[sdiy] Music Easel balanced mod, one mystery solved, others remain

[Aaron Lanterman]

A bit of an update on the notes below...

What I was reading as a 7.5K resistor I now believe is really a 1.5K  
resistor with smudge. ;)

If I make them both 1.5K, the strange high pitched scrachy sound goes  
away, even if I let N be ground. (Interestingly I can set that N to  
-15 V instead of ground, or anywhere in between, and it doesn't seem  
to change the sound at all.)

So I went on to wire up the wet/dry mixer, which consists of two  
vactrols at the top of the page. It does work, but the control range  
seems awfully small - 0 V gives dry, and something like 0.5 volts  
gives fully wet (ring mod), and something like 0.25 volts gives  
something about in between. I started stringing 10Ks in series at the  
Mod index input, and I found changing that 1K at the input to around  
40K created a control law that seemed to run 0 to 5 volts with 2.5  
volts being an even mix. Of course there's a possibility I wired up  
something else in the circuit wrong, but I've checked it over a few  
times.

Any Easel owners out there (or Easel knowledgeable people) who know  
what the control voltage range going into the Mod Index In is supposed  
to be? I've tried to track it down using the various Easel schematics,  
but I'm finding it hard to follow once I get to the how-boards-are- 
wired stage. (Someone told me it should be something like 0-5 V, but  
that's not consistent with the behavior I have on the breadboard, so I  
wanted to double check...)

Of course, I should also actually sit down and analyze that LED driver  
circuit...

- Aaron

On Nov 13, 2008, at 2:39 AM, Aaron Lanterman wrote:

> The balanced modulator is on the left part of the page. You'll see  
> the four-vactrol structure. The "lowest" vactrol LED is hooked to a  
> 1.5K which then goes to an N in a circle. I assume this is a "noisy  
> ground."

> But, when I connect it to ground, I get sort of a high pitched whine  
> superimposed on everything, and interestingly, I get the same effect  
> when I just leave it floating.
>
> But, if I ignore that "N" and instead hook the 1.5K to the -15V, the  
> whine goes away and it sounds beautiful.

...

> And actually, while we're at it, I read the resistor R22 (the  
> equivalent at the top of the ladder) as 7.5K, which seems odd. Maybe  
> I misread it. Should this be 1.5K instead so it's symmetric with the  
> other 1.5K? Any insights?