[sdiy] expo pair "heat shield?"

[Mattias Rickardsson]

Neil Harper via Synth-diy <synth-diy at synth-diy.org> skrev:

> hey all,
>

Hey!

just wondering if anybody thinks it's worth encasing an exponential pair
> for stability. while this is in my modular case, there must be some heat
> currents moving around from the other stuff that share the same space.
>

If there are very nearby objects with strong pulsating heat, which affects
one transistor more than the other, then a heat shield might do some good.
There probably aren't! (-:

(I assume that "shield" means metal foil or some other reflective material,
not just any material that is in the way but absorbs incoming heat)

What's most important is to keep the pair at identical temperatures, which
is done by having substantially less thermal resistance between the
transistors compared to between the pair and their environment. But it's
important that they are not totally isolated - if the contact between pair
and environment is very weak, such as using a shield that reflects heat
from the transistors back on themselves, then there will be increased
self-heating effects with different temperatures depending on the currents
through the transistors. Not optimal. I guess the component legs would give
enough contact in most normal cases though.

Making some kind of blob around the pair in order to increase thermal
inertia... Well, might be somewhat useful, but also potentially give some
of the self-heating problems mentioned above. Keeping them at one constant
temperature is the second important thing.

Don't forget to optimize the inter-transistor contact first. Thermal paste
between them and something like shrink tubing around them should do. :-)

/mr

>
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