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Old 29th May 2013, 07:25 PM   #1011
JRKO is offline JRKO
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Overtheairbroadcast, I use mine fins up (see post 750 for a pic) with the kit supplied heat sink

They get hot, but you can touch them for a few moments. At first I didn't have the amp on too long but now I have it on for 6/7 hours sometimes with no harm
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Old 30th May 2013, 04:02 PM   #1012
overtheairbroadcast is offline overtheairbroadcast  Canada
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Thanks for the info JKRO and good to hear it is not detrimental to the parts and components. It opens up a few other design options for me now.
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Old 30th May 2013, 08:11 PM   #1013
Variac is offline Variac  Costa Rica
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Quote:
Originally Posted by Eric View Post
Love those wooden chasses! Interesting design with heat snk configuration. It looks like you can run bias a little higher than normal with sinks that large.
Those sinks aren't particularly big for an ACA, and the horizontal fins will affect it a bit too. All and all a bit less passive cooling I think than the original or the coming chassis from the store, but close. My experience is that the most important thing is if it's in a slightly enclosed area, like with a shelf above it, the temp goes up fast! Ambient temp matters too. In a tropical climate I'd double the size!

I believe there's been some discussion that MOSFET's will tend to fail over the long term more often if they are run hot.
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Last edited by Variac; 30th May 2013 at 08:14 PM.
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Old 30th May 2013, 08:37 PM   #1014
Eric is offline Eric  United States
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Variac: Perhaps it is just the perspective of the image that makes them look larger than they are. Guess the camera was close to the amp

On another note, are there any plans for the DIYAudio store to restock those great 2U and 3U 200mm sinks?

Thanks,
Eric
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Old 31st May 2013, 05:52 PM   #1015
bcmbob is offline bcmbob  United States
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Finally got around to using something other than computer/Walkman for volume control. The first pic is a shielded box containing an Alps Blue Pot. It sounds delicious but only offers attenuation. Completely sufficient for most systems IMO.

Being a big band jazz fan/player, I like a little more SPL than that, so I hooked up a J-Fet preamp. It has more gain than I need but also sounds clean and clear. If I'm reading the power transistor spec sheet properly, which I could easily be mistaken, the high temp theoretically can go to 150 C. (TJ = Operating Junction and Storage 55 - 150 C). Can someone with more experience report what a reasonable, practical temp ceiling is. Probally will never go there but it would be nice to know.
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Old 31st May 2013, 07:13 PM   #1016
Eric is offline Eric  United States
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Hi Bob,

I wouldn't worry much about those IRFP240 devices, they are virtually indestructible. The data sheet shows the de-rating curve for them:

Click the image to open in full size.

My Aleph-X amp runs them at less than 2A bias each with a case temp that is less than 60c. This is well within the safe operating area. I don't think you can run an ACA high enough that they would break a sweat

I see you have some nice home spun cat5 speaker wires, too! Cool stuff!

Eric
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Last edited by Eric; 31st May 2013 at 07:15 PM.
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Old 31st May 2013, 07:26 PM   #1017
bcmbob is offline bcmbob  United States
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Thanks Eric. I don't do "Head Bangin" levels but i'ts nice to know there is adequate capacity for some good boost.

The cables can be a pain to build but the results are worth the effort - a very clean, warm and solid sound.
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Old 31st May 2013, 07:50 PM   #1018
Eric is offline Eric  United States
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Some followup...

There really isn't a hard-and-fast maximum power dissipation figure for the 240 devices as this is dependent upon the size of heat sink. Dissipating more power means needing a bigger sink and to some extent, larger sinks "compensate" for greater power dissipation by keeping things cooler- though there are limits.

I ran some numbers through WuffWaff's spreadsheet that he developed for the Aleph-X amp. With 20v rails and 2A bias per device (40w dissipation per device) with each device mounted on a 1.0c/w sink, you would have junction temp of about 100c and a sink temperature of about 65c - this is about as high as you would want to take it. You would have a maximum power dissipation figure per device of about 53w, so you're coming pretty close by running them at 40w each.

If you then double the size of your heat sink (0.5c/w per device), this brings the junction temp down to about 85c and sink temp down to closer to 45c - this is much more sane. With cooler temps, the "danger zone" of power dissipation becomes more like 77w per device, (instead of the previous 53w) so at 40w per device, they can breathe a little more easily on a larger sink.

Eric
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Old 31st May 2013, 08:42 PM   #1019
bcmbob is offline bcmbob  United States
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That tells me, though adequate with attenuation, my little PC CPU coolers are close to max capacity. If I continue to use the ACAs with a gain device I'll bump up to something more substantial. When my devices read ~ 52 C the heatsinks are ~38. They are hot to the touch but don't burn the fingers. I suspect the very thin fins are helping in the current configuration, but I understand the advantage of using more metal mass.

Don't know how far I'll push these as they weren't designed for high sound levels, but thanks for your replies and useful information.

P.S. Been listening to the Alps/ACA combo with a Paul Desmond/Chet Baker duo + percussion...sweet, and again - thumbs up Mr. Pass.
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Old 31st May 2013, 11:58 PM   #1020
Eric is offline Eric  United States
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Bob, for a rough approximation, divide your thermal rise by the power dissipation and that will provide you with the c/w rating of your current sinks.
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