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Old 23rd December 2003, 08:05 PM   #1
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Default heatsink question

I have a couple questions that have probably been asked before, but I couldn't find them in my searching, so I'll ask anyway. First, when puting a heatsink on the output devices of an amplifier (let's call it a class-b), do all the transistors/FETs have to be mounted to the same heatsink if an aluminum bar is mounted on the opposite side for thermal tracking? If not, would it be ok to use a large aluminum CPU heat sink such as this one for each of the output devices? For amps with fewer output devices, this might be a good, cheap option, as you can get 4 of these sinks for $20, and they are large, heavy, skived-fin sinks. They are, however, designed for use with a fan, but it should be able to keep a transistor cool enough without one. Perhaps it would work better if you connected them to each other with a thermal epoxy? Maybe this is a dumb idea, but I'm new to this. I'm trying to avoid the mammoth cost of large extruded/skived aluminum heatsinks for mid-power amps.

Another idea I had that involved a lot more work was the possibility of using a 1/4" plate of aluminum and 3+ 1/8" plates connected to eact other by short pieces of copper pipe brased/soldered to the aluminum plates?
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Old 23rd December 2003, 09:39 PM   #2
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Default Re: heatsink question

let me answer your 2nd question first. Yes, you can use a cpu fan. Here is what I did, with a pentium 2 fan cooling two irf540 working in class a, each dissipating about twenty watts. The fan, as you can see, is powered by a 12v supply.

http://www.diyaudio.com/forums/showt...544#post282544

However, if you parallel multiple devices, it is a good idea to mount them onto the same heatsink so that their temperature is about the same, avoiding overloading of one or two particular transistors (we aren't talking about l-mosfet here).

if you want to mount multiple devices onto one cpu heatsink, consider that with the fan powered, the dissipate about 20-40watts (I think. Athlons and P4 could be higher), and far less without the fan.

I would do it very carefully, but it can be done.
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Old 23rd December 2003, 09:46 PM   #3
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> large aluminum CPU heat sink such as this one

Fins that close together NEED a fan. Without forced air, fins closer than about 1/4" or about 6mm will trap stagnant air.

> do all the transistors/FETs have to be mounted to the same heatsink... for thermal tracking?

Depends a lot on your design. If all paralleled transistors are sure to carry essentially equal currents (large bias resistors) then they don't have to be on the same sink. Also some designs control bias at the pre-driver, not the output, and you rarely parallel pre-drivers.

If labor is more expensive than scrap metal or space (wiich is often true in DIY), then just get a huge slab of 1/4" aluminum and bolt your devices directly to it. 7"x7" of 1/4" Al slab will not be much worse than the CPU sink with a high-blow fan, better than the fan you get with a $5 sink, and a lot better than a fanned sink without a fan. So if you think four CPU sinks would do you, look for a 14"x14" or so slab (maybe 17"x12" rack-panel) of 1/4" Aluminum. Don't fool with L-brackets, they add more thermal resistance than they are worth. Nail the trannies right on (with greased insulator).
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Old 23rd December 2003, 09:47 PM   #4
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Default Re: Re: heatsink question

Thanks for the advice. In that case, I think it's best to go with a more traditional heatsink, unless you think the plate/pipe hybrid would dissipate a lot of heat with a lot of surface area.

Quote:
Originally posted by millwood
let me answer your 2nd question first. Yes, you can use a cpu fan. Here is what I did, with a pentium 2 fan cooling two irf540 working in class a, each dissipating about twenty watts. The fan, as you can see, is powered by a 12v supply.

http://www.diyaudio.com/forums/showt...544#post282544

However, if you parallel multiple devices, it is a good idea to mount them onto the same heatsink so that their temperature is about the same, avoiding overloading of one or two particular transistors (we aren't talking about l-mosfet here).

if you want to mount multiple devices onto one cpu heatsink, consider that with the fan powered, the dissipate about 20-40watts (I think. Athlons and P4 could be higher), and far less without the fan.

I would do it very carefully, but it can be done.
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