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Old 20th May 2012, 05:00 AM   #1
gbcomp is offline gbcomp  United States
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Default LM4780 transformer question

I plan on building a stereo amp bassed on two lm4780 ICs, one for each chanel, each set up in parelle for 120W driving an 8ohm speaker. I am wondering what power/voltage ratting I should be looking for when shopping for a transformer with a 120V primary. Currently I am looking at this transformer VPT48-10400 Triad Magnetics | 237-1358-ND | DigiKey . It is rated at 500VA with dual 24V secondaries at 20.8Amps. I believe this to be adequate with plenty of headroom but I don't want to spend more money then I have to. Any input would be greatly appreciated.

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Old 20th May 2012, 07:06 AM   #2
N Brock is offline N Brock  United States
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My paralleled LM4780 runs on a 22+22v 400VA transformer. It seems to be enough.

Step 1: Antek - Your reliable source of transformers, power supplies, and more.

Lots of people on here will vouch for these guys.

Compared to the price you were looking at before you might as well go for the 500VA.

Best,
Nelson
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Old 20th May 2012, 01:17 PM   #3
gbcomp is offline gbcomp  United States
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Thanks for the link, the prices and selection looks great. I take it then that something in the 22v+22v to 24v+24v range at 400-500VA should be enough for two lm4780. If anone would care to conferm this that would be great. I will shop around in that range. Thanks again.
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Old 20th May 2012, 02:37 PM   #4
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by gbcomp View Post
I plan on building a stereo amp based on two lm4780 ICs, one for each channel, each set up in parellel for 120W driving an 8ohm speaker.
How?
Have you read the datasheet?
Did you understand what you read in the datasheet?
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Old 20th May 2012, 02:53 PM   #5
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Yes I agree!
You will need more voltage in order to get 120watts into an 8ohm load !

jer
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Old 20th May 2012, 03:10 PM   #6
SoIL4x4 is offline SoIL4x4  United States
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22v secondary voltage and a parallel configuration is recommended for 4ohm loads. If you will be driving 8ohm speakers the literature suggests a bridged configuration and higher secondary voltage. As for va rating there has been a great deal of discussion about the antek transformers and how to calculate.
I used two AN3222 transformers for my 4780 parallel build. They do well with my 6ohm speakers and outstanding with the 4ohm pair.(though two very different sets of speakers).
Also I think in my setup any more than 300va would have required some compensation for current inrush.
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Old 20th May 2012, 03:21 PM   #7
gbcomp is offline gbcomp  United States
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Thanks for the input, I was doing some looking around earlier and I thought I spotted my misunderstanding, you guys just confirmed it. I have 8ohm speakers and was looking to get about 120Watts. I'm going to start looking into a bridged setup now. As for VA rating and inrush current, that's something that I was just beginning to look into.

I'm still in the planing stages, a few weeks from ordering parts. All of the input is greatly appreciated!
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Old 20th May 2012, 03:43 PM   #8
AndrewT is offline AndrewT  Scotland
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If you want to drive 8ohms speakers, then design each half of the 4780 as a 4ohm capable amplifier.
Take care with overheating.
The 4ohm capability requires more current from the chipamp and that requires more cooling.
BUT !!!!
the dual amp chip has very limited heat dissipation through to the heatsink. It is an extended To220 after all.

You might be better aiming for 40W to 50W into 4r0, rather than 60W into 4r0.
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Old 20th May 2012, 04:01 PM   #9
gbcomp is offline gbcomp  United States
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I was just looking at the curves in the data sheet and came to the same conclusion, I think I am going to run each at 45W and use a single 400VA 22V+22V transformer for the whole amp. I think this should work. As for the heat, I plan on trying to find a very large heat sink for both chips and using it as a sidewall of the chassis, so I have not been worrying about it as much as I probably should.
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Old 20th May 2012, 04:10 PM   #10
gbcomp is offline gbcomp  United States
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I am thinking something like this, I did not realize how inexpensive they were so probably on per chip. I am assuming that something of this size can dissipate the heat from a lm4780 running at 45-50W bridged. Aluminum Large Rectangle Heatsink | eBay
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