[sdiy] Music Easel Balanced Modulator control range mystery solved

[Aaron Lanterman]

A few weeks ago I posted about breadboarding the Balanced Modulator  
from the Music Easel:

http://rubidium.dyndns.org/~magnus/synths/companies/buchla/Buchla_2080_5_200.jpg

I lamented that with a 1K resistor at the Mod Index In point, I only  
got a useful control range from 0-0.3 volts.

The take home message here is that if you want to build one of the  
Easel subcircuits, you can't just look at the individual pages with  
the individual boards.
You also have to look here:

http://rubidium.dyndns.org/~magnus/synths/companies/buchla/Buchla_2080_13A_200.jpg
http://rubidium.dyndns.org/~magnus/synths/companies/buchla/Buchla_2080_13B_200.jpg
http://rubidium.dyndns.org/~magnus/synths/companies/buchla/Buchla_2080_14A_200.jpg
http://rubidium.dyndns.org/~magnus/synths/companies/buchla/Buchla_2080_14B_200.jpg

and check out the various resistors and pots that are hooked to the  
boards to make sense of things.

Most Easel board inputs seem to have a structure with two 120K  
resistors and a 50K pot, as well as a connection for the programming  
board. Note also a 120K resistor represents "full on" according to the  
the Easel user's manual.

In the case of the balanced modulator, the circuit driving the vactrol  
really has a bunch of 120K resistors forming a current summing node  
for the various control voltages. The 1K resistor that is there just  
look like insurance in case some nefarious voltage gets accidentally  
connected to the input.

When I run the calculations based on my old fashioned pencil and paper  
analysis, and use 120K for the input resistor instead of 1K, I compute  
the currents driving the Vactrol LEDs as

i_dry = (7.84 volts - 0.5667 vcontrol) / 2.87 kohm

i_wet = (-0.057 volts + 0.5667 vcontrol) / 3 kohm

(yeah, I know I'm kind of random about significant digits)

Basically, to get i_wet = 0 (essentially infinite R_wet, meaning a  
pure dry signal,
you use vcontrol = 0.

And if you want i_dry = 0 (essentially infinite R_dry, meaning a pure  
wet signal), you can compute that you want vcontrol = 13.8.

And you get i_dry = i_wet, and even mix, around 7.1 volts.

And it just so happens that the various control pots on the Easel seem  
to have their tops hooked to 13.5 volts (hence the conjecture in my  
previous e-mail.) The key ingredient, that I've spent all day looking  
for, is the weird 3-way switch at the lower right of board 3 in the  
big block diagram. It seems that in one seeting it hooks 13.5 to "EN  
A" which seems to mean "enable the front panel controls," and leaves  
"EN B" disconnected; in the other extreme, it hooks 13.5 to EN B, and  
leave EN A disconnected, which seems to mean "put 13.5 to the OFFSET  
line of the card," activating the card; and the middle input seems to  
put 13.5 to both, so you get a sum of card and front panel effects.

In any case, however its set, the banana jack inputs seem to always be  
active; as far as I can tell from the schematic there's no mechanism  
by which plugging in a card deactivates those.

- Aaron