[sdiy] Reverse engineering a analog synth module?

[harrybissell]

Hiya Bret

The components are there either because they are needed, or the designer is
stupid...
or they are missing because they are not needed or the designer is stupid....

Usually I assume that either the designer is stupid (not) or that I am missing
the
elegence of the design.

There is a good article by Bob Pease about reliability (MTBF) in one of his
"Pease Porridge" columns... where he explains that even though ADDING
components seems to make reliability go down... deleting the wrong components
can make the reliability go down REALLY really fast, and hard.

Like the series resistor on the outputs... you 'might' delete that... but (since
you
mentioned Doepfer) if you are using minijacks... the tip WILL short to ground
every time you plug them in... and what if you screwed up and left one halfway
in
and did not notice... the opamp 'might' be 'short circuit protected' but its
sure to
get hot, maybe damage itself eventually, maybe overlod the power supply... etc.

A lot of the unexplained components have to do with these conditions... A TTL
input
is good for 0-5VDC - negative voltages are death. SO you need to protect them...

A series resistor... maybe OK but if you plug in 12V (and it limits the current
/ voltage
to a safe level) it will lose a lot of performance at 5V.  So a series resistor
and a pair
of diodes to +5 and ground... OK... but in the case of losing the power supply
to the
TTL... you might actually power the whole circuit off the input.  If you are
using CMOS
it could be even trickier (oohh I know!!!)  Maybe a series resistor and a zener
diode...
thats my favorite, because it can't backfeed the power supply... but then you
have to worry
about the ratings of the resistor and zener... and just how much were wou
willing to protect?

I had a customer who had a circuit that would run on 24VDC (low current) but
would...during
a not uncommon failure... get hooked up to 650VDC.  He asked why we couldn't
protect
against that. I replied "we could, and I'll quote you, but its gonna cost like
$50 more, each...
maybe its cheaper NOT to put those components in and suffer a failure once in a
while)

If you have some specific schematic that you like to analyze as a learning
experience... send
me one and we'll chat...

... just DON'T make it a Bob Moog schematic.  Bob is NOT a stupid designer...
and you will
play hell trying to figure out what he's doing... Often he makes one component
serve
several simultaneous functions... that's what I meant by 'elegent'  :^P

H^) harry

Bret Truchan wrote:

> Hello everyone!
>
> I'm sorry to post so many questions.  You have all spoiled me with amazingly
> informative answers. :-)  Here's an interesting one for 'ya!!
>
> I've been very happy and successful in building a few modules (for analog
> modular gear) so far.  My designs tend to be *very* simple.  I'm also
> focused on modules that deal with logic and microcontrollers [gate stuff].
> However, when I peer into my Doepfer system, even the most simple modules
> have more components than mine.  Which makes me wonder.... what's all that
> stuff for?!?   And, um, am I missing anything? The dual logic module comes
> to mind.  It looks like there's a microcontroller in there, and some
> resistor networks, but there's a plethora of other components that baffle
> me.
>
> Can anyone explain what a typical TTL based analog module may have for basic
> components?  For example:
>
> 1. There are probably two or more capacitors for reducing noise on the power
> lines.  One of these will be located near the power input, the others near
> the vcc pins on the ICs.
>
> 2. There are probably going to be resistors to limit the current on the
> input and output jacks.  These may be in the form of resistor networks.
>
> 3. If you see zener diodes, they may be being used to limit voltage to a
> certain level.
>
> Thanks a ton!  I don't suspect that anyone will *really* go through the
> gauntlet, but it sure would be sweet if you do!!  Schematics are nice, but
> usually when I see them, I'm like a little kid saying "why? why? why?" :-)
>
> Cheers to you all.
> - Bret
>
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