Author Topic: DIY Simple Output Transformer  (Read 46816 times)

Offline Mars

  • Commercial Member
  • Trade Count: (+1)
  • *****
  • Posts: 1,034
  • Total likes: 1
  • “The path is the goal.” Mahatma Ghandi
    • Mars Tube Audio
DIY Simple Output Transformer
« on: October 26, 2009, 11:10:26 PM »
For those of you who are interested in the inner workings of an Output Transformer, I'll show you how I do it; and where to get the parts. Let us do a push-pull transformer for the EL34 tube.

A simple configuration is Class A push-pull. From the data page at http://tdsl.duncanamps.com/show.php?des=EL34WXT I get this:

A P/P (triode)    400    -    -    130 - 142    -          220 shared    5,000    16.5    3    Pair of tubes

So it is going to be a 16.5 watt output transformer that should be able to handle a max current of 142mA. The anode load is 5000 ohms and the speaker load I choose as 6 ohms.

Next I need some idea of what Afe (cross section of the rectangular core that is in the centre of the bobbin; expressed in mm^2) I need to pump out 16.5 watts.

From Turner's web page ( http://www.turneraudio.com.au/output-trans-pp-calc.htm ) I use this formula:

Afe =300Xsquare-root(power output needed)

thus: =300 X sqr(16.5)=1219mm^2

To get the tongue width if the laminations to be used I square root the Afe = sqr(1219)=35mm

Tongue sizes comes in these standard sizes at AMC: 20mm, 25mm, 32mm, 38mm, 44mm, 51mm, 63.5mm

I choose the 32mm size of the EI96 (1.25 inch). Thus I go with a 32X32mm bobbin on a 32mm stack of EI96 laminations.

Next I'll work out what thickness copper I need for the Primary and secondary. That depends to some extent on my layering arrangement I am going to use. That is a whole topic on it's own ;)......

to be continued

OEM manufacture of Tube Audio
Power and Integrated Amplifiers
Phono Pre-amplifiers
DIY Kits
DIY Workshops

Offline GearSlave

  • Peacekeeper Extraordinaire
  • Trade Count: (+3)
  • AVForums Grandmaster
  • *****
  • Posts: 8,796
  • Total likes: 0
  • Ohm's Law has no business in Audio
    • Studio B
Re: DIY Simple Output Transformer
« Reply #1 on: October 27, 2009, 07:18:52 AM »
It is very, very cool of you to start a thread like this. This subject has popped up many times in the past and I feel you deserve a case of beer ;)
Resistance is not futile; it is voltage divided by current (R=V/I)

Offline ghostinthemachine

Re: DIY Simple Output Transformer
« Reply #2 on: October 27, 2009, 08:33:19 AM »
This is great - keep it coming!
Inactive

Offline Hi-Phibian

  • Commercial Member
  • Trade Count: (+43)
  • *****
  • Posts: 11,853
  • Total likes: 33
  • I really prefer email, see my banner for address..
    • Croak Audio Exploration
Re: DIY Simple Output Transformer
« Reply #3 on: October 27, 2009, 09:15:58 AM »
yup, some would keep this close to their chest.. I think I would if I made tube amps...
maybe it is the dutch in me...
Proprietor of Croak Audio Exploration.
Fair, not crazy, cash paid for turntables and tonearms from Rega, Linn and Thorens.          http://www.croak.co.za

Offline Timber_MG

Re: DIY Simple Output Transformer
« Reply #4 on: October 27, 2009, 12:21:14 PM »
I agree wholeheartedly with Gert's sentiment. Awesome stuff, perhaps Ampdog can add something as well. I am lead to believe that OT winding is a bit of a black art.

Offline Ampdog

Re: DIY Simple Output Transformer
« Reply #5 on: October 27, 2009, 10:03:01 PM »
.... I am lead to believe that OT winding is a bit of a black art.

Not really (I would imagine that Karel will agree). It only seems so for those who do not really understand all the relevant factors. As will be evident from Karel's description, e.g. space between layers etc. has an influence on leakage reactance and interwinding capacitance, thus consistent results would require neat winding and assembly. E.g. quite a difference in leakage can be had with 'loose' windings. In that sense it can appear as an art. I take the sometimes frowned upon route of afterwards compressing the windings on the flat sides to go into the core - dimensions on the rounds indicate that the necessary space is there. There is no such thing as 2 - 3% tolerance in transformers as e.g. with resistors and capacitors. Winding ratio; Yes, but not regarding internal capacitance and leakage reactance. 10% tolerance is not unusual and should be catered for in the amplifier design.

I myself use the approach in Radio Designer's Handbook (IV) (originally from Norman Crowhurst) and has never found that wanting. That uses a number of graphs. Earlier Karel quoted a suitable PC programme. Let us bear with him as he continues. He is doing a good job.
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline Mars

  • Commercial Member
  • Trade Count: (+1)
  • *****
  • Posts: 1,034
  • Total likes: 1
  • “The path is the goal.” Mahatma Ghandi
    • Mars Tube Audio
Re: DIY Simple Output Transformer
« Reply #6 on: October 29, 2009, 10:35:48 PM »
Thanks for all the feedback. To recap: to design your OPT (output transformer) you need to know the following:

1. power you want to deliver.
2. lowest frequency you want to pass - do not go lower than you need (and save weight/money)
3. current demands of the tubes in the output stage
4. layering arrangement

1 and 3 has been covered. Let us look at topic 4:

For a push-pull transformer you need some kind of balanced layering: two anodes that want an identical load and a centre tap. The simplest transformer is obviously the following:



The core is at the bottom of the picture; so you see a layer 2 (Primary layer 2) then the secondary windings and then lastly layer 1 (Primary layer 1). Layer 1 and 2 have the same amount of windings to maintain balanced impedance. This is a very popular winding method, but the DC resistance in Layer 1 and Layer 2 are not identical. The outer layer uses a longer length of copper to do the same amount of windings on the inner layer. Technically speaking: The average winding length of the one primary is not the same as the average winding length of the other. I would rather suggest we improve the arrangement a bit to this:



Look at picture b

Here the secondary is split up into two layers with identical number of windings.

Half primary-anode 1
Half secondary
Primary-anode 2
Half secondary
Half primary-anode 1
Core

In this arrangement the DC balance is better maintained. Note that the secondaries are wired in parallel. The two halves of Primary 1 should be connected in series.

I would also suggest that you roll the layers one at a time with paper/mylar layer to separate the layers. 0.1 mm thickness is good. If you can get thinner material, let us know:-)



Another suggestion is to keep secondary windings number to full layers. Thus: If you can wind 100 windings side by side on the bobbin to fill the full width available; keep the secondary winding number to multiples of 100. On the Primary side I would also suggest sticking to winding numbers that are multiples of complete layers.

Next we explore the software and enter the data we have. To follow...
OEM manufacture of Tube Audio
Power and Integrated Amplifiers
Phono Pre-amplifiers
DIY Kits
DIY Workshops

Offline ludo

Re: DIY Simple Output Transformer
« Reply #7 on: October 29, 2009, 11:31:19 PM »
This is how I wouldn't have approached it. So it's just as well I never tried eh? ;D ;D

Thanks Karel. I'm also watching with interest and will keep the questions piling up until I have a better grasp of it. Probably the questions all disappear as you continue.

Much Respect!

If it 's going vrot, put it on the buffet ! - Fats

Offline Mars

  • Commercial Member
  • Trade Count: (+1)
  • *****
  • Posts: 1,034
  • Total likes: 1
  • “The path is the goal.” Mahatma Ghandi
    • Mars Tube Audio
Re: DIY Simple Output Transformer
« Reply #8 on: November 05, 2009, 08:32:05 AM »
OK Everybody... Let us check out the software. Download the file from here:

http://www.dissident-audio.com/OPT_da/Page.html

And install it on your computer....
OEM manufacture of Tube Audio
Power and Integrated Amplifiers
Phono Pre-amplifiers
DIY Kits
DIY Workshops

Offline Mars

  • Commercial Member
  • Trade Count: (+1)
  • *****
  • Posts: 1,034
  • Total likes: 1
  • “The path is the goal.” Mahatma Ghandi
    • Mars Tube Audio
Re: DIY Simple Output Transformer
« Reply #9 on: November 06, 2009, 10:25:18 PM »
Once you have installed the software (and hopefully all went well 8) you should be greeted with a page looking more or less like this (except for all the call-outs of course)



Look at the green block: Start here and enter all your data. Use tab to move from field to field. It works better for me than mouse clicks.

Now things are going to get hairy.  :'( You'll need some high school maths to cope. You have to enter data about your bobbins and the laminations you have in stock into the software database. I'll show the data for the EI96 example, but you should enter as much items as possible, to get a good idea of what the software suggest you use. As you can see, there are examples already in the database, but they are for laminations and bobbins that you do not necessarily have available.

So now you need your magnetics suppliers data book for dimensions: I use Alloy Magnetics Cores for my supplies. Look at the Non Oriented Silicon Steel (NOSS) Continental Range.

On the software page, go to the grey block on the left bottom and click and high light any of the items listed in the table. Now click on "Edit Highlighted"... A small windows pops up. We need to name a new entry. I called mine EI96-32-32 and all the other fields go blank. Do not worry, we'll fill them with our own measured data.



We have to find the following:

1. What is the average Magnetic path length
2. What is the average Winding length
3. What is the weight of the 32mm stack of EI96 laminations
4. How deep is the bobbin
5. How wide is the bobbin
OEM manufacture of Tube Audio
Power and Integrated Amplifiers
Phono Pre-amplifiers
DIY Kits
DIY Workshops

Offline Mars

  • Commercial Member
  • Trade Count: (+1)
  • *****
  • Posts: 1,034
  • Total likes: 1
  • “The path is the goal.” Mahatma Ghandi
    • Mars Tube Audio
Re: DIY Simple Output Transformer
« Reply #10 on: January 09, 2010, 12:10:33 AM »
Let us move on to the physics of the laminations. Here is a diagram from the supplier's site:



TYPE     NOMINAL DIMENSIONS (mm)     
         A      B      C      D      E    F    G    H    Ψ    (kg/cm)     

EI96     32     96     80     16     16     48     64     16     5.5     0.480

This is the average magnetic path (see red dotted line)



So the length of this path is

ML=2X(C-H) + 2X(E+D)

The path runs in the middle of the laminations (excepting the tongue)

Thus ML=2X(80-16) + 2X(16+16)=192mm=19.2cm

Next we need to determine the weight of the core... to follow
OEM manufacture of Tube Audio
Power and Integrated Amplifiers
Phono Pre-amplifiers
DIY Kits
DIY Workshops

Offline Mars

  • Commercial Member
  • Trade Count: (+1)
  • *****
  • Posts: 1,034
  • Total likes: 1
  • “The path is the goal.” Mahatma Ghandi
    • Mars Tube Audio
Re: DIY Simple Output Transformer
« Reply #11 on: March 10, 2010, 10:34:22 AM »
Hi All

The posts are a trickle  :-[  ... things are hopefully  ::) coming to a head with this design ;D

Next entry on the iron data table that jumped out when you clicked on a EI type. We have done average magnetic path length.

The weight/cm ratio is in the table published above:

The weight of the laminations for a 3.2cm stack will be 0.480 (kg/cm) X 3.2 = 1.536kg

Next to do is the bobbin specifications....
OEM manufacture of Tube Audio
Power and Integrated Amplifiers
Phono Pre-amplifiers
DIY Kits
DIY Workshops

Offline Yves

Re: DIY Simple Output Transformer
« Reply #12 on: March 10, 2010, 01:21:55 PM »
Hi there from France  :) Pardon my poor english.
Glad to meet you and be able to chat. . .

Just a quick info for a first post:
With so called "scrapless laminations" the Magnetic Path Lenght is simply twice the lenght of the "I" part.

Thanks to Karel for its efforts trying to explain "how to use"

Yves.


Offline Mars

  • Commercial Member
  • Trade Count: (+1)
  • *****
  • Posts: 1,034
  • Total likes: 1
  • “The path is the goal.” Mahatma Ghandi
    • Mars Tube Audio
Re: DIY Simple Output Transformer
« Reply #13 on: March 16, 2010, 06:34:41 AM »
Ok, so let us look a the bobbin.

I use the so called "loose lead nylon bobbins".



The AMC website is shy with the dimensions of the EI96 bobbins, so I consulted the printed catalog:

It is called a 78A type bobbin (or 1-1/4") . For the 32/32 bobbin we have

       A         B         C         D        E
     32.5      32       63.5     44        1.5

We want to roll copper onto the bobbin, filling as much of the available space as possible, but not exceed the height of the window, otherwise the copper windings will scratch against the laminations and cause a short circuit.

From the data item E (1.5mm) we know how thick the plastic is. So I take it that there is a 1.5mm layer of plastic around the core.

So the available height is value C - A thus: 63.5-32.5=31

This is the value for both side added together, so half for a single window 31/2=15.5mm
Subtract the thickness of nylon at the bottom of the bobbin, thus 15.5-1.5=14mm

For our calculations, I am going to assume we have 14mm bobbin height available to roll copper onto.

Bobbin width is easy: value D (44mm)

So next we have to calculate the Mean Turn Length....

OEM manufacture of Tube Audio
Power and Integrated Amplifiers
Phono Pre-amplifiers
DIY Kits
DIY Workshops

Offline ludo

Re: DIY Simple Output Transformer
« Reply #14 on: March 16, 2010, 07:27:24 AM »
Ha! we have neglected to welcome Yves. No need to worry about your English Yves, it's a lot better than our French.

And THANK YOU for making the software available.

I'm not a valve amp builder, just a newbie with all of this, but I greatly appreciate Karel's effort here, and also your interest and contribution.

If we are very quiet, don't let that bother you. The information stays as a valuable resource for the future, when I'm certainly going to need it.

There were perhaps only two (maybe 2.5 :D) people here that understand the making of valve output transformers properly. Now there are three (3.5?). As a statistic, that is beautiful.

Thank you both Gentlemen! We are watching (1020 reads until today) but for now, most of us are still too new to this to comment much. Just keep at it, one day we'll catch up.

If it 's going vrot, put it on the buffet ! - Fats