[sdiy] Filter strangeness.

[harrybissell]

LED matching would be 'better' in series, if the LED illumination is really
proportional
to input current from unit to unit...

what is SURE to be off is the photocell matching.  If you 'need' matching, a
center tapped
cell is the best route... as temperature, illumination, and manufacturing
process are all the
same. Even then it will not be real good.

If you use 'similar' vactrols expect a maybe 100% tolerance on matching. If you
need four matched
you better buy a hundred and sort them into grades.  Even then, matching more
than one point will
be tricky.

This is why more prople have not used Vactrols in gang tuning, except on a lab
basis.

BTW I'd still like to find some folk who would like to go in on a bunch of
Clairex NOS closeout
led/photocells...   I'm not going to pop the $500 min order alone  :^P

H^) harry

synth at oldmail.charlielamm.com wrote:

> This design sounded best to me, and I tried several different variations
> before settling on this one.
>
> I also hand-matched the vactrols until I found two that sounded good.
> Your suggestion may eliminate that need, but, again I have to trust my
> ears above all else.
>
> If you want to do it some other way, have at it. I encourage everyone to
> experiment and tell the group what they think sounds best.
>
> On Thu, 29 Jul 2004, Scott Gravenhorst wrote:
>
> > <synth at oldmail.charlielamm.com> wrote:
> > >> If you wanted a photo resistor to vary cutoff, how
> > >> would you setup the input
> > >> stage?
> > >
> > >You can use opamp, with a transistor to up the current it can
> > >source...examples on my site.  Look at the circuit fragment surrounding
> > >IC1D.
> > >
> > >http://www.charlielamm.com/synth/vactrolvcfsch2.pdf
> >
> > No disrespect intended here, but wouldn't it be better to have the LEDs in
> > series?  Series forces the current to be identical in both LEDs and both
> > are guaranteed to light up, whereas parallel means you could have more
> > current through one than the other, and you could even have one LED that
> > doesn't allow the other one to turn on at all when they are in parallel.
> > It looks like you could have current sharing problems.
> >
> >
> > >Vactrol filter page is here.
> > >http://www.charlielamm.com/synth/vactrolvcf.php3
> > >
> > >There are other ways to do this...eg, check out some of the buchla clones
> > >for instance on Verbos site, buchla is the grand master of using
> > >photoresistors to do clever things.
> > >
> > >http://www.simple-answer.com/DIY.html
> > >
> > >
> >
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