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Old 2nd June 2001, 12:15 AM   #1
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I'm trying to design and build a 50W Class-A amp. One of my constraints is that I want to be able to run biased for a 6 ohm load, with no more than a 30 degree C temp rise. This works out to about 200W of heat (at idle - bias 3 1/3 A).

This looks like 4 heatsinks, each having 1000 sq. in. of area, per channel. I can do that with stacked aluminum plates, but I'd like to use less.

Does anyone know what would happen if copper were used, instead of aluminum? I know, the weight will rise, and the cost will skyrocket, but I loose too much enclosure space to the aluminum stacked plates. Also, I don't like the tight spacing, and thin vanes I'd need with aluminum.

Thanks.
Thoth
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Old 2nd June 2001, 12:31 AM   #2
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OOPS. Bias should be 1 2/3 A, giving about 100W disipated (I knew something must have been wrong). This gives 4 heatsinks of about 270 sq. in. each. I think I can do this.

Biasing to a 4-ohm load should still be within the temp safety margin of the transistors. A 3-ohm bias would be too hot. Oh well; 4-ohm minimum.

It would still be interesting to know the heatsink area formula for copper.

Thanks.
Thoth
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Old 2nd June 2001, 08:00 AM   #3
hifi is offline hifi  Sweden
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hmm...just a suggestion when building heatsink look att lcaudios "patriot" amp it has diy heatsinks that look rather easy to build....


/micke
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Old 2nd June 2001, 07:21 PM   #4
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The information for the Patriot amp is gone from the LC Audio web site. Currently, the basis for my heatsink design is at 'http://home.earthlink.net/~lotusblossom/'.

LC Audio did have information on their other amps, which use extruded heatsinks ($50 each). Of particular interest was their use of Sanken transistors. I checked up on this, and it looks like these transistors will allow me to half the number of output transistors, and related components.

Thanks for the pointer.
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Old 2nd June 2001, 08:16 PM   #5
hifi is offline hifi  Sweden
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well the cooling design of the patriot amp is like this:

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I I I I MOUNTING SIDE FOR TRANSISTORS
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do u understand what i mean?

&/mcke


DO U S
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Old 2nd June 2001, 08:19 PM   #6
hifi is offline hifi  Sweden
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The dots dont exist..

I....I....I....I
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I....I....I....I
IIIIIIIIIIIIIIII
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I....I....I....I
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I....I....I....I MOUNTING SIDE FOR TRANSISTORS
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Old 2nd June 2001, 11:13 PM   #7
blmn is offline blmn  Brazil
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Thoth,

I think the difference between cooper and aluminium is basically the thermal conductance:

for pure cooper: 385 W/(m.C)
for pure aluminium: 202 W/(m.C)

where C is temperature (Celsius or Kelvin)


The book Heat Transfer - J. P. Holman, McGraw-Hill have this kind of information to help you on heat sink designs.

Regards
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Old 3rd June 2001, 02:03 AM   #8
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OK. Copper has about twice the ability to conduct heat as aluminum. I think this means that with a given heatsink design, the C/W for copper would be half that of aluminum. This, in general, would mean that to get a given C/W for a heatsink, the copper heatsink would have only about 1/4 the area. Thanks.

It's easy to see from the description of the heatsink for the Patriot amp, that ASCII art has limits. I'll E-mail LC Audio, and ask for this information.

Thanks for the ideas, and info.
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Old 3rd June 2001, 09:39 AM   #9
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Default Thermal conductvity is not the whole picture

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 3rd June 2001, 10:32 PM   #10
djk is offline djk
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>> Copper has about twice the ability to conduct heat as aluminum. I think this means that with a given heatsink design, the C/W for copper would be half that of aluminum. This, in general, would mean that to get a given C/W for a heatsink, the copper heatsink would have only about 1/4 the area. << The *C/W is largely based on surface area.The only advantage copper would have would be if here was a single point heat source and it was a large distance to the radiating area(fins).For a distributed load, equal area = equal *C/W .Keep in mind the shape of the fin design is determined by convection requirements.The copper convoluted fin designs that have very high fin density will have horrible performance in natural convection.Under forced convection(fan),copper is king.


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