DIY Output Transformer

[sethinspain]

Hey guys,

Im about to embark on this fun journey of diy output transformers but i am stuck with a couple questions.

I found a great website that explains how to calculate things but the problem is that the calculations are for a push-pull transformer and a single ended transformer.  I am planning on making input and output transformers for tube mics and mic pres, so i am not sure if these same equations can be used to calculate core dimensions, wire dimensions and turns, ect.

There are some great documents available but i still have this doubt.  When do power transformers and audio transformers design practices begin to differentiate, or when do they stay similar?  What are the beginning questions one should ask before design a transformer?  Desired ratioo, impedences, ect?

Anyone with some experience would be appreciated here.

Cheers,
Seth

 

[Marik]

Radiotron 4th edition would be a good start. It has an excellent chapter on low signal transformers (i.e. exactly your application).
I think you can find electronic copy.

Best, M

 

[MatthisD]

What are the beginning questions one should ask before design a transformer?  Desired ratioo, impedences, ect?

What core types can one get would be a starting point.

 

[PRR]

No-DC signal transformers can follow Push-Pull calculations.

Differences are typically higher impedances, and signal-levels which are silly-small for any "power" transformer.

These interact. A mike transformer on the smallest "power" core will be large enough to catch a lot of hum, and also large enough to have a lot of stray capacitance which (with high impedance) will limit treble response. A higher permeability core can be smaller and therefore better. Cost/pound will be high and overload may be "low" by power standards, but in micro-power application both can be acceptable.

Since you have to ask: start with a small 24V-120V power tranfo as a mike step-up. It will work. Figure out how it could work better. Re-wind it to your own ideas. Smaller might be a goal, but high perm iron is much harder to find.

 

[CJ]

start out with the easy to do transformers> power outputs, 6V6 Champ, pri-sec,

steel core and #38 wire or larger,

you can buy wire, lams, bobbins and end bells from Edcor,

then maybe rewind a guitar pickup with #42 so you learn small wire,

graduate class would be to wind a Telefunken choke, 20,000 turns of #48 

nickel lams are expensive now days, cheaper to buy old mic/line inputs from evilbay,

forget the math for now, just wind and check results on the bench,
that's how engineers do it, math is for college students only,
learn how to measure inductance and how to plot frequency response,
stay away from power transformers, unless you live in a brick house with ceramic tiles on the roof,
copy some of the classics, buy iron and take it apart,  ;D

 

[ruffrecords]

Just remember that +26dBm into 600 ohms is 400mW so output transformers are small power transformers.

Cheers

Ian

 

[sethinspain]

Thanks guys!  Great contributions. 

I plan to wind and study as I go, yet I do want a basis to go off of for my first try.

The radiotron book is awesome, Marik, for even more than just OPT.  Thanks for the tip-off

Seth

 

[Landins]

1. Insulate properly. Be very catious.
2. Grasp inductive reactance.
3. Present correct load when measuring.
4. Buy heatgun.
5. Buy cheap oldies, and take apart. As said.
6. Grasp turns ratio and z-ratio.

 

[gyraf]

..and go through the material in our local Transformers "Meta" thread: http://groupdiy.com/index.php?topic=467

Jakob E.

 

[sethinspain]

That Meta is a gold mine!

Seth

 

[sethinspain]

Actually, do most power transformers made today use M6 laminations?

 

[Landins]

No. Usually lower grade steel. And powerlams are thicker (0,5mm) and thus induces more eddy currents that consumes usefull energy. Audiolams are thinner, 0,35 mm and below.

Informative tubin on this subject:

http://m.youtube.com/watch?v=q3YhbugYjJY

http://m.youtube.com/watch?v=UvHCQswnjEg

http://m.youtube.com/watch?v=y11SbmXPY18

 

[CJ]

tape is very important for audio transformer work,

you need some cloth tape, like this>

http://www.ebay.com/itm/P213-PERMACEL-GLASS-CLOTH-ELECTRICAL-TAPE-3-4-X-60-YD-/120531844249?pt=LH_DefaultDomain_0&hash=item1c1041fc99

and some yellow 3M polyester tape, like this>

http://www.ebay.com/itm/3M-56-POLYESTER-YELLOW-3-16-x-72YD-ELECTRICAL-TAPE-/150415194298?pt=LH_DefaultDomain_0&hash=item230571b8ba

poly tape will loosen up with heat and varnish, so you need the cloth tape to hold the lead wires to the coil,


you need insulation,

Nomex 410 is the Cadilac  of all papers, used in high quality transformers, sounds great, yes, insulation can influence sound,

http://www.ebay.com/itm/Nomex-410-Insulation-Paper-10-mil-thick-2-each-8-x12-Sheets-Aramid-Electrical-/261450086740?pt=LH_DefaultDomain_0&hash=item3cdfa3b954

Nomex is best used in between pri and sec layers, too expensive to use as layer insulation,

for layer insl. you use kraft paper, or KP as they call it on xfmr blueprints,
you can use heavy KP from the butcher shop at your grocery store, good for using underneath lead wire splices to protect inner layers and also as a cheaper wrapper than Nomex,

http://www.ebay.com/itm/Natural-kraft-tissue-paper-Acid-free-100-sheets-15x20-/261417569612?pt=LH_DefaultDomain_0&hash=item3cddb38d4c

for a UTC A-10 which has many layers on a small coil, you use Glassine Paper,

this paper is non acidic which means it will not eat away ay fine wire,
layer insl allows smooth winds, without it, you end up with a random wind over a smooth wind as the wire will catch edges and go back and forth,

http://www.ebay.com/itm/Legion-Paper-Glassine-Onterleaving-Paper-/130885683931?pt=LH_DefaultDomain_0&var=&hash=item1e79650adb

you set up the frequency response of the coil with insl. layers, if you have too much hi end roll off, you increase the distance between pri and sec sections, this is done by adjusting the amount of insl til you get a flat response, or whatever response roll off you are hoping for,

so transformer work involves a lot of experimentation, use a recycle bin for copper wire as you will be doing as much unwinding as winding when doing prototypes,

keep a log book so you can record how many turns of each wire gauge you use can be fit onto all your different bobbin sizes, how many turns you get per layer,
they have charts for this but they only get you in the ballpark as the wire you use may vary from the wire used on the wire table in the book,

also record turns vs inductance for various size lams,

like 1000 turns = 1 Henry on 75 EI silicon, 400 turns = 10 Henries on 375 EI Super,

Processing-

bake you transformers for 2 hours at 220 F, then dip in your favorite varnish,
Poly-Verathane works good, clear semi gloss, wait til the small bubbles stop,

you can rebake the dipped transfromer if nobody is in the house, or let it air dry for a few days,

you can use wax to dip the coil in or dump in a mu can, but finding the old poly wax can be hard, some say it sounds better than varnish, using nothing at all sounds great but wires might break after a while as there is mechanical force generated inside coil wires which can mean wire vibration,

here are a few taping techniques>

 

[sethinspain]

CJ,

I have this sudden urge to buy you a bottle of some nice, Belgium beer!!

Thanks and more thanks to you!

 

[alexc]

This is some nicely informative discussion.  Thanks.

Who would've thought to need non-acidic insulation paper in transformers
And that it does influence sound!

Here's a question - how difficult is the 'voicing' of a transformer ?

In the example of a 6V6 singled ended Champ style, there are premium models out there that are said to be specially 'voiced' to be 'great' as opposed to your more 'run of the mill' models.

-  tailoring the freq response with empirically determined winding asymmetry (s)

I've seen CJ pics of them in his dissections which were used for capacitance tweeks.

also by

- exotic lam materials and geometries  ie. shape, stacking factors and thickness
- the use of virgin copper or not 

 

[alexc]

Of the couple of traffos I've used and thought were unusually 'awesome' sounding, the most recent is the cinemag 2461nico.

It seems to have a certain something in the low mids - I used the same simple triode-mode-pentode tube mic amp circuit with a couple of different types of traffos and the cinemag just leaped out and said buy 2 more. Today. The others were 'working as expected' to be sure but didn't stand right out.

Obviously the desirable response is very subjective, but I can't help thinking that it's been made by people who know a thing or two extra in the way of 'voicing magic'.

Anyway - please continue !

 

[CJ]

match the output transformer to the speaker and cabinet,

you can play with the gap on a single ended transformer to get the bottom end where you want it,

and you can play with the coil to get the high end dialed in,

smaller wire can yield a coil with less capacitance which can increase hi end,

experimentation is a fun way to get the sound you want as you do not need to use a slide rule,

no wait.......

calculator.

 

[MicDaddy]

CJ, can you go into some detail on interlayer insulation?

Is this between EVERY layer, for all types of transformers?  I've seen some windings where there was none, where there is only interwinding insulation.

How to know when to use? 

What would you expect the differences in say two identical stack lams and coils built with same turns/layers with the exception being one has interlayer insulation with nice pretty lays and the other does not and is "free wound"?

Do you varnish under a vacuum?

 

[CJ]

wire insulation is a lot better these days, so you can random wind wire right on top of each other, this will lower the interlayer capacitance a bit as there is more space between turns than in a flat winding,

you might get a coil with slightly better HF response if random wound, depends on the transformer, there is a youtube video of a guy winding guitar pickups and comparing random wound to even wound, he has a frequency analyzer that does not show much difference in response or the resonant peak of the pickup, but that is thin wire, thicker wire which is random wound will put more air in between turns, thus making the difference between random and flat wound a little bigger,

if you use, say,  #34 wire or bigger, you will probably want to wind it flat, in this case, you need insulation between every layer to get a smooth platform to wind on,

if you have a high voltage winding, then you probably want to use insulation between every layer no matter what, especially in power transformers,

 

[MicDaddy]

Thanks CJ! 

Say is there any concern with wire insulation build, say a single poly nylon 155C vs a heavy poly nylon?  Probably something to do with what you went into with the decreasing in interlayer capacitance (how much) as the build of insulation is slightly more on heavy vs single.... eh it's possibly not discernible in audio.  I'd be curious to see the same built transformers with the magnet wire insulation differences.  This is most likely digging in the weeds.  Thanks again for all the help, almost know enough to be dangerous over here.