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Old 23rd April 2005, 12:47 AM   #11
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by valveitude
Well.. Its looking like my original impulse to hold off posting on this topic until I was at the point of testing the electrical/magnetic properties of an actual core was the correct one.
Ah, now don't let a few skeptics keep you from experimenting. And I hope you didn't take my chuckle at Schematic's post as any critique of your plans. I don't know enough to criticize. I just found humor the image of cooking up stuff with hydrogen in the back yard. It's the kind of crazy thing I would be tempted to try.

Sheldon
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Old 23rd April 2005, 03:15 AM   #12
jlsem is offline jlsem  United States
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If you want to make an output transformer fast, easy and cheap, disassemble a military surplus modulation transformer (can be had for $10-$20). The laminations are of excellent audio quality.

I don't want to dampen your enthusiasm, but making your own powdered iron cores for audio transformers is a complete waste of time and will only result in frustration. Purity and exact formulation are absolutely critical in Fe-Si magnetic materials. Any sintered powdered core, even permalloy, is unsuitable for an OPT. I had a couple of good-sized permalloy powder cores (free) I wanted to use in the worst way and, after doing all the research and calculations, I gave it up.

Why try to re-invent the wheel?

John
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Old 23rd April 2005, 07:36 AM   #13
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Ok..heres the challenge. Find a powdered core for a 80-100 watt tranny (40 watt SE running hard class A) with a useful freq. range of 5hz-100khz with a permeability of 250-450. No Toroids please. Oh yea, it has to be affordable,you know $50-$100

Or..a source of 4% silicon iron powder under $2.00 a pound in smallish (100lb or smaller) quantitys.

For bonus points find a source at any price that can provide a "cup" core that meets the above specs. At any price.

Here's a hint, Micrometals nor Ebay have 'em.

If anybody can source either of these, I will stop my folly post haste, and speak highly of you at all future social gatherings.
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Old 23rd April 2005, 09:20 AM   #14
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My last post was originally up a little ways in the thread. Being a new member I got stuck in moderator pergatory for a while. Ergo, I'm a little out of sequence.



Quote:
If you want to make an output transformer fast, easy and cheap, disassemble a military surplus modulation transformer (can be had for $10-$20). The laminations are of excellent audio quality.

Yep...fast and easy...Thats what diy Ultra-Fi means to me.


Quote:
I don't want to dampen your enthusiasm, but making your own powdered iron cores for audio transformers is a complete waste of time and will only result in frustration.

Wow..thanks. Clearly I would have saved a lot of undue stress if I had only asked first.


Quote:
Purity and exact formulation are absolutely critical in Fe-Si magnetic materials

Well... If your in the biz of producing cores, and you want the cores you sold, in say, 2004 to perform exactly the same as the cores you sold back in 1997 this is very true. On the other hand, if you want to make a "one off" pair of cores for a project...not so much.


Quote:
Any sintered powdered core, even permalloy, is unsuitable for an OPT

Were to start..a)as mentioned earlier, I am not sintering b)of all the materials, Permalloy would be one of my last choices for an OPT. It was designed for low level signals and saturates quickly. I made some good coin back in the early eighty's relpacing my customers permalloy heads in their tape decks with sendust heads..the increase in dynamic range was stunning.


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I had a couple of good-sized permalloy powder cores (free) I wanted to use in the worst way and, after doing all the research and calculations, I gave it up.

There you have it...it simply CAN'T...BE...DONE!!!.


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Why try to re-invent the wheel?

We haven't actually re-invented the wheel since Ork was whooping it up around the cave with his round rock, but dare I say, we have made an improvement or two on the concept since then.

I admit this post got a little ..umm..snarky, but at this point in the thread the defeatism is starting to wear on me. Maybe by page 4 or 5 somebody will have somthing positive to contribute. At this point a simple "welcome to our forum" would help my attitude.
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Old 23rd April 2005, 02:08 PM   #15
jlsem is offline jlsem  United States
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Quote:
I admit this post got a little ..umm..snarky, but at this point in the thread the defeatism is starting to wear on me. Maybe by page 4 or 5 somebody will have somthing positive to contribute. At this point a simple "welcome to our forum" would help my attitude.
I apologize for sounding defeatist, but I was only trying to save you a lot of heartache.

Quote:
Yep...fast and easy...Thats what diy Ultra-Fi means to me.
I see now. I assumed you were trying a project that had little cost and made from found materials - you know, resistors from old pencils, capacitors from Reynolds Wrap and brown paper sacks, tubes made from old projection lamps, etc.

Quote:
core for a 80-100 watt tranny (40 watt SE running hard class A)
For an output transformer that large, you really only have one choice for material - silicon steel, either amorphous strip or grain-oriented, preferably Hypersil c-core.

John
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Old 23rd April 2005, 02:09 PM   #16
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"Ok..heres the challenge"

O....ooKay......y, here's my answer. Visit the town dump and sequester a quantity of worn out steel belted tires. Burn or strip the rubber off to retrieve the steel belts. Use a torch to slightly oxidize the steel surfaces. Can either try using as-is for a giant toroid or can strip out the steel wire to use as a more compact wound core.

Now, back to reality. If you want to make a low cost one-of xfmr., just find a good scrap metal yard in a big city. Locally, I can get nice used xfmrs of almost any size or shape for $.50 a pound. My problem is finding two alike.

If you are more interested in bizarre xfmr construction, then try this one on for size. Wind your xfmr. windings on a large plastic spool (maybe 12 inch ID). Then tape the windings over well to seal them, and drop the whole assembly into a bucket of ferro-fluid. Not cheap though unless you make your own fluid.

Welcome to DiyAudio by the way!

Don
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Old 23rd April 2005, 02:38 PM   #17
Mark Kravchenko --- www.kravchenko-audio.com
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Default A glimmer of light

You don't know untill you try it. So in the spirit of be optimistic I offer my 2 cents to this idea.

To make your C core or donut you need a form. I understand that you will probably be using a slow set epoxy and a muller to mix your dough. You will need a form which you already alluded to. To get the pressure you could use a vacuun pump and a form and a bag. If you don't have a vacuum pump a compressor will also draw a vacuum on the intake side. A bag needs to be thick plastic. Like the stuff old ladies covered their sofas with. A bag can be constructed quite cheaply. Fold it over and seal the edges with silicone. Use duct tape to hold it together along the edges untill it dries. Keep it flat for at least 24 hours. A method to get the air out is up to you. You will need some sort of coupling. I personally use air hose fittings and an adapter to seal to the bag that I made up. Standard air hose works well to draw the vacuum with.

As for pressure figure it this way. Atmospheric pressure is roughly 15 psi. Or about 2102 pounds/ ft squared. Figure out the surface area of your mould and you have the toal pressure all nice and uniform. If you have a slow enough setting epoxy you will also draw out any air bubbles in the mix. But don't go to slow as you have to keep a vacuum the whole setup time for maximum effect.

Food for thought.

Mark
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Old 23rd April 2005, 02:41 PM   #18
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Don't listen to all the armchair experts that have not done what you are doing. There is also a quieter group here that read many posts and only rarely speak up. You will gradually get to know them.

I have played with normal ungapped ferrite cores as interstage coupling transformers and gotten quite acceptable results from them but you do need rather more turns than you would for iron. What I would like to try for a *gapped* transformer is a number of EE type ferrite cores stacked together side by side (sort of like thick laminations) so you can get any desired cross sectional area you want. EE type cores have flat sides and square limbs so they will stack together perfectly without any waste space.

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Old 23rd April 2005, 03:09 PM   #19
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Why bother with epoxy. Just wind the xfmr. windings on a very large plastic spool. Then suspend it halfway in a 55 gallon drum (with castor wheels on the bottom so it can be moved). Pour in the powdered iron to the top. (suggest letting the powder rust some by heating it in air first to oxidize the surface, maybe can feed it thru an old oil burner assembly.) With such a large volume of material, the magnetic field strength can be kept low. Losses scale as the square of field strength. Low Mu will be compensated for by the large cross sectional area. (You do need to get sufficient inductance for low frequency bandpass) Roll this rust bucket onto the train or subway and ride around with it for a few hours to get the powder to settle optimally. Maybe can just ride around with it in the back of a pickup truck over a dirt road.

An old utility xfmr. would be a lot simpler though. 10KV to 120V is just about right for SE 100W. Divide voltage ratings by 3 for 20 Hz rating.

Don
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Old 23rd April 2005, 04:17 PM   #20
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Off the shelf xfmr for $9.99, 2000V to 120V/240V @ 150 mA, these often have a CT on the HV winding for the lower voltage ranges.

A source of HV power supplies and components (including xfmrs and capacitors) that is largely unknown in the tube audio realm are electrophoresis power supplies.

Here's one on Ebay for $9.99
http://cgi.ebay.com/ws/eBayISAPI.dll...508954815&rd=1

I often see these go by without a single bid on them. Just have to find a cheap one near you, so you can pick it up without shipping charges.
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