[sdiy] Music Easel slider analysis, in detail

[Aaron Lanterman]

I posted this to the buchla_modular list, and am reposting it here in  
case there's some interest.

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I redid my equations and scanned them, and also wrote some FreeMat  
(MATLAB equivalent) code to make a plot. I did have the 120K resistor  
in there. Here are my results:

http://users.ece.gatech.edu/~lanterma/buchla/sliders

The upshot is that a 120K shaping resistor is too high to linearize  
the input current if you use logarithmic slider. If you put in a  
linear slider, the 120K shaping resistor counteracts the loading  
effect of the 120K input resistor so you get a line that lies almost  
on top of the line you get using the card resistances (if you plot  
everything vs. "level.") If you leave out the shaping resistor, the  
curve droops below the straight line created by the card resistances.  
I don't think it's a coincidence that both the input and shaping  
resistors are 120K.

When I try to put in a log-pot type curve, everything droops too low.  
I cannot make it match the card resistor values.

If Easels out there have log pots, and more to the point the assembly  
instructions say they have log pots, then of course I can't argue with  
that - but then the resistor values given in the Meta-Programming part  
in the back of the Easel manual don't match up. Or at least, I cannot  
get them to match up.

If anyone can see a flaw in my analysis, or even better has an  
alternative analysis in which a log pot makes sense in this design,  
I'd love to see it.

One worry is that I understand that there can be a lot of variation  
from unit to unit among the same module type. Maybe they built some  
with linear pots and then decided they like the feel of log pots  
better, and didn't bother to change the resistance values in the table  
in the Easel manual. Maybe they ran out of linear pots and switched to  
log pots, and then lowered the value of the shaping resistor to try to  
balance out the log pot, but didn't make that change on the schematic.  
Maybe I've done something drastically wrong. I really have no idea.

- Aaron