM.

All good fortune,
Chris