New Wiard Module: Borg "Assimilator" Filters

[Grant Richter]

Hi All,

I might as well kick off the flood of new products
with the announcement of the Borg (Buchla-Korg) "Assimilator" filter
module.
This is the Vactrol design of the Buchla 292 low-pass gate
with pannable mode response, regeneration and exponential
control voltage response. It has low, band and high pass response
continuously pannable in each section, with resonance up
to self oscillation available through out the entire pan.

The design came about because I had a dream where
Don Buchla was explaining to me at a party that the
292 WAS a Korg MS-20 filter (That is a Sallen-Key).

The Vactrols also have slew rate limited control voltage 
response that gives everything a very acoustic tone
character and that HUGE Buchla 200 sound.
I have prototyped 9 different filter designs so far using
Vactrols and have built state variables, four pole low
and high pass, Sallen-Keys and comb filters.

I have also built VCOs and VCAs with them,
but the VCOs don't calibrate too well and the
VCAs are "soft" sounding. But the filters are bitchin.

There are three features that are nice with the Vactrols.

1. The control path is optically isolated from the audio path.
That means it is impossible to get control voltage feed through.

2. The control path is slew rate limited. The Vactrols will
slew about three decades in five milliseconds. This produces
a noticeable, but fortunately pleasing, artifact in the processed sound.
It rounds the envelope attacks and gives an acoustic character
to notes, while still percussive, their more marimba like than metalophone.

3. The LDR is just a resistor and so does not contribute any
additional noise. Built with discrete FETs and the LM837
op-amp the designs are quiet enough to use in line
with a guitar or microphone. It has become apparent to
me that fewer parts = less noise and I have been concentrating
on designs that have the fewest possible number of parts in
the audio path.

Grant Richter
http://www.wiard.com