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Old 7th May 2004, 07:00 AM   #1
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Default LM 4780 amplifier heat question

Hi there!

I've build an 6 channel amplifier useing 3 LM4780 chips.
They're mounted on an aluminium sink (see attachment) cooled by an 80mm fan commanded by a 60 degrees celsius bimetal.

The board has 20cm/10 cm double layer(see attachment).
The ploblem is the heat. Even if muted the sink reaches 60 degrees and the fan starts blowing.
It's that normal for this kind of amplifiers?
Thanks
Jean
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Old 7th May 2004, 07:02 AM   #2
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Default here's the sink

the sink
(dimensions are restricted by the VCR case where is mounted)
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Old 7th May 2004, 07:10 AM   #3
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To answer your question:

How much power will these LM's dissipate?

How much thermal resistance must the heatsink have at the most?

6 x 60 W = 360 watts

=> at least 200 watts in heat
=> max 70 deg C
=> 0.25 deg /watts
=> A big beast!
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Old 7th May 2004, 07:30 AM   #4
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In that case I think I'm gonna cut the cover and mount a higher sink. It's gonna look like a catholic organ

Too bad I found out too late (the board was allready made) about LM4781 (3 channel amp) wich would be much more suitable for my needs.

Anyway LM4780 thermal protection works at 150 degrees celsius and there's no way to go that high.
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Old 7th May 2004, 08:37 AM   #5
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If you play at the level you without the heat protection is falling out, you don't have to worry altough it's good to keep the temperature down for life-time reasons. A cool amp will last longer probably.
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Old 8th May 2004, 09:08 AM   #6
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Does your still amp overheat with the input signal turned right down? If so you may have a stability/oscillation problem.
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Old 8th May 2004, 10:22 AM   #7
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I also get suspcious when someone complains about heat with these chips, I have found that these chip (LM3875T & LM1875) produce negligable heat, even when pumping. My current LM3875 amp barely strays above ambient even when going hard. A lot of people get away with only using the case as a heatsink. I would agree that you might well have some serious stability issues. Try bypassing the powersupply pins with ceramic disk caps.

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Old 8th May 2004, 01:09 PM   #8
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You may have stability problems but that heatsink isn't anywhere near what National recommends in their tech. notes. Granted...they give a very conservative stance on sizing the heatsink but it is at least a starting point.

From their notes notes you need total supply voltage and output load to calculate the maximum power dissipated in each amplifier. After you have that number they give a simple series of equations to solve for a thermal reistance of a given heatsink.

I'm using a HS with 1.75" tall fins, 5" wide by 3" tall on a 3875 and it is way more heatsink than needed. I'd expect with natural convection a heatsing about this size would be suitable for the stereo 4780 but you should at least run the calculations first.
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Old 9th May 2004, 02:46 AM   #9
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I'm running 86 volts rail-to-rail and using each chip in parallel for single-channel, and at 1 watt of output it takes about half an hour for my heatsink to reach 40 deg. C. The pictured heatsink is the one I was testing with.

You're probably oscillating somewhere. As Shoog said, bybass the heck outta your power supply pins.

oO
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Old 13th May 2004, 06:35 AM   #10
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Oscilation is my guess too.
I'll use an osciloscope to find out where the oscilation is.
It's something wrong with the heat with no signal on inputs or even if muted.

I told you that the size of the sink is limited by the case I use.

In full power when the temperature excedes 60 degrees the cooler startes blowing.

Thanks for support!
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