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Old 9th February 2011, 10:37 AM   #1
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Question Heatsinks class A amp question

Wich heatsinks need an amp in class A with a total heat disipation of 520W with a total bias 11A giving 120W RMS at 8 ohms load?
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Old 9th February 2011, 02:38 PM   #2
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See Extrusion Profiles - Heat Dissipating, pick an extrusion profile similar to one you can find locally and change the length in the calculator to suit your desired height. Note that each extrusion side by side will give you half of the thermal resistance, but doubling the height will not.

I have no interest in Aavid Thermalloy, I just find the calculator useful. Be sure to check B and C stock for a match to a profile you find surplus. You'll often find an exact match.

You're going to need a lot of heat sink or a fan to dissipate 540W
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Old 9th February 2011, 02:39 PM   #3
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Quote:
Originally Posted by merlin el mago View Post
Wich heatsinks need an amp in class A with a total heat disipation of 520W with a total bias 11A giving 120W RMS at 8 ohms load?
A very large heat sink. Force cooled would be advisable. Firstly you must decide what temperature rise above ambient would be acceptable, then you look for a heat sink that tells you how much K/watt per length it dissipates and you calculate how many meters of it you need to stay withing the temperature rise you anticipate to be safe.

Lets say you find a heat sink that states 0.5K/watt (per some length) you dissipate 560 watt, thus the temperature rise would be 56 degrees centigrade above ambient. If your worst case summer ambient is say 40 degrees centigrade, the final heat sink temperature would be 40 + 56 = 96 degrees centigrade. Besides you will have to consider what would the heat sinks heat up. If you have capacitors rated at 86 degrees centigrade, obviously they will not last. Plastic parts may all go floppy.

Even a small fan circulating the air would make a significant difference.

That is a serious amount of heat just to listen at moderate levels.
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Old 10th February 2011, 11:46 AM   #4
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Thanks guys.
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Old 10th February 2011, 12:01 PM   #5
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by merlin el mago View Post
Which heatsinks for an amp in class A with a total heat dissipation of 520W with a total bias 11A giving 120W RMS at 8 ohms load?
Have you looked at a Krell KSA100? It is a ClassA amp with similar 8ohm output but running on 2.6A output bias spread over 4pairs of 250W 200degC devices.
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Old 10th February 2011, 12:35 PM   #6
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The Krell is pushpull so twice as efficient and needs half the sinking.
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Old 10th February 2011, 12:37 PM   #7
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To add, from the figures originally quoted I did assume this is a single ended design.
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Old 10th February 2011, 12:51 PM   #8
AndrewT is offline AndrewT  Scotland
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Yes Krell has a push-pull topology.
But has Merlin even looked at the heatsinks?
Can't be SE.
Pq=520W and bias=11A gives 47V
To get 44Vpk into the load it must be some form of balanced push pull. Each half with 5.5A and +-23.6Vdc
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Last edited by AndrewT; 10th February 2011 at 01:00 PM.
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Old 10th February 2011, 12:55 PM   #9
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Now there's a question....
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Old 10th February 2011, 12:58 PM   #10
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Hi Andrew, hope all is well with you. Must away now as I have an unwell Quad 44 pre to fix. catch up with this thread later.
Henry
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