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Old 18th August 2011, 10:01 PM   #1
emosms is offline emosms  Bulgaria
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Default PP OT made of 2 transformers

Hi,
Is it possible to couple 2 transformers and make "one" for a PP output?
Connection in series, to get the center tap and make a double more powerful unit.
Best Regards
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Old 18th August 2011, 10:17 PM   #2
tomchr is offline tomchr  United States
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If you split the trafo like that, each trafo will end up carrying a DC current. Hence, their cores will saturate. Unless, you design them not to - as done for SE trafos.

So I think the answer to your question is no.

~Tom
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Old 18th August 2011, 10:34 PM   #3
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Originally Posted by tomchr View Post
If you split the trafo like that, each trafo will end up carrying a DC current. Hence, their cores will saturate. Unless, you design them not to - as done for SE trafos.

So I think the answer to your question is no.

~Tom
This is interesting.
The windings have to 'share' the same core?
I'm thinking about using a toroid PT as a PP output transformer, connecting the two 120vac primaries to form a 'center-tapped' primary. Others have reported success with this.
It does seem similar to the idea that emosms is considering.
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Old 18th August 2011, 10:41 PM   #4
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by VictoriaGuy View Post
This is interesting.
The windings have to 'share' the same core?
I'm thinking about using a toroid PT as a PP output transformer, connecting the two 120vac primaries to form a 'center-tapped' primary. Others have reported success with this.
It does seem similar to the idea that emosms is considering.
Yes in a PP transformer the windings have to share the same core, the opposing dc fields in the two halves of the primary cancel each other leaving only the AC component - this allows for a significantly smaller core.

Several people here have extensive experience with toroid power transformers as OPTs.
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Old 18th August 2011, 10:58 PM   #5
AJT is offline AJT  Philippines
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Quote:
Originally Posted by emosms View Post
Hi,
Is it possible to couple 2 transformers and make "one" for a PP output?
Connection in series, to get the center tap and make a double more powerful unit.
Best Regards
perhaps this is a better alternative:

Crowhurst's Twin Coupled Amplifier
http://www.audioxpress.com/magsdirx/...0crowhurst.pdf

Last edited by AJT; 18th August 2011 at 11:12 PM.
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Old 18th August 2011, 11:25 PM   #6
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Why not take two E-I transformers, Strip off the lamms and rearrange so they can face each other with the 'E's used in each one, and the 'I''s discarded....

Make up a frame to keep them held together, and you can arrange the 'primaries' to magnetically cancel the 'DC' component in the coil/iron...

I did this many years ago with some SE transformers from TV sets, Worked out pretty good too....
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Old 19th August 2011, 12:12 AM   #7
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Maybe a kludge of this sort could work:
The center tapped low voltage (2 Volt maybe) DC supply (T3 and bridge rectifier) provides current to the secondaries so as to null out the primary DC tube currents in each OT xfmr. (The audio AC is nulled out at the DC supply insertion point) The speaker has the DC nulled out by connecting to the center tap of T3.

edit: The Rlimit resistors for current control would be better put on the DC sides of the bridge rect. than on the AC sides as shown. Then any mismatch between them would only inject a bit of DC to the speaker (rather than 60 Hz hum as configured).
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Last edited by smoking-amp; 19th August 2011 at 12:27 AM.
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Old 19th August 2011, 12:29 AM   #8
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Quote:
Originally Posted by emosms View Post
Hi,
Is it possible to couple 2 transformers and make "one" for a PP output?
Connection in series, to get the center tap and make a double more powerful unit.
Best Regards

Do you mean to use 2 big expensive transformers instead of one smaller and cheaper?

Yes, why not?
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Old 19th August 2011, 12:39 AM   #9
Gilgy is offline Gilgy  United Kingdom
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The only time I've heard of this being done was in the Marshall JTM45

Big thread about it HERE with schematics
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Old 19th August 2011, 01:05 AM   #10
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"Connection in series, to get the center tap "

For two "OTs" with no primary center tap on each and no air gap, the only practical way is in post #7, or go Circlotron mode, or use a totem pole tube output.

If the "OTs" do have dual primaries, then it's much easier, since each side of the P-P tubes can go thru a primary on both xfmrs to get DC balance. The Crowhurst Twin Coupled will work then too.

note:
Another improvement on the post #7 diagram: replace the C with two C in series and connect their center point to the CT on T3. That will eliminate any chance of bridge generated glitches from 60 Hz AC commutation.
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Last edited by smoking-amp; 19th August 2011 at 01:23 AM.
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