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Old 12th January 2011, 07:27 AM   #561
h_a is offline h_a  Europe
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I think your heatsink but with twice its height could be barely sufficient for the ~75 W power dissipation.

However, do not forget, you can always reduce outputstage bias to fit your heatsinking.
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Old 12th January 2011, 11:31 AM   #562
AndrewT is offline AndrewT  Scotland
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Hi,
two of these, one on top of the other will increase dissipation by about 40%.
Two side by side or back to back will increase dissipation by ~100%.

Side by side spaces the FETs too far apart.
Back to back with the sinks clamping the devices would work if this is the top most part of the amplifier.

Two channels at full bias would probably require 4 of those sinks.
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Old 12th January 2011, 12:11 PM   #563
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Andrew,
I have a heatsink here which measures 20cm across by 4cm deep with height of 15.5cm.
Is this enough?
Best wishes,
Chris
I'll post a photo in a minute.
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Old 12th January 2011, 12:42 PM   #564
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As you can see there are two of these heatsinks bolted together.
Chris
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File Type: jpg Large heatsink.jpg (90.2 KB, 290 views)
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Old 12th January 2011, 01:06 PM   #565
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If these heat sinks (from picture) are for one channel they will bi sufficient. Do you know theirs Rth
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Old 12th January 2011, 01:16 PM   #566
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No. These heatsinks come from the Italian company MODU with the larger model of the Pesante Dissipante.
Chris
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Old 12th January 2011, 01:43 PM   #567
AndrewT is offline AndrewT  Scotland
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download the free software for simulating heatsink performance.
or
look up datasheets until you find a very similar heatsink and read off the data.
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Old 12th January 2011, 03:54 PM   #568
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I didn't think they would have the data but they have. Well done MODU.

The data (which is attached) for a heatsink 400x 160x 40, (which means two of the 200x160x40 bolted to a joining bracket), at 25 degrees C ambient and Rt((degrees C/W) of 0.23.
For a TO3-P device, multiplied by 6, Max power dissipation (6xTO3-P) is 375W per heatsink.
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File Type: pdf Modu heatsink data.pdf (27.5 KB, 45 views)
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Old 12th January 2011, 04:28 PM   #569
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Having looked through Farnell heatsinks I have found one that looks very similar to those on the other chassis I was hoping to use for the amp.
This chassis has smaller heatsinks than the MODU design, measuring 300 width, 120 depth and 40 height similar to Forsman's.
I am getting a K/W of about 0.35 for this heatsink, looking at the graph supplied on the datasheet from Farnell.
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Old 12th January 2011, 04:31 PM   #570
AndrewT is offline AndrewT  Scotland
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delta T s-a = Rth s-a * P
Ta = ambient.

375W into a pair of sinks gives delta T s-a ~ 86C
When room is hot say Ta=30degC the heatsink will be at 116degC and Tc ~130degC.

0.23C/W does not come close to being adequate.
Does each in the 6pair dissipate >30W?
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Last edited by AndrewT; 12th January 2011 at 04:34 PM.
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