[sdiy] So-called Vactrol "ringing" [Was: Re: VTL vs. H11F]

[lanterma at ece.gatech.edu]

On Jul 2, 2011, at 4:54 PM, Tim Stinchcombe wrote:

>> lanterma at ece.gatech.edu wrote:
>>> IIRC it didn't do that vactrol ringing thing - it responded much faster. So it was definely its own thing.
>> 
>> Do vactrols "ring"?  What part do they store the energy?  
> 
> Ah, one of my pet peeves! Short answer: No. I regard this as a misuse/confusion of terminology

....

> These of course are fabled for producing the famed 'Buchla bongo' sounds, which as far as I can tell arises because the lowpass filter closes slowly due to the lag in the vactrol's response, giving the characteristic 'hollow'/near-clangorous metallic-type sounds reminiscent of ring mods etc., hence the reason (I suppose) for calling it 'ringing'.

....

> (And I apologize in advance to Aaron, I don't mean to single you out!

Hah, no worries. ;)

You are of course correct; I've generally use the term "ring," when talking about Vactrols, in the generic sense of the decay of a "ringing bell" or something like that. It's not "ringing" in the sense of a sinusoidal response of a second-order filter, as might be exploited in a analog drum machine or something. Perhaps some word other than "ring," when talking about Vactrols, would be more appropriate.

One thing I've found is that the slew of the vactrol is direction-dependent; it changes whether you're going up or down. In particular, it's much faster going up than going down, which is why you get the so-called "ring" but not a "swell."

It's funny this exact issue came up with a senior design team that I was informally advising - they were using a LPG with the vactrols driven by a PWM output from a microcontroller, as they talked about the ringing, and their formal advisors asked about whether the "ring" came from parasitic capacitance in the vactrol. Luckily the students got it right, and explained it wasn't a capacitance effect - but I could totally see how one could make that mistake.

- Aaron