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Old 4th June 2001, 09:38 AM   #11
Alex M is offline Alex M  United Kingdom
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Rod Elliott has an excellent tutorial on heatsink design at

http://sound.au.com/heatsinks.htm

This, as far as I recall, includes a discussion on the
choice of heatsink material.

Alex
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Old 22nd June 2001, 08:57 PM   #12
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Talking Surplus to the rescue

I guess I lucked out. I walked into the local industrial metals supplier, and started finding out how much the sheet aluminum for heatsinks would cost. Then, I saw 3 sticks of Thermalloy 62725 (unfinished) in the 'scrap' bin (priced by the pound; all 3 were ~$1.50 per pound). I promptly bought all 3, and I now have 31 6.75" long by 9.75" wide heatsinks, and 1 13" long piece with some bent fins. Now I just need to sand the edges, deburr them, and have them black anodized.

Thermalloy's heatsink calculator gives me a thermal resistance of 0.48 deg. C/watt for these. I'll be using a copper heat spreader on the back, so the effective thremal resistance should be lower (line source vs. point source).

Thanks for all the information/help.
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Old 22nd June 2001, 09:10 PM   #13
grataku is offline grataku  United States
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You lucky Bastard!!!

I would have given you the 'other' finger but that wasn't available!
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Old 24th June 2001, 05:55 AM   #14
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Default Re: Thermal conductvity is not the whole picture

Yes Aluminum has a better emissivity than copper, and yet, copper has a better heat conductance. From my point of view, which is materials research at MIT, I think there is a practical compromise. One may use the emissivity of Aluminum by making a large surface area of Al, but embedding a sheet of copper across the inside of the assembly. So, the copper ( a thin sheet .025" ) will conduct the heat to a large bank of Aluminum fins with superior emissitivity. Thus, the best of both worlds...

Quote:
Originally posted by Petter
Aluminium has better "emissivity" (new word?) which means that a give surface area of Alu is better than that of copper all other things being equal. Therefore, a Copper heatsink can be made better if engineered properly, but this may not be tha case if it is the surface area favours Alu and conductivity in the metal is not the issue. For the Patriot, you should be able to use thinner sheets, and more of them to enhance thermal performance.

Copper is also very very dense (reads heavy), and quite costly. I have some busbars that I am planning to use in a power amp so I know what I am talking about! A given volume of copper will thus set you back perhaps as much as 10 times as much as that of Aluminium.

Machinability: Copper is much harder to work with.

Colour: Probably not much to choose between them except Alu can be anodized black with about 10% increase in performance.

Availability: Alu is much easier to find

Magnetic properties: Copper is better than Aluminim. This is probably because Alu is diamagnetic and as I recall Copper does not have this limited liability.

Sound: Due to magnetic properties above, Copper is generally considered to allow for better sounding equipment (ie cabinets made out of ...)

Looks, corrosion etc. -- consider for yourself what is best.

Petter
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Old 24th June 2001, 08:38 PM   #15
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Default Now that MIT materials are involved ....

Now that MIT materials are involved, it would be useful with some thoughts on material choice versus magnetic properties which should be much more of a general concern for DIY audio people than it currently seems to be.

With limited understanding of the issues, I believe magnetism of any type is a detriment to sound quality.

How do the "secondary" types of magnetism conceivably affect sound quality? (That is dimagnetic and paramagnetic. ferromagnetic properties are fairly well understood by laypeople)

I guess we can all safely assume there is some loss element involved? How do these factors work with the materials in question?

Petter
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Old 24th June 2001, 08:42 PM   #16
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Default Emissivity improvements

Can one improve emissivity by doing a surface etch, similar to the way electrolytic cap alu is etched to radically increase capacitance (surface area?)

If yes, how should such an etch be performed on Alu or Cu?

Petter
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Old 25th June 2001, 07:05 PM   #17
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Moby,

> One may use the emissivity of Aluminum by making a large
> surface area of Al, but embedding a sheet of copper across
> the inside of the assembly. So, the copper ( a thin sheet
> .025" ) will conduct the heat to a large bank of Aluminum
> fins with superior emissitivity.

Right. That's why I intend to use a copper heat spreader with the heatsink. The heat spreader I'm looking at is 3/16x3x8.5 copper for a single heatsink (3/16x3x18 for a double). It will be oriented perpendicular to the fins, and be centered on the back of the heatsink(s).

Petter,

> Can one improve emissivity by doing a surface etch,
> similar to the way electrolytic cap alu is etched to
> radically increase capacitance (surface area?)

The people I talked to about black anodizing said that with a smooth surface, the result would be slightly reflective. They suggested that the heatsink be bead blasted for a flatter finish.
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