[sdiy] Re: Music Easel balanced modulator on the breadboard

Aaron Lanterman lanterma at ece.gatech.edu
Thu Nov 13 09:05:58 CET 2008

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

Huh. It works. It works like a ring mod is supposed to work, one I  
carefully tweaked the little balancing trimmer. I don't think it  
sounds particularly different than any other ring mod, but then I  
haven't spent a lot of time trying out different ring mods.

It's a strange contraption with four Vactrols. Why do it this way, I  
wonder? Vactrols are expensive, and I know Buchla had many other ring  
mod designs. Back In The Day, were Vactrols cheaper in such a way that  
this was cost effective relative to a transistor-based design of some  

One thing that's odd though is I'm unsure about one of the voltage  
notations. See here:


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  
scratchy 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.

Any insights into what's going on here?

My next task is to wire up the stuff to the right, which appears to  
offer voltage-controlled mixing of the original signal and the  
modulated output.

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?

I've posted about this circuit before, and there was some interesting  
follow-on discussion, but didn't get around to breadboarding it until  
just now.

- Aaron

P.S. I should also note that I used TL07x op amps instead of the  
original 4-something-something op amps.

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