[sdiy] vactrol compressor schematics?

[Thomas Hudson]

The Craig Anderton compressor in EPFM didn't use a precision  
rectifier, just fed the signal into the LED, but this was using the  
CLM6000.

I know the discrete photo resistors like those used in the univibe  
have very slow fall times. I picked up a bunch of the "assorted photo  
resistor" packs at Radio Shack hoping to find four that I could match  
for a univibe clone. They were all over the place, but you might be  
able to hand select one that would work. As long as you're willing to  
live with the fact you'll never be able to find a second one that  
will behave the same, no matter how many packs you buy. :-)

  Tomy


On Oct 26, 2007, at 1:23 PM, mike ruberto wrote:

> I noticed this in the datasheet:
>
> "Speed is limited by the response
> time of the photocell. With rise and fall times on the order of 2.5 to
> 1500 msec, most AOIs have bandwidths between 1 Hz and 200 Hz."
>
> I'm thinking a little passive filter should easily deal with any
> ripple on the output.
>
> On 10/26/07, harrybissell at wowway.com <harrybissell at wowway.com> wrote:
>> Even with a tiny amount of ripple in the LED... the result will
>> couple directly into your audio path. The response of the cell needs
>> to be fast enough to be useful... at that point the rejection of
>> CV by the filtering will not be good enough for critical audio.
>>
>> For monophonic instruments, it might well be good enough and the
>> distortion will add ~character~... in other cases it adds IM
>> distortion. (essentially its an AM modulator)
>>
>> The ripple is almost sure to be a sawtooth if you use the cell
>> as the filter... the rise time is usually MUCH faster than the fall
>> time.
>>
>> H^) harry (its a boy) bissell
>>
>>
>> On Thu, 25 Oct 2007 21:17:36 -0400, mike ruberto wrote
>>> Well, I have spent a few hours studying various compressor designs -
>>> some good, some bad. I see that in the better designs more serious
>>> effort is put into the sidechain circuits. RMS converters and
>>> precision rectifiers of varying complexity and trickyness in most of
>>> the better circuits. However, these are mostly designed for devices
>>> using a VCA or FET as the gain varying element.
>>> Even the "What" circuit has a significant amount of circuitry to
>>> process the CV. It seems to me that all of this is rather useless  
>>> and
>>> unnecessary when using a vactrol for the gain element. Due to the
>>> sluggish response of the CDS cell in the vactrol any pre-processing
>>> of the CV is going to be compounded and deformed by the response
>>> curve of the CDS cell. This seems to defeat the purpose of using
>>> elaborate RMS converters and such.
>>>
>>> So my question is, why not use another Vactrol in the sidechain
>>> circuit? Get rid of the precision rectifiers and let the slow CDS
>>> response curve act as the "rectification". By biasing the the LED
>>> drive to a certain range it might be possible to get an RMS voltage
>>> and rectification at the same time.
>>>
>>> Anybody have any thoughts on this? See any glaring problems with the
>>> idea?
>>>
>>> M. A. Ruberto
>>>
>>> On 10/25/07, mike ruberto <somnium7 at gmail.com> wrote:
>>>> This is great! Thank you Dave!
>>>>
>>>> On 10/24/07, Dave Manley <dlmanley at sonic.net> wrote:
>>>>> mike ruberto wrote:
>>>>>> I have a few extra VTL5C3/2 vactrols and thought it would be an
>>>>>> interesting project to build an opto-compressor with them.  
>>>>>> Anybody
>>>>>> know of any schematic I might use for a starting point?
>>>>>>
>>>>>
>>>>> There is the "What Compressor":
>>>>>
>>>>> http://dt.prohosting.com/hacks/what.html
>>>>>
>>>>> There are two different circuits on that page, the "Joe Cheep
>>>>> Compressor" and the "What" near the bottom.
>>>>>
>>>>> -dave
>>>>>
>>>>>
>>>>
>>> _______________________________________________
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>>> Synth-diy at dropmix.xs4all.nl
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>>
>>
>> Harry Bissell & Nora Abdullah 4eva
>>
>>
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