# Buchla Music Easel Slider Analysis

There’s some confusion over whether the sliders in the Buchla Music Easel

should be linear or logarithmic.

Mark Verbos said that the ones in the Easels he’s seen have been logarithmic,

and the assembly instructions say they are logarithmic. Luther says the

ones his his Easel are linear.

The sliders in the Easel have their tops hooked to +13.5 V (which I’m

generically calling V_EN in the notes below). The wiper is connected

to the modules through a 120K resistor, which I’m generically calling R_in,

and the wiper is also connected to the V_EN supply, which I’m calling R_sh

(for “shaping”). I am calling these R_card.

The Easel also has a plug in card that the user puts resistors into. The

resistors connect the V_EN supply to the module inputs.

The Easel sliders have “1” through “10” settings, and the back of the Easel

manual lists card resistor values that correspond with these setting.

One clue as to the designer’s intent is that the “1” thourgh “10” level

settings are specifically called “Conductance Values” in the table in the Easel

manual. Indeed, if you plot the level settings vs. reciprocal resistance,

you get a straight line (slighly off in a few places, I assume due to

rounding desired resistances to nearest easily available resistance values).

Assuming the card input is held at a fixed voltage

via an op amp feedback loop, this means the input current is a linear function

of the level setting (the blue line below, assuming the input is at ground.)

Again assuming an input held at ground, a linear slider (assuming equally

spaced tick marks) gives us the red line, which centers nicely on the blue

line – in fact, at level setting 5, they are identical. Higher levels dip

a little lower, and lower levels rise a little higher.

If we set R_sh to a “near infinite” value, essentially taking it out of the

circuit, we get the green line, which consistently dips below the blue line.

From this we can suppose that R_sh is intended to balance the loading

of R_in.

Here’s my handwritten analysis:

page 1,

page 2.

Here is my FreeMat code

(should work in MATLAB and Octave) used to make these plots. You’ll notice

there’s some other experiments I commented out. I couldn’t find a particular

equation relating travel to resistance for logarithmic pots, so I guessed one

using a base I could vary. Any log pot I put in created a line that drooped

well below the blue line. It might be possible to counteract some of

the droop by lowering the value of R_sh, but I didn’t try that. I also tried

to read pixel locations off a photograph of the Easel front panel to take

into account that the pixels on the front panel aren’t evenly spaced. This

resulted in a curve that was a little higher than the red line at the lower end

and a little lower at the upper end. I suspect maybe the original sliders,

if they are linear, aren’t quite as linear near the ends, so maybe the varying

tick marks account for that. In any case, I cannot counteract the effect

of using a log slider by using the tick marks.

The upshot is that if the assembly manual says to use logarithmic sliders,

I can’t argue with that, but I cannot reconcile that with the resistor values

given in the Easel manual – which fit so nicely assuming a linear pot.

If anyone can provide an analysis in which log pots make sense, I’d love

to see it!

Aaron Lanterman

Last updated 6/3/09