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Old 30th December 2011, 06:36 AM   #1
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Default Back to back autotransformers for B+?

I think I know the answer already, I'm trying to get my B+...I have a large enough transformer core to rewind to get an isolation type TX but wire in bulk is virtually impossible to find & a reel of suitable wire is god-awful priced. Plus the 'old' bobbin got roasted. Bouncing around ideas, would back to back autotransformers work for this application? Autotransformers here are a dime a dozen. If not I'll be chasing around for wires till the cows come home.

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Old 30th December 2011, 07:56 AM   #2
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If the input of the first transformer is directly connected to the mains, I would not use it. Because when touching the amplifiers chassis you directly touches the mains, and that is not healtly!!
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Old 30th December 2011, 08:33 AM   #3
Bone is offline Bone  United Kingdom
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NEVER use auto transformers connected to the mains. It will result in a LIVE chassis
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Old 30th December 2011, 09:17 AM   #4
godfrey is offline godfrey  South Africa
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Originally Posted by Richard Ellis View Post
Autotransformers here are a dime a dozen.
How about unwinding one of those to get the wire you need?
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Old 30th December 2011, 10:24 AM   #5
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There´s usually not enough room on the bobbin to rewind an autoformer with an additional winding.

Check if they have soldered the different parts of the winding outside the bobbin.

Then you can split the windings and connect 2 transformers for an 1:1 isolation.

It´s not as safe as a regular isolation tx since wire insulation is the only thing making insulation.

Last edited by flatheadmurre; 30th December 2011 at 10:37 AM.
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Old 30th December 2011, 10:24 AM   #6
dhaen is offline dhaen  Europe
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Please don't use autotransformers. Even if you can arrange for a "neutral" chassis, you will never get the noise down as much as you'd like.
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Old 30th December 2011, 10:42 AM   #7
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Reusing the wire is a gamble when you bend it the insulation cracks.

Causing problems that can lead to arcover.

Best use for an auto former is to gap the core and use it for choke.
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Old 30th December 2011, 10:42 AM   #8
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As high voltage transformers are becoming "Audiophile" and "Expensive", why not try Back-Backing simple mains transformers.

For example.

If you have two 115+115 / 12 transformers.

I'm assuming that you have 115V mains.

Parallel the primaries of the first transformer.

Connect the secondary to the secondary of the second transformer.

The primaries of the second transformer will then have 115V each on them.

In series they will provide 240V AC or 340V DC.
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Old 30th December 2011, 11:35 AM   #9
Aiace is offline Aiace  Italy
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Originally Posted by KatieandDad View Post
As high voltage transformers are becoming "Audiophile" and "Expensive", why not try Back-Backing simple mains transformers.

For example.

If you have two 115+115 / 12 transformers.

I'm assuming that you have 115V mains.

Parallel the primaries of the first transformer.

Connect the secondary to the secondary of the second transformer.

The primaries of the second transformer will then have 115V each on them.

In series they will provide 240V AC or 340V DC.
Right but the transformers will be very hot soon due the big current flowing between the secondaries.
To reduce the heat yuo have to reduce the current, so don't use 12V secondary but a voltage as hight as you can. More is better.
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Old 30th December 2011, 11:44 AM   #10
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I don't that would work. Actually, autotransformers are just like big resistors. And resistors can't increase voltage.
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