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Old 14th February 2001, 12:25 AM   #11
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FYI: Website posted re Aleph4 progress.

http://www.adelaide.net.au/~mefinnis/passlabs

Hope this is helpful to all and sundry.
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Old 14th February 2001, 01:20 AM   #12
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Cool site MeFinnis.
Everything looks very cool.
Wish I could start faster with my aleph amp.
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Old 23rd February 2001, 08:52 PM   #13
peted is offline peted  United Kingdom
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Hi - A few findings of my own with heatsinking - the BIGGEST issue with Class A. I'm doing Zen Revisited - 2 sets of paralleled amps for 4ohm spkrs, each running 4amp bias, 50v rail, thats 128w on the gain stage MOSFET (for symetric clipping it runs at ~60% rail, not 50%). So MOSFET at 128w = non-trivial h/s problem. Using 250W TO3 devices (internal junc/case is 0.5C/W), which with temp. derate makes a max safe junc temp of around 120C. Yep, that's 800w total! Winter listening only?

I found some TO3 plastic washers from http://www.bergquist.com - 0.2C/W (yep, 0.TWO)(confirmed by my measurements), SILPAD K10 or SILPAD 2000 series, used dry. To get this heat out, the problem with few devices - they are mounted on to 12.5mm Aluminium plate - works a treat. Then I have a system of alternate 6*25mm Al spacers bars and more 3mm plates (the fins)- all gunged up with thermal grease (beware - not too much) and clamped together with 3 runs of 6mm studding to the 12.5mm plate.

Performance (natural convection)0.17C/W, (with fan) 0.11 C/W. Not bad - but I'd not recommend the metal bashing involved. All mating surfaces were wet rubbed with 400 grade wet/dry to ensure as flat as possible (= good thermal joint). The grease and clamping does the rest. (I asked about welding - the guy said no-go as the heatsink effect was too great - ironic really). The amp is built around the heatsink, no other chassis apart from a supporting cradle. Fan is a stripped down 30cm desk fan, run ever so slowly so it's silent (a design factor of mine - NO noise apart from music) - at around 1/2 voltage. Seems to be a good approach generally for lo/no noise fans. It still blows enough. Pics of this monster will hopefully be on NP's site in due course.

So at 22C ambient my devices are running at case temps around 56C (junc temps 56+(0.5*128) = 120, not too bad. Remember NP's comment on MOSFET service life - low temp = long life.

A 50C thermal switch on the sink protects against those hot summer days (in the UK?) or fan failure.

Hope this is useful to someone.

Pete

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Old 23rd February 2001, 10:20 PM   #14
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Default BeO washers

Nice

Have you checked out BeO washers for electrical isolation?

That is the absolute best you can get unless you bolt on to a piece of copper and use a larger area washer.

Another cool thing about BeO washers is that they are thick and so reduce capacitances of device to heatsink ....

The only problem with BeO is that they may be illegal in the EU and that if sanded they produce dust which gets you berylliosis, similar to astbestosis from asbestos ...

I have worked with this material in the past for it's nuclear properties, should not be a problem unless you find out sanding it is i a good idea.
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Old 24th February 2001, 11:42 AM   #15
peted is offline peted  United Kingdom
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On washers: The washers I used are not BeO (as far as I know) - they are white, flexible plastic, yes, quite thick, but squish down when bolted up. I'd assume totally safe to handle. I believe the stuff is available in sheet form for you to cut to shape. Usual warnings though about not over tightening MOSFET/sink mounting bolts, to avoid distorting the device and losing a good flat contact with sink. Perhaps more so with these washers as I believe they are designed to give a little.

Insulation? Guess it doesn't matter too much at solid state voltages, but their thickness must be good for this. Can't remember the specs - check the web wite. Sorry - got their URL wrong above - should be http://www.bergquistcompany.com. (Got mine as samples from UK office)

Cheers

Pete

[Edited by peted on 02-24-2001 at 06:54 AM]
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Old 4th March 2001, 05:04 PM   #16
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Question IRFP250

Is it possible to use IRFP250 instead of IRFP240 for the Aleph 4 project?
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Old 4th March 2001, 07:13 PM   #17
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Of course it is. They don´t have such a big difference.
The IRF250 has a smaller on resistance so can handle bigger current.
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Old 5th March 2001, 03:10 AM   #18
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The Safe Operating Region looks good...go for it.
IRF (www.irf.com) has info online. The SOA (Safe Operating Area--same thing--aka SOAR) chart is generally several pages back.

Grey

[Edited by GRollins on 03-04-2001 at 10:12 PM]
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Old 6th March 2001, 03:38 PM   #19
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Then IRFP250 it is!
Thanks
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Old 4th April 2001, 08:11 PM   #20
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Question aleph 5

with all the confusion over the number of output devices on the Aleph 4, I'm still confused! Now the Aleph5 still says X3 for output devices is that correct?
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