[sdiy] vactrol compressor schematics?

[mike ruberto]

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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>
>
> Harry Bissell & Nora Abdullah 4eva
>
>