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Old 25th October 2001, 11:42 AM   #1
Edgar is offline Edgar  Norway
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Hi!
My first question on this forum. Been watching it for some time now, its really good!
I have a transformer I ripped from an old amp. It looks like it's a 72V with a center tap (two red leads=72V, and black lead=CT). I was wondering if its possible to "parallel" it to give 36V (tie Tie together the two red leads and use red/red and black as the outputs)? This means I could get a +/- 25V supply after rectification. (Want to make a small cheap amp to make some noise until my Alep3 is finished:)

B B-|-----------
B B |
B B | 36V
B B-| ----------
B B |
B B |
B B-|


I don't know if the center tap are meant to be used this way…
Any comments appreciated.

Edgar
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Old 25th October 2001, 06:50 PM   #2
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No. But if the amplifier has twin 120V primaries
as many do, you can series them to halve the secondary
voltages.
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Old 25th October 2001, 07:39 PM   #3
paulb is offline paulb  Canada
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Yes, if you connect the red wires together you'll blow the fuse. You will use a fuse, right?

I don't understand how wiring two 120 V primaries in a series bucking arrangement will help, maybe I'm missing something.

You can't get +/- 25 directly from a 72 VCT transformer, but you can get a +50V single supply by using a full-wave centre-tap arrangement. Perhaps your small cheap amp could be one with a single supply. If it's based on a power amp IC, it can probably be wired either way. Even if not, it may not be difficult to modify the schematic to run from a single supply. You'll need an speaker output capacitor for each channel, but that's really not a big issue in my mind. And it's only till you get your Aleph done, right?
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Old 25th October 2001, 11:36 PM   #4
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Cool Primary series

Well,

I presume Nelson meant that when the amp has a 120V / 230V switch somewhere it is most likely there are two primary windings.

When your local voltage is 120V and you use the two 120V primaries in series you end up with 36V on the output.

The reason for putting in two 120V windings is that one way (230V running through 2 120V windings ) or the other (120V running through 2 sets of windings) keeps the current flowing through all the primary wire within safe limits.

Succes
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Old 26th October 2001, 01:50 AM   #5
paulb is offline paulb  Canada
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Yes, apparently I was missing something obvious. Doh.
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Old 26th October 2001, 02:24 AM   #6
blmn is offline blmn  Brazil
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If the primary of the transformer has two 115/120v primaries to be used in parallel for 115/120 VAC operation, will not be a good idea to use this series arrangement in 115/120v in case of loads over 50% of full nominal capacity of the transformer, because the wire gauge of the primary arrangement will be the half area designed for correct operation. The efficiency of the transformer will be reduced also, even in operations under 50% nominal capacity.



regards
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Old 26th October 2001, 10:56 AM   #7
Edgar is offline Edgar  Norway
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Thanks for the replies!
In retrospect it was a bit optimistic thought! I'll go with paulb's suggestion, run an cheap tda2050 at a single 50V, a bit dangerous but hopefully I'll soon finish the Aleph. (the fear of it tda blowing up will only make me work harder on the aleph, he he..)

Edgar
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