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Old 5th October 2001, 07:58 AM   #1
ergo is offline ergo  Estonia
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I'm working on my class A amp design and I had an idea to use aluminium oxide washers for the output stage transistors. These washers have impressive thermal resistance <0.3 deg/W....

The question that I have is as follows. Has anyone used them and are the results positive? Also what is the appropriate method of using them? Do they provide a good contact on their own or should one still use a very thin layer of standard thermal paste to quarantee a good contact.

I have not seen these washers in real life. I'm just thinking of ordering them...

Thanks for any feedback
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Ergo
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Old 5th October 2001, 08:00 AM   #2
hifi is offline hifi  Sweden
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not an answer but...

the SILPADS2000 from Bergquist have a thermal resistans of 0,2c/w and i think they are cheaper....

hmmm am i being paranoid now? is there a difference in sound between washers??

/micke
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Old 5th October 2001, 08:10 AM   #3
ergo is offline ergo  Estonia
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to /micke

If your question is whether I'm thinking of these washers because of the sound then no. As I will drive the output stage in pure class A I just want to make sure the transistors have the best conditions possible from thermal point of view....

Ergo
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Old 5th October 2001, 09:05 AM   #4
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Default There probably is a difference

What you need to use for "optimal dielectric conditions" is Beryllium Oxide washers (ref radio frequency stuff). Illegal in Europe, but I think you can get them from Canada in TO3 form factor. TO247 washers are unavailable. More on this under the X-100 thread in solid state.

These should be better because they help reducing the apparrent capacitance of the device.

Aluminum Oxide washers are just alu washers which have an oxide layer on both sides and are probably no better than Silpad for dielectric properties.

AluO and BeO units are prone to performance reduction given the need for thermal grease.

Petter
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Old 5th October 2001, 10:36 AM   #5
peted is offline peted  United Kingdom
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Quote:
Originally posted by hifi
the SILPADS2000 from Bergquist have a thermal resistans of 0,2c/w and i think they are cheaper....
/micke
Ergo, I used SIL-PAD K10 (or SIL-PAD 2000) in my Zen (I obtained samples of each). They are the same thermally, one is a higher (MIL?) spec. They are used dry. Being a born sceptic, I confirmed the 0.2C/W by my own measurements - good stuff! Bolt them down reasonably tight, but don't go mad - you'll distort the flange on the TO3 and compromise the thermal joint. These washers 'squish' down a little and are therefore also non-reusable. They are non toxic.

Good luck.

Pete
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Old 5th October 2001, 10:51 AM   #6
hifi is offline hifi  Sweden
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beryllum oxide is prohibited in europe for a reaseon...if you get it in your lungs then you would probobly need an iron lung......


/micke
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Old 5th October 2001, 11:47 AM   #7
ergo is offline ergo  Estonia
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Thanks to everyone for your replies.

The reason I consider the aluminium oxide washers is that these are the best solution offered by ELFA, a part supplier in Sweden... As I live in Estonia (near Sweden and Finland) it is very hard to find suppliers for such small orders with reasonable shipping costs.... If someone knows an online part supplier (especially in Scandinavia or Europe) that accepts international orders I would be interested.

Ergo

PS. to /micke
In our country the beryllium problem most likely wouldn't scare people as there are so many hazardous materials, building materials, etc around as a legacy from Soviet Union times that most don't think of it much as one would most likely be exposed to it somewhere anyway...


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Old 5th October 2001, 12:32 PM   #8
peted is offline peted  United Kingdom
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Ergo - check out your local Bergquist supplier (are they not a Scandinavian outfit with a name like that?) tell 'em your'e a DIY nut and ask for samples - Bergquist (UK) were dead helpful - I got 10 of each type!

Pete
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Old 5th October 2001, 01:16 PM   #9
hifi is offline hifi  Sweden
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http://www.farnell.com

they are a lot better than elfa. quicker orders and cheaper price they are uk based...its cheaper and the shipping goes faster (also the shipping cost is less) for me to order fron farnell than elfa altough i live in sweden!

/micke
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Old 5th October 2001, 04:09 PM   #10
tvi is offline tvi  Australia
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If the Aluminium Oxide washers are the one I'm thinking of, white 2-3mm thick ceramic, I have heard them called 91 day wonders, 1 day after the warranty end the washer fails. Of course they heve most likley improved in the past few years.

As to the sound, I believe YBA used a three layer insulator in their amps, the middle layer was copper foil, supposedly helped with capacitance to the heatsink/chassis?

I found a similar scheme in an old mainframe PSU, the power transistors were mounted via 2 layer ceramic washers, when I seperated the washers I found one of the interal faces was metalised nd included a grounding wire.

aavid/thermalloy make a pad called A-Dux

<a href="http://www.aavidthermalloy.com/products/options/pads.shtml">A-Dux</a> without Adhesive</b>
Thermal Impedance 0.16C/in<sup>2</sup>/W

Swedish Distributors for aavid/thermalloy are listed <a href="http://www.aavidthermalloy.com/bin/catdbctry.pl?Country=Sweden&GoCountry.x=12&GoCount ry.y=16"><b>HERE</b></a>
theirs a sample request form on their site also.


<center><font size="+1" face="fantasy">Regards
James</font></center>
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