[sdiy] Music Easel sliders determined to be linear

[Aaron Lanterman]

On Jun 5, 2009, at 1:39 AM, Aaron Lanterman wrote:

> Anyone have any particular knowledge of which pots and sliders on  
> the Music Easel are linear taper and which are audio (log) taper?

I was able to figure this out using the resistor chart for the plug-in  
board near the back of the Easel manual.

They are LINEAR. So if you need to replace a slider on your Easel. now  
you know which one to get.

The Easel uses a 13.5 V source to drive the various CV controls, and  
just connects or disconnects this voltage from the card or the front  
panel controls to turn things on and off.

Using the resistors values in the chart for settings 10, 8, 6, and 4,  
supposing things go to a virtual ground summing node, for the given  
resistor values of 120K, 150K, 200K, 300K, and 620K respectively, I  
get currents of 0.1125 mA, 0.09 mA, 0.0675 mA, 0.045 mA, and 0,021 mA.

The 50K sliders in the Easel are hooked to a couple of 120K resistors;  
the current flowing through one of the 120K resistors essentially  
(with a lot of handwaving) controls the various controlling bits.  
Again, let's assume these go to a virtual ground summing node. (This  
is rarely actually the case but it's a place to work from.) Calling  
one leg of the pot R, and the other 50K - R, we wind up with Ra = (50K  
- R) || 120 K and Rb = 50K || 120K, and then the current through the  
120K resistor going into the Easel boards is

(13.5 V / 120 K) Rb / (Rb + Ra).

When I do this calculation, assuming a linear pot, I get currents of  
0.1125, 0.086, 0.066, 0.046, and 0.026 mA.

Ladies and gentlemen, we have a match!

Whew, I dreaded trying to work out the case of assuming an logarithmic  
pot...

- Aaron

P.S. I will post this finding a couple of other relevant lists.  
Apologies if you get multiple hits.