[sdiy] LDR optocoupler blowout

[mark verbos]

The Buchla 292c has a .002uF cap and 470k resistor in parallel with the regular 100k resistor from the CV in to the control summing amp. I guess it speeds up the response a bit, of course that spits in the eye of all the love of Vactrol slowness. ;)
It also should make a 292c sound different from a 292b that doesn't have the compensation, but I haven't noticed.

Mark



On Sep 28, 2012, at 11:43 AM, John Speth wrote:

>>>> With the slow response of LDRs and the regular means of controlling
> LED
>> brightness being PWM, I wonder if these could be used as a digitally
>> controlled resistor, assuming that one clocked the PWM comfortably above
> the
>> response time of the LDR. Just a thought.
>> 
>> Some senior design students I was informally advising did exactly that,
> using
>> PWM to control the Vactrols in a Buchla-style lowpass gate circuit. It
> worked
>> pretty well IIRC.
> 
> These two comments got me to thinking: I wonder if one could use a speedup
> circuit to compensate for slow LDR response by driving it harder for quick
> changes and less for slow changes.
> 
> At my professional job we had to speed up output response of thermocouples
> measuring the heating of a thermal mass from the instantaneous turn on of
> a laser.  Since the heating equation was linear we applied a simple high
> pass RC circuit at the output to compensate for the slow response from the
> thermocouples.  R and C where chosen to match the required response from
> the mass thermal time constant.  Later on we used a digital filter to
> compensate but the theory was the same.  We also applied the theory to
> speeding up analog panel meters with slow time response.
> 
> Is that feasible or am I ignoring some fundamental fact that makes this
> impossible?
> 
> JJS
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