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Old 7th May 2005, 03:48 PM   #1
Mr Teal is offline Mr Teal  Canada
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Default Simple/Cheap computer speaker chipamp

Here at work, we have a few surplused power supplies for laptops. 20V, 3.5A.

How feasable do you guys think it is to take two of them in series for my pos and neg rails, and use a stereo LM4780 to power a couple drivers?

I can't see any reason why it shouldn't work, maybe toss in some extra capacitance inside the speaker box, but that's about it. Anything I should watch out for?
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Old 8th May 2005, 01:49 AM   #2
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are they AC or DC output?
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Old 8th May 2005, 10:10 AM   #3
Giaime is offline Giaime  Italy
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They most of the time are SMPS power supplies, and with those things, avoid connection between them (either series or parallel)... you're looking for smoke
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Old 9th May 2005, 07:17 PM   #4
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20V is still enough to run some chip amps. It its 20Vac you can get 27.6Vdc ish. you wont get as much power but you can bridge a pair together if you need the power. Perhaps a tda2040W I have been inpressed by them.
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Old 9th May 2005, 09:32 PM   #5
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Originally posted by Giaime
They most of the time are SMPS power supplies, and with those things, avoid connection between them (either series or parallel)... you're looking for smoke
I have to disagree - they (SMPSs) normally work fine in series. The only chance of problems is if they share a common ground on the output side, which is very unusual. I have succesfully used 3 5volt switch mode power supplies in series (tweaked down to 4.6v each) to form a 120 amp battery charger for Slot Car Racing. It works faultlessly. --- Those things would make a lot of mess if they couldn't handle being connected like that!

Anyway, I think this would work fine for your amp, as long as you don't expect too much power out of your amp. I would add some capacitance at the chip pins, and maybe some larger caps further back, just to make sure the system can handle transients well. -- SMPS's do like to vary their output current very quickly.

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