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Old 10th December 2005, 08:36 PM   #31
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Question Output transformer redesign help

I have the duncan amps program; it's really cool. But, I would like to shift the conversation to the subject of an amp's output transformers. Specifically the subject of re-design, and transformer quality.

As far as transformer re-winding is concerned, I am confident that I could do it, (especially w/the magnetics design handbook availible on The Texas Insturments webpage) because I am already experienced in AC Motor re-winding and redesign. But all of my knowledge is industrial; I'm wondering what makes an output transformer "high end". What makes tranformer quality go beyond the formula "Turns in primary/Turns in secondary=V primary/V secondary"?

I don't have any projects on the agenda, but I was considering building my own amp sometime soon; and I was thinking that I would transplant and redesign the transformers from my current amp; if the cost should end up being less vs. the cost of brand new transformers.

So, what makes a transformer "high-end"?
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Old 10th December 2005, 08:57 PM   #32
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Sir Trefor,

Do you have access to the Radiotron Designers' Handbook 4th ed.?

The chapters there are somewhat dated as some of the magnetic materials have changed (pretty much the same for audio though), but otherwise the methods are sound.

Material choices aside;

You extend the bottom range of a transformer by increasing inductance... more turns and therfore more iron

You extend the high frequency performance by interleaving the primaries & secondaries... your goal there is to minimize the leakage inductance.

Interleaving the windings increases the capacitive coupling between the primary & secondary... so a comprimise is in order.

Capacitive coupling may or may not be an issue in an OPT... depends. Shields can be placed between the layers to reduce coupling but this adds capacitance to ground and consumes space in the bobbin that you would like to fill with copper.

All of these things lead to a bigger transformer... hence, why the better ones are usually bigger. You could certainly improve the performance of transformer by rewinding it... in the end you will have to settle for less power though.

I am sure someone here knows of link to the Radiotron book. I know it can be purchased in electronic format. It will have an angle that the TI site night not... TI is pprobably all about switching power supply stuff??
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Old 10th December 2005, 09:50 PM   #33
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Quote:
Originally posted by poobah

Material choices aside;

You extend the bottom range of a transformer by increasing inductance... more turns and therfore more iron


All of these things lead to a bigger transformer... hence, why the better ones are usually bigger. You could certainly improve the performance of transformer by rewinding it... in the end you will have to settle for less power though.
That's why the TI page is valuable; it has all of the magnetic core information needed to know when re-designing a transformer. Even though you can re-design a trasformer to whatever spec you like, the iron needs to be there; so there is a limit to what you can do with a transformer of a certain size. I imagine you can over-saturate the iron with too many lines of flux just like you can an AC motor core.

Quote:
Originally posted by poobah
Do you have access to the Radiotron Designers' Handbook 4th ed.?

I am sure someone here knows of link to the Radiotron book. I know it can be purchased in electronic format.
Sounds interesting... I'll google it, and check on eBay.
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Old 10th December 2005, 09:59 PM   #34
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HERE it is.

Before I buy it, though, is it worth it? Does it have enough information in it relative to my needs and intended applications?

Just wanted some feedback first.
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Old 10th December 2005, 10:13 PM   #35
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I would let some other people reply first. The Radiotron Designer's Handbook 4th ed. is a classic book though... considered a bible many tube nutz. There is all kinds of stuff in it... not just transformers... tubes, math, radio, audio.

I picked up a real one on Ebay for $40 US. I think I saw electronic ones for r$20.

There are certainly more books on just transformer design... see what other say too. Some will say EVERY tube nut should have RDH4.

Remember though, if you rewind for better quality, you lose power... nothing is free! Except advice on DYIAudio.

Cheers,
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Old 10th December 2005, 10:21 PM   #36
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Sir Trefor

On Ebay,

http://cgi.ebay.com/Radiotron-Design...QQcmdZViewItem

$6

I sure this guy has lot's of copies... might be hard to read and use the charts though... you should ask for some sample pages

Ask for page 219 Fig. 5.13H

Some on Amazon too... $30 and up... make sure you get 4th edition.

Later
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Old 10th December 2005, 10:22 PM   #37
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http://cgi.ebay.com/Radiotron-Design...QQcmdZViewItem
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Old 10th December 2005, 11:07 PM   #38
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Third edition of RDH is worth having ESPECIALLY for a beginner - but it's only 1/5 the size - 300pp vs. the 1500 pages of the later edition. Don't pay over $10 for it... you can download it from here
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Old 11th December 2005, 01:31 AM   #39
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If you go to the web site of forum member "GEEK" the entire RDH4 manual is available for download. The link is in the upper right hand corner of the page. There is loads of information in that book, however the book was written for engineers. Some of it is pretty technical.

http://geek.scorpiorising.ca/

Basically if re-designing an OPT, you are trying to minimize parasitic losses, and extend the frequency response. Loss is reduced by using a better grade laminations, and wire. A really high end transformer will use silver wire to reduce resistive losses. Frequency response is improved on the low end by increasing the primary inductance. It is improved on the high end by reducing unwanted capacitance, and leakage inductance. However these terms tend to be mutually exclusive, so every transformer is always a compromize. Be prepared to experiment a few times to get a good sounding transformer. From my experience, a push - pull transformer is easier than a single ended transformer, and the "success window" gets smaller as the power level goes up.

My attempts at a 100 watt SE transformer proved disastrous, so I had one built by a transformer winder who makes excellent 10 watt SE transformers. It was far better than mine, but not good enough for HiFi. It will make a serious guitar amp though.

One more observation. I tried using Kapton tape for the insulation between windings. It killed the high frequency response. The dielectric constant of Kapton is much higher than paper, which makes the distributed capacitance go way up.
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Old 11th December 2005, 02:23 AM   #40
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Quote:
Originally posted by tubelab.com
If you go to the web site of forum member "GEEK" the entire RDH4 manual is available for download.
Went to his website and, I'll be damned, there is quite a bit of great info and articles on winding your own transformers.

Thanks to tubelab and Geek
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