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Heatsink Help!

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Super Wammer

I am almost done compiling the parts and components for a new DIY amplifier project but I am struggling to get my head around heatsinks!

The amplifier is a DIY Nelson Pass A40, but will be split in to 2 enclosures a-la monoblocks, rather than in a large single enclosure.

The only advice I have gleaned out of anyone up to now is “use the largest heatsink you can”, which isn’t much guidance!

My plan is / was to mount each of the 4 TO3 power transistors on its own individual heatsink to the outside of the amp case (2 on the left side and 2 on the right side) and then cap them with there own small heatsink, but I’m not sure whether this would provide sufficient ambient cooling. 

The transistors I’m using are:

MJ11032G & MJ11033G TO3 Darlingtons. 

The main heatsinks are rated at 3 degrees c/w and look like this:

and the heatsink caps are rated at 14 degrees c/w and look like this:

I know that the Class A Pass A40 will run hot, supposedly very hot, but can anyone comment on whether my planned heatsink set-up will be up to scratch??

Thanks in advance!

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As a class A design it will be dumping a lot of energy into the heatsink, you need to find out exactly how much. I don't think 3 degrees/W will cut it, at 50W they would reach a temperature of 150C. If space is at a premium, you could go for a heatsink with integral fan cooling but you would need to have a thermal cutout in case of fan failure. It's also advisable to mount all the output transistors for each channel on the same heatsink so they are thermally coupled to compensate for drift problems.

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