Vactrols (was: New Stuff)

[Haible Juergen]

	>As the discussion touches vactrols, I have a question regarding
these
	>buggers: what are the original ones ?
	>
	>I suspect it is not just a combination of a LED and a LDR (CdS?),
but
	>a some kind of special photocells.

	>Is there something really special in these semiconductors ? Is it
possible
	>to achieve similar results using a LED+LDR ?

Vactrols *are* just a LED + LDR.
I've occasionally made my own ones, gluing parts together, painting it with
black colour, painting it again because there were some spots where the
colour was too thin, covering it with black heat shrink tube ... you get the
idea.

Buying a Vactrol means that the manufacturer has done the potting, and
probably
selecting, for you. Dual vactrols are what you need for certain filters, you
can buy
them as center tapped LDR versions. (Don't ask me where to buy - I got all
of
mine from friends, not from a shop.)

As for CdS, or CdSe, or mixtures of both materials, each has its specific
properties (rise time, impedance range, spectral sensitivity ...).

	> I remember there were some points
	>risen (JH?) about their sonic superiority in bandpass/formant
filters
	>(vide KORG PS3300, MAM Resonator).

There have been some attempts (by myself and others) to track down the
"sonic superiority" to certain parameters, but in the end I think it's one
of
these cases where the sum is greater than its parts, and a well-balanced
circuit has some pleasant over-all performance. Change something of
importance,
and you loose the "charm".

As far as I know, both the MAM an d the MOTM versions have the "charm" of
the original, while VCA-based resonators are "cold" and sterile in
comparison.

Sorry that I can't be more technical at this point. 
(Can anybody explain why these old Vactrol-based compressors have such a 
pleasant sound ?)

JH.