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Old 11th November 2006, 02:45 PM   #1
yoke is offline yoke  Croatia
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Default Aleph X - mosfet temperature

Hi I have plan to build some active loudspeakers and would like to use Aleph X for bass , Aleph 30 for mids and mini for twiters...

Now I have build case for it, which is based on Fischer SK 41 heatsinks. In spreadsheat it states that c/w at lenght of 150mm is 0.28...muhahahahhhhaaa...

In my case the temperature at heatsinks is 72*C...
on picture is one cahnel of Aleph X with 8 mosfets per chanel, 18V and 4A bias ( 1A per mosfet )

so is this 72* too hoot for mosfets ???

how much can it take ???

I know that it's recomended not above 60-65 , but :-(


Thanks
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Old 11th November 2006, 02:55 PM   #2
yoke is offline yoke  Croatia
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and one picture of case ( inside would be Aleph X,30, and mini )
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Old 11th November 2006, 03:17 PM   #3
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Might be time to consider forced air cooling.

Grey
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Old 11th November 2006, 09:46 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
18V @ 4A is only 72W.
The temp rise for a 0.28C/W sink should be about 20Cdegree.

Measuring 72degC indicates there is something far wrong.

What is ambient?

Could there be oscillation?

Is the airflow blocked, maybe at the bottom?
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Old 11th November 2006, 11:17 PM   #5
moamps is offline moamps  Croatia
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Hi yokash:

If each transistor draws 1A and the voltage is 18V then the total dissipation for 8 transistors is 8x18x1=144W. If the heatsink has a coefficient of 0.28 deg C/W, it will heat up at 40 degrees above the ambient temperature (in theory). In practice, things are much worse. From my experience with Pass amps, your chassis might accomodate maximum 1 channel of AX or Aleph 5.

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Milan
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Old 12th November 2006, 12:00 AM   #6
Blues is offline Blues  United States
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You can reduce your bias to about 0.75A for each transistor or about 3A total. This will theoretically give you about 30C above ambient. Total dissipation for a channel of your X will be about 110W.
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Old 12th November 2006, 06:55 AM   #7
AndrewT is offline AndrewT  Scotland
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post1 pic shows four devices on each of the two sinks.

But are they really 6inches tall?
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Old 12th November 2006, 08:05 AM   #8
yoke is offline yoke  Croatia
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Hi all...
I have problem with Internet at home, seams like my router have drop dead ...

so...I will not access to web all the time...( writing from office )

The heat sinks are fischer SK41 at length of 150mm ~ 6 inch...ambient at around 20*C...

AndrewT _ I don't think that there is any thing oscillating, it's just the bead spreadsheet from fischer...I believe...

here is the tread I ask about those heat sinks
Fischer Elektonik SK41 Heatsinks

O.K. the butom of heat sink do lay on the floor but , could that relay affect that much ? ( I mean all amplifiers that I see a not far from surface, less then 1 inch )

How tall should I rise it from the floor ?

I have place the top sheat of case just to see what I can expect if I add more surface...the case would be all aluminum, as you can see from pictures...

so now with this one, temperature drops to 64*C...which is not that bad ...I hope
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Old 12th November 2006, 08:51 AM   #9
vuki is online now vuki  Croatia
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Friend and I made AX with SK110 (150mm, ca. 0.35C/W) and 4 IRFs on each heatsink. There are approx. 90W dissipated on each SK110 and the temperature is ca. 60C with room temerature at 22-24C.
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Old 12th November 2006, 08:56 AM   #10
yoke is offline yoke  Croatia
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Gray ... could you please tell me, if you know, what resistor value should I use to set ac current gain to 60%...
It says that I should place trim pot in place of R12/R34 ( schematic from wiki tread revision 1.0 )...but last time I played with this I manage to burn one of my output mosfets :-(...

well, at lest should I go below this 1.2 or higher ?

since this will be driving bass driver I think that I can sacrifice sound quality regarding power, at 60% I will get more power to control bass ... am I right ?
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