Aleph X - mosfet temperature

yoke · 11th November 2006, 02:45 PM

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

yoke · 11th November 2006, 02:55 PM

and one picture of case ( inside would be Aleph X,30, and mini )

GRollins · 11th November 2006, 03:17 PM

Might be time to consider forced air cooling.

Grey

AndrewT · 11th November 2006, 09:46 PM

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?

moamps · 11th November 2006, 11:17 PM

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.

Regards,
Milan

Blues · 12th November 2006, 12:00 AM

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.

AndrewT · 12th November 2006, 06:55 AM

post1 pic shows four devices on each of the two sinks.

But are they really 6inches tall?

yoke · 12th November 2006, 08:05 AM

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

vuki · 12th November 2006, 08:51 AM

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.

yoke · 12th November 2006, 08:56 AM

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 ?

12th November 2006, 12:00 AM	#6
Blues Lightning In A Bottle diyAudio Member Join Date: Sep 2003 Location: The Milky Way	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. __________________ Leap into the Warpspeed wormhole... http://www.6moons.com/audioreviews/warpspeed/1.html

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11th November 2006, 03:17 PM	#3
GRollins diyAudio Member Join Date: Feb 2001 Location: Columbia, SC	Might be time to consider forced air cooling. Grey

11th November 2006, 09:46 PM	#4
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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?

11th November 2006, 11:17 PM	#5
moamps diyAudio Member Join Date: Oct 2002 Location: Croatia	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. Regards, Milan

12th November 2006, 06:55 AM	#7
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	post1 pic shows four devices on each of the two sinks. But are they really 6inches tall?

12th November 2006, 08:56 AM	#10
yoke diyAudio Member Join Date: Jan 2004 Location: Split	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 ?