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Old 27th June 2006, 02:24 PM   #1
j9184p is offline j9184p  United States
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Default lm4780 p/s

I need help designing a power supply to operate a lm4780ta amplifier. The maximum supply voltage is 84v and it needs a split power supply. i know the basics of what a power supply contains but i don't know how to determine the size of the components. any help is appreciated.
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Old 27th June 2006, 07:30 PM   #2
preiter is offline preiter  United States
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Look through the whole data sheet to determine the supply voltage and current that you need.

The supply voltage should be about 25-35 volts depending on the impedance of the speakers you will be driving. The transformer is going to be somewhere from 160VA to 220VA with secondary windings between 18-0-18 and 25-0-25.

Lots of power supply designs out there, try a search. In it's simplest form you want a transformer -> rectifying diodes -> smoothing capacitor. But there are many improvements on that basic design.
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Old 28th June 2006, 03:01 PM   #3
j9184p is offline j9184p  United States
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Is this the graph to figure how much current is needed to supply the amp?
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Old 28th June 2006, 03:31 PM   #4
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lm4780 is something, but not enough...speaker impedance?
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Old 28th June 2006, 04:18 PM   #5
sangram is offline sangram  India
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Quote:
Originally posted by j9184p
Is this the graph to figure how much current is needed to supply the amp?

No, that's the idle current.

To find required current we usually use the power output formula. Pwer = Current^2*Load resistance.

For the rated 60 watts into 8 ohms, that is about 2.7 amps per channel, or about 5.5 amps total.

I use 30 volt DC rails so I can stay safe over a large range of speakers, and since the design uses more than one chip per channel I manage to keep the power output at high levels without needing to run very high supply rails.

I think a transformer capable of powering one chip should be able to deliver at least 6 amps, and deliver 25 + 25 volt AC. That's 300 VA or thereabouts, a pretty common transformer by any standards. You can either opt for a center-tapped transformer or one with two windings. There are many posts and sites about how they should be connected, so I won't get into it here.

Now the bridge rectifiers - should be able to take the max load, so 6 amps seems sufficient, but it's really not. The supply capacitors need large charging currents, and the diodes should be able to keep it up without blowing. 10 amp diodes or bridges are the bare minimum, with 16 amps preferred. I use the diodes that are supplied with the audiosector kit, those are 8 amp schottkies and work just fine (two bridges per transformer).

Then the reservoir caps. Andrew has just made a long post on capacitor size in the Electronics and Parts forum, so take a look. I myself prefer very low capacitance, 1500uF per rail per chip, whereas he's an advocate of lots of capacitance. It's up to you to decide.

In reality, there are more than one way to skin the cat, the best is to try yourself and see what works. With these chips, it's easy, cheap and fun to experiment. And they sound great too. All the best.
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