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Old 2nd May 2009, 03:52 AM   #11
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some manufacturers glop on the heatsink grease, too, like Pioneer and their output chips in their "mid-fi" receivers. i usually don't even need to add any grease when i repair those amps, just spread out what's already there. if you wonder how bio labs let viruses leak out, just consider how fast just a tiny bit of heatsink grease gets all over everything when you're working on a repair. a company i used to work for actually used clear heatsink grease. it still got on your clothing, etc..., but it didn't leave white splotches when it did. we wear grey work uniforms where i work, and one of my team members was replacing one of those Pioneer amp chips, and left it "buttered" side up on his work bench. he turned to answer the phone, and caught the leads on his uniform and it left a large rectangular white blotch. at break time i said "i can tell you were working on a Pioneer......". he told me later his wife was a bit upset, because she washed that shirt twice to get the blotch out.

Sony tends to use it sparingly, and it looks like some machine stripes it on their heat sinks. Yamaha seems to use about the right amount, and a lot of manufacturers have gone to using strips of sil-pad. i'm starting to see these very thick, but VERY heat conductive squishy rubber pads on a lot of HDMI and DVD processor chips, as well as on class D amp chips. according to what i have seen in trade magazines, they're not silicone, but some other type of rubber that is so heat conductive that it even draws away the heat from the chip's leads as it gets squished over the edge of the chip. as i NERTS (scrap) DVD players that aren't cost effective to repair, i've been gathering a few of these pads to test for their thermal conductivity. right away, i've noticed that when they're on a heat sink, they are "colder" to the touch than sil-pads are, but i want to do actual temperature gradient testing to be sure. i'll probably use a couple of diodes as thermal sensors, and a transistor as the heat source (or even better, monitor the transistor Vbe directly as a built-in thermal sensor). the pads are squishy enough that i could use 1N4148s as sensors and not create any large gaps between the pads and metal surfaces.
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Old 2nd May 2009, 04:43 AM   #12
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Hello unclejed, I'm rather interested in your results of your tests, and in particular, anymore information you get as to what the material is and who makes it.

Peace,

Dave
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Old 2nd May 2009, 09:54 AM   #13
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Quote:
Originally posted by wwenze
[B]IMHO the thermal resistance of the small heatsink inside a non-active-cooled case is too high for the insulating pad's thermal resistance to be of any significance. We're not talking about 130W CPUs that need to be kept below 60 degrees by a huge active cooler.
Actually, the application is not so relevant, sizing of heatsink and sufficient airflow are.

I once, foolishly, used an insulation pad instead of thermal paste between a hothead CPU ("Prescott" Pentium D) and the actively cooled heatsink. It died within days. Here the pad's thermal reisistance was the determining factor. The stock cooler was borderline acceptable and barely able to keep things cool. It's no wonder a huge (after)market emerged aimed at CPU-cooling. We're talking about huge power dissipation through a small surface area here, so there's very little margin for error.

When you're talking about a class AB amp, there's only appreciable power dissipation at high power levels during a long time (and if you have neighbours, that's unlikely to happen).

A class A amp is a different story. The heatsink needs to be properly sized and the transistors mounted with electrical insulation and thermal paste. My Marantz PM80SE is a good example, a large part of space inside this amp is used for the heatsinks. In class A mode (it can also be switched into AB mode) it needs a lot of free space around it and especially above it to ventilate.
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Old 4th May 2009, 03:18 AM   #14
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Quote:
Originally posted by dave_gerecke
Hello unclejed, I'm rather interested in your results of your tests, and in particular, anymore information you get as to what the material is and who makes it.

Peace,

Dave

found it, they're called Gap-Pads.
http://www.bergquistcompany.com/ther...ls/gap-pad.htm

same company that makes Sil-Pads

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Old 4th May 2009, 03:37 AM   #15
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Cool. Thanks for the link uncle jed. Guess I'm going to have to do some reading, and some research as to who carries these.

Peace,

Dave
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