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

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.

:D


Patrick

looks good
i am sure it works very well, too

where do you buy those ... ?

Custom made to my drawings by a friend who has a workshop with CNC wire cut facilities.

Patrick

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.

Will they be available for sale?

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

If there is enough interest to justify a run i would take 25 of them.

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?

;)