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Old 20th December 2008, 06:03 AM   #1
EUVL is offline EUVL  Europe
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Default The next best thing to 2SK389 / 2SJ109 ?

Just as everyone else, I am running down on my reserve of 2SK389/2SJ109s, and am looking for the best possible substitute.

The LSK389 is only available in N-JFET, and is not pin compatible. So for me no solution.

The most obvious solution is to use still available 2SK170/2SJ74 matched pairs. If you have a large pool, you can get them even better matched than the 2SK389/2SJ74 (spec at 3% match).

http://www.diyaudio.com/forums/showt...79#post1689279

But how about thermal coupling ?

You can glue them on the flat surfaces with silver epoxy, but you mess up the pin compatiblity to the duals.

So this is my solution.

It has a factor of 3 more thermal contact surfaces than just gluing on the flat surface, for much improved thermal coupling, and is still small enough to fit in most applications, with a footprint of 16x8mm.

The heatsink helps to increase thermal insertia and hence thermal stability, and keep the temperature a bit lower than without.

They are not extrusions (with a minimum order of 1 ton in weight), but rather CNC wire-cut from solid. This also explains the rough surface finish, which is an added advantage as it increase surface area for heat exchange.


Merry Christmas,
Patrick
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Old 20th December 2008, 06:08 AM   #2
EUVL is offline EUVL  Europe
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And of course you can use the same heatsink for any TO92 devices -- MPSA18, 2SK369, ........

One more fantastic application I am using them for is the JohnCurl Complementary JFET Buffer with a Idss-matched pair of 2SK170 / 2SJ74. You will get next to zero thermal drift in DC offset even if you use the V grade at full dissipation.




Patrick
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Old 20th December 2008, 06:08 AM   #3
lineup is offline lineup  Sweden
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looks good
i am sure it works very well, too

where do you buy those ... ?
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lineup
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Old 20th December 2008, 06:11 AM   #4
EUVL is offline EUVL  Europe
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Custom made to my drawings by a friend who has a workshop with CNC wire cut facilities.

Patrick
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Old 20th December 2008, 10:54 AM   #5
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My absolute offset is less than 10mv after warm up.

The relative offset remains -0.5 volt in warm up mode then0.15mv in normal operation.

So I devise warm up/stand by 1/2 bias current and let amp await in warm up mode like Pedator then full bias via DPDT switch...prepare for some excitement.... Big Bang.
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Old 20th December 2008, 03:54 PM   #6
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Will they be available for sale?
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Old 20th December 2008, 07:35 PM   #7
EUVL is offline EUVL  Europe
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If there is sufficient interest (say 500-1000 pcs), I shall see whether I can arrange a small production run. But it would probably be not less than US$ 2.50 per piece.


Patrick
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Old 20th December 2008, 07:44 PM   #8
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If there is enough interest to justify a run i would take 25 of them.
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Old 21st December 2008, 04:15 AM   #9
EUVL is offline EUVL  Europe
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Doesn't look like people think thermal coupling is important enough.

If you are the only one, I can probably let you have some from my own reserve. Give it a day and I shall let you know.


Patrick
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Old 21st December 2008, 05:49 AM   #10
housing is offline housing  Hong Kong
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Quote:
Originally posted by EUVL
Doesn't look like people think thermal coupling is important enough.

If you are the only one, I can probably let you have some from my own reserve. Give it a day and I shall let you know.


Patrick
Hi Patrick,

Can I have some also?

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