So has anybody ever designed their own output transformer?

[PositiveNegativeMan]

Attempting a 50w bass amp build and I thought it might be fun to wind my own OT.

I wanted a 3.2k ohm to 16 ohm with an 8 ohm tap.
I worked out the impedance and turn ratios because that maths is simple enough for me.
Then I hit a snag. I designed a power transformer a while back and before I tried to assemble it I was able to compare my design with some real ones. They were old things that had failed so nobody cared if I took them apart. Picking those apart told me that I was there or thereabouts with my choices of core area, lamination type, and number of primary turns. But for this I have no 50w bass amp OT to compare to. Not even one.
I have tried to find out information about these things, but other than impedence ratios, I can't find a great amount of detail.

I was hoping to use EI laminations left over from the PT I did. They are very similar to the "EI 108" laminations shown here ASCO Components - Transformer Laminations, Bobbins, Angles, Frames, Terminals and Enclosures..
I was going to "square stack" them. Can anybody tell me if that seems about appropriate for a 50W bass OT?
Also, I was going to go for 2400 turns on the primary, that's in total. Or 1200 turns each side of the centre tap if that's a better way of saying it. To me it seems adequate, but does anybody know if that is fairly typical? I'm not asking anybody to pick apart an OT, but if anybody here has ever had to rewind one and can speak from experience and and can say one way or the other it would greatly set my mind at rest before I start winding the thing.
Cheers!!

[Steve Conner]

You need to know about interleaving. Even bass amp OTs were probably interleaved once.

[Chuck H]

Never done it. But from what I've read there's a lot to know and experience is best. So many
factors that our blunt DMMs can't measure effect the performance and every parameter seems to be a tradeoff with something else. I guess what I'm saying is that if "I" were going to try it I would probably want to copy a known design. And thats tough because I don't know of any archive of OT blueprints.

It's a simple enough thing to wind a transformer that will "work". How will it sound and perform??? I think the outcome without the benefit of a known design or experience is unpredictable. That's why I've never done it. I don't make my own cheese either. And people who haven't studied mycology shouldn't pick wild mushrooms. Of course the consequences of a bad sounding transformer are not as grave. My point is, unless you want to do this as a beginning to learning how to do it, and so plan to wind more and more, I don't think you should expect that by making your own OT that you'll achieve the performance level of already available units.

JM2C

Chuck

[defaced]

Ran across this while searching for something else. Looks like it covers how to calculate the number of turns required. http://www.geofex.com/article_folder...es/xformer.htm

[Gregg]

Calculating the number of turns, wire diameter etc is only basic/raw data.
More important is the "Fit wire to core window" section from that article.
If you have EI108 laminations and if you don't know the steel data go for 40-50mm stack size just to be on the safe side although bass amp OT's don't go that low as you may think.

[tubeswell]

FWIW here's another link on this topic

output-trans-winding.html

[J M Fahey]

Short answer, a square stack of EI 108 and your turns calculated will work well.[end of short answer]
I use our "#125" lamination, square stacked (29x29mm bobbin core), which would be about your "EI96" , with good results.
Tried and true 1900 turns primary; secondary to match desired speaker impedance, so your 2400 turns on a somewhat larger core will be fine for bass.
I see you have designed for a lower frequency cutoff (¿40 hz?), yet it still looks reasonable.
You will not have much highs, but don't go overbord with interleaving; the simplest one would be 1/2 the primary (your 1200 turns), the secondary, then the other half primary.
Just wind it , put it in the amplifier, and test/play it.
Calculations can be very precise "mathematically", but we don't know what iron we are getting, so that reality cripples them big way.
We end designing something in the ballpark, testing it, and adjusting empirically.
Then hopefully that correction holds for a whole range of transformers.
I don't go crazy on specs or expecting audiophile grade iron, have adjusted designs for my "commercial reality" which is good power transformer grade iron, 0.35mm thickness, 1.8 or 2.2 W/Kg loss, non grain oriented, and get good, repeatable results.
Well, just another two cents.
This way, you'll get rich .

[Steve Conner]

In the RDH4, it says something like, "A really generously sized core of ordinary iron" is better than the fancy audiophile grade alloys.

I don't know if grain-oriented steel has any advantage in EI laminations: the grains are all oriented one way in the sheet that they're stamped on, but the magnetic flux goes in a bunch of different directions as it makes its way round. You only get the advantages when the flux is going the right direction relative to the grain structure.

In toroidal and C-cores it makes a big difference, these are made from a single strip wound into a big coil, so the grains can point the right way at all times.

If there isn't enough interleaving, you won't be able to use NFB: the amp will go unstable and start to squeal when you try. Again, RDH4 has a whole chapter on this. I think most guitar and bass OTs are interleaved once in the way JM says.

[Diablo]

I don't know if grain-oriented steel has any advantage in EI laminations: the grains are all oriented one way in the sheet that they're stamped on, but the magnetic flux goes in a bunch of different directions as it makes its way round. You only get the advantages when the flux is going the right direction relative to the grain structure.

In toroidal and C-cores it makes a big difference, these are made from a single strip wound into a big coil, so the grains can point the right way at all times.

Grain oriented will have much higher permeability and lower coreloss in the rolling direction of the sheet. An E and I lamination can take advantage of grain oriented steel. The legs of the E are in the rolling direction, and the I lamination is in the rolling direction. The only part of the lamination that is in the transverse direction is the back side of the E. You can compensate for the poorer magnetic properties of the back side of the E by increasing its width. You are correct about the C-cores where everything is in the rolling direction.

[R.G.]

The short answer to this is: experienced professional transformer designers usually expect to do more than one, up to several iterations on a design if it's a new (to them) design and not really close to something else they've done or tried before. You are correct in what you just thought that the experienced pros and companies doing this keep very detailed records of every design, even the "failures" as a starting point for the next funny thing that comes in.

It is trivially easy to get some output transformer to work as an output transformer. In fact, I did try and get an off-the-shelf power transformer to work as an output transformer. Many Fenders from the "cheap, the-customer-can't-tell" era are just non-interleaved transformers which might as well be power transformers for all the care taken with them. There is such a huge window of "works as an output transformer" that you really have to mess up to have it not do something.

Getting it to meet specific power output, power bandwidth, frequency response, and distortion specifications and at the same time making it easy/practical to manufacture in quantity and also minimize the money spent on both materials and labor is quite another thing. Getting hi-fi frequency response and distortion performance in anything that can just be manufactured in quantity at all is about an order of magnitude harder than that even.

I used to design power and switching transformers for a living. I've read extensively on output transformers and actually done some autopsies for fun as well as winding or "finding" them as victims. I've specified output transformers for the professionals to take and make manufacturable. But I would not take a job actually designing output transformers for money because I don't have the experience base that is absolutely critical to having this work well.

I'm not trying to dissuade you from doing something you will enjoy. But don't beat yourself up if it takes you several tries to get something you like. It's not rocket surgery - but it is tricky to do elegantly and well while you are hitting a predetermined performance target. Go read the RDH4 on audio output transformers and interleaving. Or "Transformers for Electronic Circuits" for some theory.

[PositiveNegativeMan]

Big Thanks to everybody for all the replies!!! I find much of the information to be encouraging. Maybe I'm mad for wanting to give it a go, I'm almost certainly mad for thinking winding the thing will be "fun", but I think the learning through the process will be enough of a reward and if I like the sound of the end result then that will be a considerable bonus!!
I had intended interleaving 1/4 by 1/4. I did try to design for 40hz cutoff (Well spotted J M!!!) But it's all on paper at the moment and I admit I'm toying with the idea of revising that upwards.
Thanks to everybody though. The advice, information, and above all your time are very much appreciated!!

[Steve Conner]

Maybe I'm mad for wanting to give it a go, I'm almost certainly mad for thinking winding the thing will be "fun"

Oh, it'll be great fun If you started trying to sell them for a living, that's when the nightmare would start.

For a one-off hobbyist thing, you can just make everything a bit bigger than it needs to be, and that'll increase your chances of success greatly.

[R.G.]

For a one-off hobbyist thing, you can just make everything a bit bigger than it needs to be, and that'll increase your chances of success greatly.

Dead-on accurate. Not having to meet some arbitrary size, weight, or cost number for the guys in the front office make the chances of success hugely better. Not having to *defend* your decisions to do it right instead of cheap is priceless.