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Old 26th March 2004, 03:34 AM   #1
batee is offline batee  United States
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Default Giant Heatsink - How large an Aleph X will it tolerate?

I found a giant chunk of heatsink at my local scrap dealer. It measures 9.75" wide, 2.25" high and 72" long. The base plate is 1/4" thick.

Click the image to open in full size.

Click the image to open in full size.

I'm interested in building a two-channel Aleph X amp. I'd like to build it as one component rather than as two monoblocks. I really like the Pass XA200 form factor chassis (rack width and about a foot high. I was thinking about cutting the heatsink into two 9.75" x 36" heatsinks.

The problem I see with this is that the fins would be running horizontally. It's been a while since my Thermo classes, but it seems that they'll be less efficient than if the fins were horizontal since the convective flow won't pull cool air across the fins. It seems to be a waste to cut this into a bunch of small pieces, and I really like the sleek look of having the fins running horizontally.

Does anyone have suggestions about the best usage for this material? I'm interested in building the largest stereo amp I can with the heatsink material I have.

Should I use it with the fins horizontally? Or would I be waaay better off to cut it into a bunch of 12" high sections and turn the fins vertically? Or just sell it and buy some stock similar to the stuff that Nelson uses on the XA200s?

Thanks in advance for your help,

Bryan A. Thompson
batee@roadslessphotographed.com
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Old 26th March 2004, 06:31 AM   #2
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I would suggest cutting it into smaller lengths and using it with the fins vertically. You do say that you want to build the largest amp you can and it is indeed more efficient from a heat transfer standpoint with the fins running vertically.

Some time ago, I spent quite a while trying to come to conclusions about what size heat sinks are required for a given dissipation. I took a look at lots of results reported on the forum, and also built a mock-up of a single channel Zen IV. When I measured temperatures, it confirmed my estimates.

I found that starting at room temperature you can dissipate about 1W per 10in^2 of heatsink area while ending up with the heatsinks at about 50C. (that's about a 25-30 degree rise) That's of course with the fins vertical. And, Aleph or Aleph X circuits dissipate about 3x the rated power output.

So, I'd say you've got 2.25 in. x 2 sides per fin x 20 fins = 90 in^2 per inch of length, plus a little safety factor from the contribution of one side of the baseplate.

Multiply by 72 in. long and you've got 6480 in^2.

That should be good to dissipate 600+ watts of power, or enough to get you 200 watts RMS of amplifier power.

If you dice it into 6 sections, you'll get one big amp when you're done. I'd cut one side of the protruding backplate off so that when you butt the sections up against each other, the fins are all equally spaced rather than having a visible gap where the sections meet.

Keep in mind... these are MY conclusions, and consider what you paid for this advice.

I suggest that you do some homework to make it your own and to avoid dissapointment. It's part of the fun
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Old 26th March 2004, 11:51 AM   #3
Coulomb is offline Coulomb  England
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I have attached a link below that will take you to an extrusion very close to the one you have. I have assumed you want to build one Stereo chassis, this would give you 36" per channel.

If you cut the 72" length into 6 equal sections turn them 90 Deg and clamp them together mechanically you will have 2 Heat sinks approximately 12" x 27".

This configuration would offer you the greatest convection flow with the fins running vertically. Based on the C/W you should be able to dissapate 240 watts per side at 30 Deg C. rise above ambient.

http://www.thermaflo.com/bin/exdatas...=Change+Length

Regards

Anthony
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Old 26th March 2004, 12:08 PM   #4
Jennice is offline Jennice  Denmark
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Default Cut it up

Hi,

I would definitely cut it into 4 pcs or more, making it lower and wider, placing the sections side by side.
My experience is that doubling the height of a sink will only give you approximately 30% - or so - more energy dissapation capability(again, remember what you paid for the advice, as a kind fellow stated).

Keep in mind, that air has a fairly low thermal capacity. It heats up fairly quickly when rising through the "channel" between two fins of the heat sink, and therefore you'll reach a point where you just have warm air running along the heat sink without heat energy reansfer from metal to air. We don't want that. Contrary, we want a LOT of shorter air-"channels", in stead of few long channels. Make it low and wide, in stead of tall and narrow.

Jennice
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Old 26th March 2004, 01:00 PM   #5
SineEra is offline SineEra  United Kingdom
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Default Heatsink Calcs

In my research for which power AlephX to build. ie one my other half would let into the lounge! I came across the following site.

http://www.aavidthermalloy.com/produ...de/index.shtml

Looking at the European standard extrusion catalogue you should find an appropriate match and using the correction factors listed the simple maths will give you the overall C/W.

I found it easier to use and more detailed than the Thermaflo site Coulomb refers to.

Simon
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Old 26th March 2004, 01:02 PM   #6
Magura is offline Magura  Denmark
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The effect you describe as air running hot at the upper end of a heatsink, is cancelled out by the factor that the air speed increases by a higher factor than the temperature for a given height of a heatsink.


Magura
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Old 26th March 2004, 04:16 PM   #7
batee is offline batee  United States
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Consensus seems to be function over form. Dang.

The above suggestions yielded the following results:

My ambient room temp is 22-25C. I found nothing at the Aavid site as far as a close match for my sink. Coulomb found something close, but with slightly shorter fins, at the Thermaflo site. Looks like a 35C increase will dissipate 320Watts (per channel), for a thermal resistance of ~0.11C/W. This matches the numbers provided by vpharris, so I'm inclined to believe the calculations.

Assuming I cut it into six sections (three for each channel), what would be the best way to distribute the heat generated by one channel over the three sections?

I was thinking of building a chassis out of 1/2" - 3/4" aluminum bar stock, then covering the top and bottom with vented aluminum plate and the front and back with solid aluminum plate, and just bolting the heatsinks to the bar stock to form the cabinet sides.

Is it possible to arrange the FETs for each channel so that they're on the different pieces? Or should I use some sort of a backing plate across the backs of the three plates, or are the large bars enough? If a backing plate, how thick?
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Old 26th March 2004, 05:14 PM   #8
Jennice is offline Jennice  Denmark
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Marqura,

If you like your theory, enjoy... I prefer my audio gear to survive.

Point is, that I have made some tests myself, which tend to give results not too far from the data sheets for the industrial heat sinks I've tried.
they come in up to 1 meter length, and their data sheet shows almost no improvement in heat sinking capability over the last 50 cm. It's not a linear function of height... theory or not ;-)

Jennice
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Old 26th March 2004, 05:18 PM   #9
Coulomb is offline Coulomb  England
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Quote:
Originally posted by Jennice
Marqura,

If you like your theory, enjoy... I prefer my audio gear to survive.

Point is, that I have made some tests myself, which tend to give results not too far from the data sheets for the industrial heat sinks I've tried.
they come in up to 1 meter length, and their data sheet shows almost no improvement in heat sinking capability over the last 50 cm. It's not a linear function of height... theory or not ;-)

Jennice
Seems a little Harsh Jennice!!!


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Old 26th March 2004, 05:30 PM   #10
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I think your plan for the chassis is fine, and practical.

You can distribute the FET's across the heatsinks. Assuming that you have 12 per channel, then you get 4 per heatsink piece. If you are going to have the pieces 12 in. high, then you might be best with two rows of 2 on each piece. Spread them out so that each is using the same area on each heatsink. Like, 4 quadrants with one in the middle of each.

Can you put them all on one plate that spans the heatsinks? Yes, but no matter what, you'll lose some efficiency. That said, when I made my Zen IV, I was bound and determined to do it the true DIY way, with 'found' heatsinks (free from a local electronics recycler). They were too small and I bolted two together. Used a strip of Al where they overlapped, and coated the joint with heatsink compound. I later measured temperature right at the back side of the joint, opposite the FETs and compared that to an inch away from the joint and saw very little temperature drop, so I think it worked quite well.

If you choose to use a backer, probably 3/16 or 1/4 in. thick is good enough. If you have it screwed to the heatsinks securely enough (read: lots of screws) then the effect will be as though there is only one heatsink. I think the surface area of the connection is key. I'd probably want it to be at least 3 in. wide for something as big as you're considering.

Here's what that looked like on the mockup, with details to follow:
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File Type: jpg dscn0066.jpg (90.8 KB, 702 views)
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