[sdiy] Ultrarough draft of my hot-rodding of the Music Easel ring mod

[Aaron Lanterman]

On Nov 27, 2008, at 10:38 PM, Dave Manley wrote:

> 1. Perhaps obvious: Buchla always uses two grounds - one labeled N,  
> for the Vactrol's LED return current, and a second, labeled Q, for  
> the signal path.   I assume these labels stand for "noisy" and  
> "quiet".  You may have already been planning to do something with  
> the grounds, but it isn't documented on the schematics.

I was originally planning on using a common ground for everything,  
since ultimately I was planning on using this in a modular in which  
there wasn't the same control/signal separation that Buchla uses. But  
if I do want to retain full-as-much-as-feasible "backward  
compatibility" with the original Easel, I should separate them.

> 2. The feedback path that has the VT5 and VT6 LEDs is missing a  
> resistor 33K resistor (R33 in the original).

Good catch! Thanks, I just fixed it.

> 3. Not sure about this one: A common resistor, R11, is used  
> connected to the + input of IC1B, IC2B, and IC2A.  R31 is used in a  
> similar manner on IC3A, IC3B and IC4A.  I don't think you want to do  
> this as it becomes a summing point for the bias/offset currents of  
> the connected opamps.  Don't you really want separate resistors for  
> all of these?  I defer to the analog masters on sdiy :-)

I was wondering about that too, actually. I did it solely because  
Buchla did it - but I have no idea if it's a good idea or not! I don't  
know if he was trying to save pennies on resistors, save space, or if  
there was a good reason to do it...

- Aaron