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Old 26th May 2013, 06:39 PM   #1
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Default Power supply for chip amp ?

Hi, i salvaged this old transformer from an old Bang and Olufsen amp. I want to use it for a gainclone project, but i am wondering if the VA value is big enough. How is it possible to determine this value? There i no information on the transformer itself. I do have the diagram of the amp it was salvaged from, but i cant figure out if it can tell me anything.

Thanks
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Last edited by TroelsG; 26th May 2013 at 06:54 PM.
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Old 26th May 2013, 07:38 PM   #2
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If you are not able to look it up, or measure it, then as a last resort you can weigh the transformer.

I put the catalog data for (these Triad transformers) into a spreadsheet and performed linear regression.

Not surprisingly, there is a very good straight line relationship (R^2 = 0.998!) between weight and VA rating. I think Bob Cordell's book on amplifier design mentions this linear relationship, for toroidal transformers. However these Triad transformers in the attached plot are laminated, non-toroidal types.
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Old 26th May 2013, 08:06 PM   #3
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Ok thanks, the transformer weighs 1721 grams, so i could possibly assume that the VA value is about 120 right?
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Old 26th May 2013, 09:24 PM   #4
tinitus is offline tinitus  Europe
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may be a bit more than 120VA, but surely less than 200VA
somewhere in between

nice C-core
should be ok for a modest chipamp
if voltage fits
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Old 27th May 2013, 05:06 PM   #5
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I am considering using it with some of the kits around, audiospector or chipamp.com. Do you know if there is any guides on how to use a center tapped transformer with these kits. Is it even possible?

Thanks
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Old 28th May 2013, 06:23 PM   #6
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by TroelsG View Post
Do you know if there is any guides on how to use a center tapped transformer with these kits. Is it even possible?
Thanks
amp kits should be ok with that
but their power supply kits seem to be designed with double bridge for dual secondaries trafo

but you may take a look at Dantimax LM4780 kit
open schematic file, and you will see how it uses a CT trafo

Dantimax (elektronik) - Preamps_-_poweramps

other power supply kits may be converted to using one bridge
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Old 28th May 2013, 06:34 PM   #7
tinitus is offline tinitus  Europe
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I moved your thread to Chip Amps forum

that should give you better answers


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Old 28th May 2013, 07:31 PM   #8
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Quote:
Originally Posted by tinitus View Post
amp kits should be ok with that
but their power supply kits seem to be designed with double bridge for dual secondaries trafo

but you may take a look at Dantimax LM4780 kit
open schematic file, and you will see how it uses a CT trafo

Dantimax (elektronik) - Preamps_-_poweramps

other power supply kits may be converted to using one bridge
Well, thats very convenient. I live in Denmark and i have already sent Dantimax a mail asking them if their kits are compatible with CT transformers. Thats not necessary anymore .
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Old 29th May 2013, 02:42 AM   #9
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In that diagram one of the secondaries is listed as 37V AC. That is FAR too high for most chipamps, definitely for the LM4780/LM3886.
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Old 29th May 2013, 04:16 AM   #10
JMFahey is offline JMFahey  Argentina
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Sorry but 37VAC means around 52V , well within a LM3886 ratings.

Of course, having a single secondary, it means a single supply amp, with output coupling capacitor.

It will give you around 30W into 8 ohms and 55W into 4 .

Not bad at all.

Check the Power vs Voltage (figure 37) on the datasheet.

http://www.ti.com/lit/ds/symlink/lm3886.pdf
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