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Old 1st November 2005, 03:54 PM   #1
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Default Aleph-X heatsinking

Hi Guys,

I have my AX boards assembled and ready to go. I bought for of these heatsinks.

For testing I just mounted one board to one of those. I'm running just the 4 outputs that mount to the board and a 12V 600Va per channel transformer.

With 0R15 source resistors at .5V It runs a bit too hot. So I decided to use two heasinks per side.

Well I sat everything side by side last night and If I go with what I have, I will end up with a case that is almost 12" high, 19" wide and 20" deep weighing about 50lbs.

So I'm asking those of you who have built these. Can I cut down the height of the heatsinks some without losing too much cooling? I'm not sure I'll be happy with a case this size. Have any of you tried fan cooling? I hate the thought of fan noise but this thing is huge.

Any suggestions?

Thanks, Terry
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Old 1st November 2005, 04:09 PM   #2
Blues is offline Blues  United States
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terry,

You're running your AX at about 12A/ch (4 output mosfets @0.5V/0.15R). You can reduce this by using 0.3R Source Rs and bias will be about 1.5A for each branch x4=6A per ch. This will cut your heat. It might still require 2 sets of HS for each ch.

allan
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Old 1st November 2005, 04:52 PM   #3
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Hi Blues,

Thanks for the response. According to the AXE-1 spread sheet I am running about 6A/channel with 0R17 source resistors. 0R15 was as close as I could find. I guess I'm OK with two heatsinks per side but I wonder if I need to keep them 11" tall. I've been trying to keep my amps at standard rack sizes so that I can mount them in a case someday if necessary. Besides, if I can cut down the height it will take some weight off as well.

Blessings, Terry
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Old 1st November 2005, 05:09 PM   #4
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Hi Terry,

0,15R source resistors mean around 3-3,5A bias per fet wich at 15V? supply voltage means 45-52Watts dissipation per fet wich is quite risky! I would go for 8 devices to be on the safer side.

If you are using the AXE sheet you must also have a look at the dissipation per device and the heatsink calculation at the bottom.

Did you measure the heatsink temperature?
Did you measure the voltage across the source resistors?
What is your supply voltage?

Doing a small calculation you junction temperature will be 60-70C over the heatsink temperature!!!

William
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Old 1st November 2005, 05:38 PM   #5
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Hi William,

I could use higher value source resistors if I have to. I bought 0R22 first.

I am using the MTW32N20E transistors for outputs. Nelson recommended running them higher than the IR transistors. He suggested using only 4 outputs due to their high capacitance.

Post #824

I am planning on using this amp with 4ohm speakers which was the reason for going to a higher bias.

I did measure the heatsink. I had one board mounted and after about a 1/2 hour, the heatsink was at 50c and the devfice measured 78c. After about an hour the heatsink was up to about 65c and the device was past what my gauge could read. It just said "hi".

Supply voltage under load is about +/-14.75

0.5v across source resistors.


That's when I decided that I would probably need two sinks.

I'm using Kapton tape.

I have more devices that are matched to these so going to 8 devices is possible. I was staying with 4 based on Nelson's recommendation.

Blessing, Terry
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Old 1st November 2005, 05:52 PM   #6
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Hi Terry,

I forgot about the different devices you are using........

If you double up the heatsinks the temperature rise will almost be halve of what you measured (from 65 down to 45?) wich should work fine.

You can certainly cut something of the heatsinks without losing to much cooling. The relation between heatsink height and Rth is not linear. You can loose about 20% of cooling capacity (from 20 rise to 25 rise) so I guess you can reduce the height by 30% and still have enough cooling.

Painting them black will also help.

William
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Old 1st November 2005, 09:05 PM   #7
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Default Re: Aleph-X heatsinking

Quote:
Originally posted by still4given
...
Well I sat everything side by side last night and If I go with what I have, I will end up with a case that is almost 12" high, 19" wide and 20" deep weighing about 50lbs.

...
A manly size, but a little light weight compared to the XA series -

the XA160 (from Passlabs.com)
Dimensions 19"W x 11"H x 22"D
Weight 150lbs
the XA200 gains 25 pounds.

Why do you want to make it smaller? the big sinks will give you room to bump up the bias, should you want to try that. Black anodizing might be a better choice than painting.
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Old 2nd November 2005, 12:48 AM   #8
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Default Re: Re: Aleph-X heatsinking

Quote:
Originally posted by BobEllis


A manly size, but a little light weight compared to the XA series -

the XA160 (from Passlabs.com)
Dimensions 19"W x 11"H x 22"D
Weight 150lbs
the XA200 gains 25 pounds.

Why do you want to make it smaller? the big sinks will give you room to bump up the bias, should you want to try that. Black anodizing might be a better choice than painting.

Hi Bob,

I had guessed at the weight. I just piled most of it on a scale and it was over 60lbs. I would love to get the stuff black anodized. Anyone know a source for that kind of work?

I'm hoping to have this weekend off so I can get the cases made. Can't really do much more without them.

It looks funny having only two little MOSFETs on a 10" x 11" heatsink.

Got a question. I'm using Kapton tape for my insulators. Do those of you who use it use grease too?

Thanks, Terry
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Old 2nd November 2005, 01:37 AM   #9
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check the yellow pages under plating and/or anodizing. A yp.yahoo.com returned 46 potential vendors within 50 miles of you - the closest 9 miles from town center.
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Old 2nd November 2005, 02:22 AM   #10
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RE: Anodizers & hardcoaters...

Speak up if you find a good one...

"Your parts will be ready Tuesday".

That don't tell you which year.
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