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Old 14th June 2010, 08:08 PM   #1
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Default help with transformer

Hi,
I'm planning on building a basic amp using an LM3875 kit from audiosector, and pulled a transformer from an old powered subwoofer for the project, as pictured below. I'm picking all this up as I go along, so I was wondering if anyone could tell me if this is a suitable transformer, or if I should keep looking. Based on what I've read, for my 8ohm speakers around 23V AC would be suitable, so I'm guessing I can use a red and white in series to achieve this ?!

Thanks very much for the help everyone!
caleb

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Old 15th June 2010, 12:50 AM   #2
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Your question is not very clear. What DC voltage do you want after the rectifiers?
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Old 15th June 2010, 01:09 AM   #3
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Sorry, as I mentioned, I'm new to this

I think anywhere from 28 to 35 V DC is what I'm after.
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Old 15th June 2010, 11:33 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
you could use the two 15.8Vac wires as a temporary supply. ~66VA
This will give ~+-20Vdc after passing through a single bridge rectifier and into the dual smoothing capacitors.

If using 8ohm speaker.
Look for a 20+20Vac to 26+26Vac dual secondary transformer or a centre tapped 40Vac to 52vac transformer.
For a single channel 100VA to 160VA will work reasonably well.
For a two channel, 120VA to 250VA will work well.
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Old 15th June 2010, 12:00 PM   #5
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Quote:
Originally Posted by AndrewT View Post
Hi,
you could use the two 15.8Vac wires as a temporary supply. ~66VA
This will give ~+-20Vdc after passing through a single bridge rectifier and into the dual smoothing capacitors.

If using 8ohm speaker.
Look for a 20+20Vac to 26+26Vac dual secondary transformer or a centre tapped 40Vac to 52vac transformer.
For a single channel 100VA to 160VA will work reasonably well.
For a two channel, 120VA to 250VA will work well.
Hi Andrew, Thanks for the response. What are the consequences of using a lower VA transformer like this one? Just a lower maximum volume, or will it not work at all? Basically I'm using the amp to run a small pair of speakers to use with my computer, so playing mp3's, videos, that sort of thing, in which case I'm not to concerned about really high volumes, just that it works
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Old 15th June 2010, 03:55 PM   #6
AndrewT is offline AndrewT  Scotland
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few consequences.
It just sounds better with slightly bigger than adequate transformer.
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Old 15th June 2010, 04:03 PM   #7
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Quote:
Originally Posted by AndrewT View Post
few consequences.
It just sounds better with slightly bigger than adequate transformer.
Thanks for letting me know. I guess I could always upgrade it in the future once I'm a little more competent!

caleb
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