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Old 9th January 2014, 06:16 AM   #1
fr1s is offline fr1s  Canada
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Default Toroidal transformer question [Gain Clone]

Hello everyone!
I'm a bit of a newb but in a mid of rebuilding my old high-school project. My old transformer died so I found this toroidal transformer that I got no idea if it will even work with this amp.. does it work? yup I checked.

here's the pic
Click the image to open in full size.

and if it will work how should I connect it?
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Old 9th January 2014, 06:32 AM   #2
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It appears to be a 28-0-28 volt 100va tranny. Impossible to tell you how to connect it or whether its suitable without knowing the amp its for.

28-0-28v AC will give -/+40 volts DC after rectifying and smoothing.
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Old 9th January 2014, 06:49 AM   #3
fr1s is offline fr1s  Canada
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sorry forgot to mention its for lm1875
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Old 9th January 2014, 07:04 AM   #4
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It may not be suitable. The LM1875 has a maximum rating of 60 volts dc.

You need to know if your LM1875 circuit is configured as a "single rail" amplifier which means there is just one positive supply voltage. The amplifier would be AC coupled with a large capacitor to the speaker. If it were single rail you could use one half of the tranny to get 40 volts DC. You would still need to check the rest of the components (caps) were rated for this voltage.

Or, is the amp DC coupled with "split" supplies where there is a minus and a plus voltage rail. If it is then this tranny won't do.
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Old 9th January 2014, 07:05 AM   #5
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Maybe post a picture of the amp and the old tranny
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Old 9th January 2014, 07:15 AM   #6
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I used TI's dual "split supply" schematic.
4.7uF
47uF
1000uF all 50v

.22 cap is 63v

Click the image to open in full size.

EDIT: old one got thrown away last year it was huge and old.. center tap.. dont remember the specs at all
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Old 9th January 2014, 07:51 AM   #7
Mooly is offline Mooly  United Kingdom
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OK, so spilt supply means you need an absolute max of -/+ 30 volts DC. Knock that down a little for a good safety margin to say -/+25 volts DC and that means you need either a centre tapped tranny or one with two separate windings of -/+18 volts AC.

Very strange that old tranny failed, particularly if it was a big 'un If it had a thermal fuse in it then could there be a short on your amp somewhere that overheated it ?
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Old 9th January 2014, 02:18 PM   #8
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SO if its Center tapped then something like 25v-0-25v how many amps?? 4A? and about 160va??
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Old 9th January 2014, 02:57 PM   #9
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Quote:
Originally Posted by Mooly View Post
you need either a centre tapped tranny or one with two separate windings of -/+18 volts AC.
i.e. 18-0-18

4 amps will be fine for a stereo amplifier with 8 ohm speakers.
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Old 9th January 2014, 05:55 PM   #10
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alright what about VA ratings?? 160VA is good?? or can I go 120??
n whats the smallest I can go?? my enclosure is about 45mm to 50mm so size does matter

http://www.rapidonline.com/electrica...-0-18v-88-3816

???
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Last edited by fr1s; 9th January 2014 at 06:10 PM.
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