This post might be a tad long. I am responding to two
comments regarding vactrols:

A) Has Dieter ever fooled around with vactrols

Answer: TRUE (yes)

B) Vactrols are sonically unexciting.

Answer: FALSE (no!!!!!)

I come from a point of confidence on both these as I have
discussed this with Dieter and as well I designed the circuit
modifications for the Cynthia Buchla Low Pass Gate module in
order to improve the response with non-Buchla voltage
configurations. I also have about a billion hours clocked on
Buchla synthesizers.

Item A explained:

Yes, the Doepfer folks have looked into using vactrols. Dieter
has even experimented with making his own. This in itself ain't
all that hard, as a vactrol is nothing but an led and a photo
resistor positioned so the light path is pointing to the business
end of the photo resistor, and the whole mess potted into a
plastic enclosure so that ambient light does not effect the
resistor. Usually this results in this 'thing' that has two led leads
sticking out of one end, and the resistor leads out the other.

How a vactrol works is the brighter the light, the lower the
resistance of the photo resistor. There are few different types,
each having it's pwn characteristics: delcay time, on/off
resistance values, dual or single (some are confiugured as
pre-wired voltage dividers consisting of two photo sensitive
resistors in series).

So there's that.

B explained:

OK, I understand that this comment was an opinion and therefor,
completely acceptable. But given that everyone is due their own
opinion, here is mine:

Vactrols are friggin' wonderful in synthesis applications.

Will they give you the snap response of a non-vactrol circuit
equivelent NO. Are they the best thing to have in a VCA which
(most engineers wil tell you) have to be fast to be good NO.

In analogy: Is the Wasp filter a good design NO. It's a horrbile
design by engineering standards, it's a mistake - but that
mistake sounds good.

Such is the case with Vacs as well, although they were
intentionally introduced by Don for the purpose of noise isolation.
Remember, the Buchla signal level is just about line level. In
order for it to be heard, it has to be turned up like made and this
is where noise can cause you grief.

Someone also mentioned they were in a Buch to keep the
control voltages isolated form the singal path, this is 99.9% true
in all cases accept (to my knowledge) the envelope detector in
which the control voltage generated from the external signal is
not isolated from the signal path electircally.

Remember, not only is the Buchla signals and controls
separated by two distinct patch cord systems, they run off of
completely different supply systems and different voltages in
some cases. While the signal paths is fed with 15 volts, some
of the control modules are powered off of 24.

Let take the 292 Low Pass gate as an example. It is, by nature
and due to the vactrols, slower than shit. Most design engineers
would tell you that this makes for a very bad VCA design which
are supposed to be fast to be efficient. Due to the vactrol's
naturally slow decay time (35 MS), and the fact that the main
mode of operation of a 292 is a low pass filter, vactrols afford a
unique ringing of the low frequency spectra of a signal which
provides the percussive phrases with a lot of life. I will post a
link by day's end of a comparison of the same signal gated by a
standard VCA and by a lowpass gate - you'll know what I am
talking about in a second.

Why is the decay so long in a vactrol It's the light. Turn an led
on. Watch it snap to attention. No turn it off. Ah-HA - it fades out
even if the signal going into it cuts instantly.

So no, in my opinion, the while use of vactrols in analog
synthesizers provides a marketably different result than
functionally identical circuits not using them, they are no worse,
just different and in some cases, that makes for a pleasent
results.

VIVA LA DIFFERENCE!

Look for a posting with a link to sonic exmaples of the 292 in
action soon.

best,

Peter






--- In Doepfer_a100@yahoogroups.com , "klamb000"
<lamb@c...> wrote:

> Actually, I think it was said that the Buchla designs weren't as
> sonically pleasing as other designs.
>
> And I also wonder if any vactrol modules are planned for next

year.

>
> Regards,
> Kevin
>
> --- In Doepfer_a100@yahoogroups.com , "ethanzer0"

<ethanzer0@y...>

> wrote:
> > A while back there was some discussion
> > on vactrol based modules. I believe
> > the thoughts were that vactrols in the
> > audio path are not that sonically exciting.
> > Looking at buchla schematics I find that
> > vactrols are used quite often to isolate
> > the control path from the audio path
> > (optoisolator ) I understand the ultra-smooth
> > response of vactrols make a modules response
> > to CV something special. I wonder if the
> > design folks over at doepfer have considered
> > this
> >
> > Ethan