Power supply for chip amp ?

TroelsG

Member
2013-05-23 8:20 am
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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Mark Johnson

Member
Paid Member
2011-05-27 3:27 pm
Silicon Valley
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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tinitus

Ex-Moderator R.I.P.
2005-11-24 1:47 am
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
 

TroelsG

Member
2013-05-23 8:20 am
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 :).
 

TroelsG

Member
2013-05-23 8:20 am
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.

You mention one of the secondaries, the one at 37 VAC (Could be the gray-gray or perhaps Blue-Blue). The other secondary, the CT secondary (Blue-Black-Blue) i measured to be at 18 VAC(Blue-Black). Is it even possible to use a single secondary for a stereo amp. I though it would have to be at least a CT secondary? If not, was is preferable. The single secondary at 37 VAC or the CT secondary at 18.5 VAC? Thanks
 
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tinitus

Ex-Moderator R.I.P.
2005-11-24 1:47 am
its a C-core with two seperate coils;)

funny thing is, toroids may not have been available yet, so B&O have used these small sized trafos that would fit into the well known flat B&O designs

that it is a C-core is just a sideeffect, but to your benefit now

a C-core have a small airgap, and is said to better resist DC/noise coming through mains voltage

ought to be perfect for a small chip amp
 

TroelsG

Member
2013-05-23 8:20 am
You have to specify @ which current.
Probably it's the opposite : the Grey-grey carries the current necessary for the
amplifier ( and I would restore the original, if possible)
and the 18.5-0-18.5 is for small current such preamplifier etc.

But is it possible to run a two channel stereo amp, with just one secondary?

PS. Just measured the grey-grey. It is 37 VAC as assumed. The blue blue is around 39-40 VAC. Blue Black 18.5-19.5 VAC.
 
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Nowadays most of SS amplifiers run with dual tensions (provided by a CT trafo)
which allows more CMRR and other things, like the elimination of the output capacitor. If your transformer is done that way, it meets the specifications of that project, which is quite common. I have lots of ( no, I have thrown them away !) 30-40 V transformers from amplifiers like that. That voltage is what the output devices of the time supported; also the current is limited.
As JM Fahey in #10 said, expect some 20-30 W from each channel with such
transformer.
 
Sorry for my lack of knowlegde on this subject, but just to clear up. If i want to use this transformer with a stereo amp (Dantimax) kit, I will have to use the blue-black-blue secondary at 18.5 VAC? The grey-grey is not useable for anything right?

you need thickest wire secondary and hopefully it delivers acceptable voltage
16-22 VAC suits LM3886 gainclone nicely.