[sdiy] Vactrols subbed for dual-gang pot

[Scott Nordlund]

> well, given that it's 'sort of' like a low-pass filter, and feedback
> around a low pass filter adds resonance..

You're missing the point of the feedback for starters.  The vactrol
in the feedback loop will improve the response of the one controlling 
the audio.  It helps the output track the input.  It's completely 
different from adding resonance to a VCF.  Feedback is used all the 
time (like in op amps), you can't just automatically conclude the 
result is resonance.  Outside of synthesizers, often the goal is to
reduce "resonance" (only here it's usually referred to as overshoot
or ringing).

It's likely dominated by a 1 pole lowpass response, which won't
resonate anyway.

> From what I understand, technically, this whole network is then a
> single LPF with a higher order, but I'm probably wrong on that, since
> it's just intuitive understanding..

I'd guess it's a sort of slew on rising inputs and a 1 pole exponential
decay on falling inputs.  The reason is that incident light creates
charge carriers in the CdS material that permit conduction.  But these 
don't disappear immediately when light is removed.  This is probably
also dependent on the voltage across the photoresistor.  The response
of the LED is also nonlinear and the negative feedback helps with that
too.

Compared to many systems that are controlled by negative feedback, a 
vactrol is quite a crude and idiosyncratic device.  The feedback 
doesn't fix it completely, but makes it a little less bad.  I don't 
think people usually worry about precise characterization, the goal 
is just to make it a bit more usable.
 		 	   		  
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