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 Vidi 5th December 2006 05:13 PM

Heat sink for Aleph 5 / Aleph 4

Hello!
We are a couple of students from UIS (University of Stavanger), who wants to build an aleph amplifyer each. Some of us wants to build Aleph 4, and some wants to build Aleph 5. The problem is (as usual with class A designs), the choosing of heat sinks.

We have looked into some heatsinks delivered through elfa (http://www.elfa.se/no).

My problem is that i can't really figure out how to read the datasheet, because the graph showing the temperature rise vs effect is only shown for defined lengths, and we want to use 1 meter of the heat sink (4x25 cm)

Could anyone bee so kind to look into the datasheet, to see if one of the element called KS160 or KS200 is suitable for the aplication?

The link to the datasheet for both heatsinks (each in 100mm length):
http://www.elfa.se/pdf/75/07562267.pdf

Regards Vidar

 Buhl 5th December 2006 09:08 PM

Hello Vidar
There are several ways of "calculating" the heatsink needed for an amp - one is to screw the calculations and make it really big - NP is a fan of big heatsinks for sure, but he also needs to run a buisness, so he needs his calculator, as sinks are expensive - but I think you have figured that out by now ;-)

Beeing engineering students you migth want to follow the calculator way.

I cant give you precise conclusions based on the datasheet, but mabye others can. What I can tell you, is that you want a max temp incline on your amps sinks at around 30 kelvin/centigrades. I assume a ambient temp @ 20 (can be lower in norway I know ;-) )and a max temp om the sinks @50 degrees centigrades. This will mean chip temps in the order of mabye 60 degrees. This you donw want higher for long term operation. Search the forum for this subject, as it is of high relevance in here, and often discussed.
Ok - 30 degrees rise - that means for an amp dispassing say 100 watts you need a sink with a C/W value around 0,3 - that means for every watt sinked, you get a 0,3 degree rise in temperature. This curve is not linear, so imply the safe rule of thump. Also, the datasheets are often optimistic, and I have read in here that a 0,3C/W heatsink is more often 0,35 or 0,40 - but again, search.

This means that for a A5 that throws 300 watts of heat (excluding losses in trannie and bridges) the total C/W value of the amp needs to be in the order of 0,1c/W - here an aluminium chassis, mabye with holes in top and bottom plate helps the heatsinks considerely

I hope this helped you along the way - if you search www and this forum, you should find what else you need.

Cheers !
Buhl

By the way - do you have any snow yet ???

 RoboMan 5th December 2006 10:45 PM

Hi Vidi,

The graphs in the data sheet give you the relationship between power dissipation and temperature rise (Temperature of Heat Sink - Room Temperature) when a certain length of the heat sink is used.

Take KS 300.6 as an example:

If the total power dissipation of all transistors mounted on the heat sink is 300W and the Temperature rise should be less than 40 Deg.(just for example, usually we would like to keep the temperature of the device below 70 deg C), you should choose a 500 mm long heat sink. Why? From the graph, take the x axis and find P = 300, the 500 graph gives you 33 deg. and the 300 graph gives you 42 deg. You may choose a 300 mm heat sink, but the devices will run a little hotter. At least this gives you some ideas how big the heat sink should be.

This is only the theoretical figures. There are other factors that will increase the thermal resistance, like the contact between transistor and heat sink, coatings in heat sink etc. Usually you will select something bigger to ensure the devices are running cool.

Heat sink spec. is usually given in terms of "deg. C/watt" for a certain length of that kind of heat sink. For example, a heat sink of 75mm long is 0.5 C/W. If you have 300W dissipation, the temperature rise will be 300W * 0.5 = 150 deg C. You can use a longer heat sink to lower the temperature. If you use 300mm long heat sink, the temperature rise will be 150 / 300 * 75 = 37.5 Again this is only theoretical figures.

Hope this helps.

 wuffwaff 6th December 2006 10:13 AM

Hello Vidar,

I just looked at the datasheet. The KS 200 is a bit thin and has a thermal resistance that is too high.
The KS160 in 200mm pieces (a convenient height for an Aleph) will have a thermal resistance of around 0,73°C/w.
For an Aleph 5 with 34V supply and 2A of bias you will need to dissipate 34x2x2x2=272watts

For 35°C above ambient this means an Rth of 0,13
So six pieces of KS160 (200mm high) would be just enough. The chassis would then be 480mm deep and 200mm high (plus feet).
Making it higher won´t reduce Rth very much more.

If you look at the KS300.6 this would be better suited. Here 2 200mm pieces could keep the amp cool.

For an Aleph 4 you will need to dissipate a bit more than 400 watts. Building two monos with KS300.6 x150 would be more practical in this case.

The heatsinks do look very much like the Fisher parts......

William

 Vidi 6th December 2006 07:38 PM

Hi everybody!

Thank's for all your advice! I'm feeling a bit smarter now :)
I was not shure if I could just add the value of the heatsinks together, but giving it a closer thought it seems logically.

Then my new question will be:

Building an Aleph 5 would be a little cheaper, and considering the minimal effect gained (40w, that would be below 3dB extra volume?), you would not notice the limitations of power between an Aleph 5 and an Aleph 4? Does anyone have any thoughts about this?
Is there any reason to build the Aleph 4 in stead of the Aleph 5?

William: Then I will take your advice, and use 6 KS160 at 200 mm if we choose to build Aleph 5 (The rest of the chassi will be in aluminium, so I'm guessing that Rth will be slightly better than 0,13), and if we choose Aleph 4, I guess we have to use 4 KS300.6 at 200mm. And for insulation, I'm planning the use of metaloxide insulators, wich has a Rth less than 0.3 degrees/W.
This could be very exciting!

Buhl: I'm sorry to disappoint you. We don't have snow in Norway yet (except in the highest mountains). None of the ski resorts (downhill) has opened yet, because of the lack of snow. All we get is rain. ALOT of rain! We have 10 degrees 6th december! So, no snow on christmas eve this year! Blame the global warming!
:(

 wuffwaff 6th December 2006 08:30 PM

Hi Vidi,

you could also have a look at the kapton insulators. Their Rth is only 0,07 and they won´t break as easily as the metaloxide ones.

I´ve used them in my Aleph5 and X with good results. They come ready for use with a thin film of thermal grease and keep your fingers and everything you touch clean:D

William

 Fuling 6th December 2006 10:49 PM

Quote:
 Blame the global warming!
...a funny statement coming from someone who´s planning to build a big class A amp:D

 folkeb 7th December 2006 04:31 PM

Hi,

I have built the Aleph 1.2 monoblocks. For this I used the Fischer SK535 heatsinks. I payed about 8000-10000Nkr for four of these including VAT and freight. Two of these would be enough for one A4. I used a lot of time finding good heatsinks. Their dimensions are: 500x200x80 mm.

If these are to expensive Seifert have somewhat cheaper heatsinks. However, for the A4 it will be big.

Best regards,
Folke

 steenoe 7th December 2006 06:19 PM

Quote:
 Is there any reason to build the Aleph 4 in stead of the Aleph 5?
Depends on your speakers, really. If the speakers are pretty normal and not excessivelly powerhungry, no one would ever notice the 40 watt difference:) Furthermore the Aleph 5 is pretty much the sweet sounding favourite of a host of people, me included;)

Steen:)

 Vidi 7th December 2006 08:09 PM

2 Attachment(s)
Hi!

I'm pretty shure that my speakers are not excessivelly powerhungry (they are self built with Scan Speak speakers), they have an min impedance of 4 ohm, and about 90 dB/W.

But, regarding cabinett to the amplifiers. I have built two The End Millennium power amplifiers from LC Audio a couple of years ago (still very happy with the result!), and just for fun I visited their web site today. I was very excited when I found that they are selling the cabinet alone (without powersupply and amplifier), at 5000 DKK.
Looking into this, I see that the cabinet uses four heatsinks rated at 0,33K/W. That would make a cabinet that has a total Rth of 0.33/4 = 0,0825, pluss the Rth of the cabinet itself (front plate, and top and bottom plates)? This would be enough for either an Aleph 5 or an Aleph 4!?

Here follows the link to the cabinet:

LC Audio Cabinet

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