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two toroids in series as OPT
two toroids in series as OPT
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Old 18th June 2019, 07:48 PM   #21
Wavebourn is offline Wavebourn  United States
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two toroids in series as OPT
You can use 2 toroids shorted by one massive common turn. Such a way I used to make line level transformers, shorting them by soldered copper foil. However, loss inductance between them will be pretty big.
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Old 19th June 2019, 12:45 AM   #22
smoking-amp is offline smoking-amp  United States
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I think you will need to make that "one massive turn" around the two toroids into a many turn winding around both toroids. Then put a LV DC supply (like 0.5 V or so) across that winding to neutralize the DC ampere turns of the tube primary in each toroid. A BIG cap across the DC supply to short AC. The AC from the two toroids is out of phase between them, so no AC resulting in the combined winding.

The AC from the two toroids needs to cancel in the extra winding, but the DC current applied should cancel the DC in the two toroids. The low V DC voltage will need precise adjustment, since only the winding resistance limits it. This technique could be extended to many 2X toroids used in series, one for each tube. Very little power need be used for the DC compensation, since the VDC is only the copper loss to achieve the ampere turns requirement.

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Old 19th June 2019, 05:42 AM   #23
Wavebourn is offline Wavebourn  United States
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two toroids in series as OPT
You are right Don, the common turn would not cancel DC bias. I was not thinking straight.

My apologies.
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Old 20th June 2019, 09:06 AM   #24
emk2 is offline emk2
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Still not made a decision but enough good information now, thank you all.

DF96, why class A is the worst case? more current but primary supposed to be balanced in all cases? what did I miss?

I unwinded two 50VA 12V indel toroids, didn't see dual primaries in series so I'll use them for heaters 3.15-0-3.15.

They have been put in varnish (for wood floor purpose..) and are totally silent now (at least when unloaded). 3mA AC RMS at primary unloaded.

It may be the right time to add a shield between primary and secondary but I don't know how it is made? does a single turn (short circuit) of copper tape connected to safety ground mains could do it?
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Old 20th June 2019, 11:53 AM   #25
Dinolobe is offline Dinolobe  Canada
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People that say it can’t be done should have a look at this amp :
Return of the Darling

It’s not the same circuit, but it seems that there’s at least one way to use toroidal power transformers as output transformers. Does it sound good ? I don’t know, but it’s an interesting design.

Last edited by Dinolobe; 20th June 2019 at 11:56 AM.
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