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Old 16th December 2005, 04:35 AM   #1
dfdye is offline dfdye  United States
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Default Quick LM4780 Transformer Question

I was thinking of using a 330VA 25V+25V toroidal transformer (specifically part number 122-640 from Parts Express) for a pair of parallel LM4780's. After looking at the datasheet for the chips for a while, this seems to be right on the edge of what the chips would handle. I will be driving a 7.5+/-3 ohm speaker off of each parallel chip, and I was wondering if anyone had any comments or suggestions as to whether the rectified voltage of this transformer (which would be ~35V according to my math) is too high or if the chips can take it.

If anybody has suggestions or comments, they would be greatly appreciated! You guys have obviously had a pretty good amount of experience with these chips, so before I jumped in I figured I would ask.

Thanks!

PS The reason I ask is because everybody seems to be suggesting 22V transformers, but the power on the 22V they have at PE is only rated at 160VA, which I thought was a little low. So, if anyone has suggestions as to where to get a reasonably priced 22V transformer rated at something a little more reasonable, I would also appreciate that!
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Old 16th December 2005, 04:59 AM   #2
hongrn is offline hongrn  United States
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You'll be fine. I use the exact same toroid from PE to power my parallel LM4780. Been running for 2 months now without problems.

Hong
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Old 16th December 2005, 04:24 PM   #3
dfdye is offline dfdye  United States
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Thanks! I appreciate the feedback.
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Old 16th December 2005, 04:29 PM   #4
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we should qualify that -- you will be "fine" as long as the thermal impedance of the heat sink is low enough -- use the Overture Series interactive design tool to calculate the thermal impedance (C/W) , then go to the AavidThermalloy website and use their interactive calculator to see whether the heat sink is actually big enough.
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Old 16th December 2005, 07:46 PM   #5
dfdye is offline dfdye  United States
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I have gone a little overboard for cooling, so I don't think there is a possibility of the chips getting too hot, but thanks for the heads up!
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