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Old 30th March 2005, 12:06 AM   #1
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Default LM4780 - Heatsink and isolation

I've built a dual LM4780 amplifier, and I have a quick question before I power it up. I have both LM4780s bolted to the same heatsink. I don't think this is a problem since the datasheet shows that the case is attached to the V- rail and both chips are powered from the same supply. Can anyone let me know if I've caused myself a problem doing it this way? Thanks!
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Old 30th March 2005, 12:58 AM   #2
homer09 is offline homer09  Canada
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It doesnt sound like a good idea to me. But you may be able to get away with it... Everyone i've seen using uninsulated chips ALWAYS isolates chip from heatsink. Your asking for trouble with this kind of setup.
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Old 30th March 2005, 01:48 AM   #3
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It's no problem as long as there is a physical barrier between the heatsinks and the outside world. If your heatsink is exposed and the chassis is grounded (probably) it is very easy to short out the power supply by any random piece of metal falling on the amp (a pen, a paper clip, etc.). In a higher powered amp the voltage difference could be dangerous to people touching the chassis and heatsink, and it is still not a good idea to allow people to come into contact with the 30V (or so) supply in your amp.

If the heatsink is inside the chassis, and you have some screens covering the openings in the chassis, there should be no problem at all. Many commercial amps are made this way.

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Old 30th March 2005, 01:49 AM   #4
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My direct experience is that things get ugly when you use one heatsink with two LM4780s and do not isolate. Even rudimentary isolation like a piece of antistatic plastic will be better than leaving them naked.
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Old 30th March 2005, 02:34 AM   #5
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Yes I agree... inductive loop is coming to mind for some reason...

Anyways, just use an insulating washer of some kind, then you will be aiight.
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Old 30th March 2005, 02:36 AM   #6
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Quote:
Originally posted by homer09
It doesnt sound like a good idea to me. But you may be able to get away with it... Everyone i've seen using uninsulated chips ALWAYS isolates chip from heatsink. Your asking for trouble with this kind of setup.
The LM4780 only comes in the non-insulated version.

If you look at the various thermal impedances there is a lot to be gained by attaching the LM4780 directly to the heatsink. An insulating washer adds a bit to the thermal impedance and if you are going to run the LM4780 hot, every 0.1 C/W that you have to compensate for means a much larger heat sink (or more lineal feet of air per minute) -- (if I had the time I would work out the math.)

My bridged amp has the heat sink attached to a sheet of birch ply via nylon screws -- i tapped the heat sink so as to not worry about the 6-32nd nuts.
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Old 30th March 2005, 04:04 AM   #7
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The problem does not lie with fact that it is not isolatied in itself, but that when you connect the two negaive leads of two chips you get some nasty loop effects and crosstalk. Isolate them. You will be happier.
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Old 30th March 2005, 04:44 PM   #8
G4ME is offline G4ME  United States
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i remember a discussion a while back about using electro static bag cut to fit over the back of the chip, is this considered a simple solution?

also if i do notice noise and loops with out insulating, i imagine that i could just later install a type of insulator right.
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Old 30th March 2005, 04:50 PM   #9
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I have used the plastic from antistatic bags with good success, but I only used them as a temporary solution until my isolator sheets arived from digikey. Just be sure to use silicon grease or some suitable alternative.

As long as you are sure that your heastink has no connection to ground it is safe to attach the LM4780s to it directly, but you had better be sure. I have blown up chips this way.

I have used 2 LM4780s unisolated on a single heatsink and they ran fine, but sound quality was not as good as after I isolated them.
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Old 30th March 2005, 05:39 PM   #10
G4ME is offline G4ME  United States
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do you happen to have that part number from digi key?
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