[sdiy] Discrete OTA

[Neil Johnson]

David G Dixon wrote:
> The potting is an annoyance, and for $10, it would be nice to be able to
> fix a bum resistor or something.

Yes, I don't think it is appropriate for something like a synthesizer
where repairability is an important buyer requirement.  Just look at
all the complaints about how small and "unrepairable" surface mount
components are.

My view on potting is that it is done for several reasons (sometimes a
combination):

1/ Robustness - it seals the circuit against a harsh environment, and
provides mechanical support for all components.  Downside is it's a
real pain to do failure analysis on failed units as you have to pick
away at the encapsulant to get to relevant circuit nodes for analysis.

2/ It can also be useful as a construction technique in making small
modules, such as George Hearn's modules, or the aforementioned
discrete op-amp.  No need for expensive custom packaging, you just use
a standard off-the-shelf potting box, a big dollop of goop, and you're
done.

3/ Thermal uniformity - with the right compound you can spread any
generated heat throughout the potted module, so good for
thermally-sensitive circuits, in effect slugging any drift due to
changes in ambient temperature, circuit dissipation, etc.

4/ Keeping it hidden - ARP modules, Yamaha hybrids, Korg 35 filter
module.  A more recent example being the Schippmann VCF-02 filter.

In all cases it is difficult to repair the potted module.
Or as Roman wrote a few posts back:
> potting is dissed here, so watch out ;)

Neil (has potted modules, and done failure analysis on potted units,
and cursed potting compound)
--
http://www.njohnson.co.uk