Need some help with tweaking CGS Synthacon VCF
2005-06-14 by Adam Schabtach
I have a CGS Synthacon VCF assembled and running. I'm trying to make what I
think is a simple modification, and tweak it in the manner described under
"Notes from John Loffink". I've realized that I've gotten to a point at
which I might end up chasing my tail in circles, substituting resistors and
never ending up quite where I'd like to be.
The instructions and schematic show a pot for controlling the frequency, and
a CV input jack. I would like to add a second CV input with a pot for
attenuating its signal. The idea is to use the unattenuated input jack for
keyboard tracking at 1V/oct, and the attenuated input for adding contour
with an envelope generator. I'm not fussy about the tracking being exactly
1V/oct, but something close to that would be nice.
John's notes suggest that he took a similar approach. I have a couple of
questions, which are largely directed at John himself, but perhaps someone
else knows the answers.
The first question concerns the statement "I also added 220K of series
resistance to the FREQ knob." Does this mean in series with the resistive
element of the pot, that is, between the pot and +15V? Or does this mean in
series with the pot's wiper, that is, between the wiper and the CV pad on
the PC board?
The second question is, how should I add another pot to create the
attenuated input? Should I just reproduce the FREQ pot and circuit shown on
the schematic, but connect the "upper" end of the pot's element to the (new)
input jack, rather than to +15V? Does the base of the transistor act as a
summing node? Would I be better off using a simple op-amp summer to add the
three control voltages and apply their sum to the base of the transistor
instead?
Thanks in advance for any help. This filter sounds really great--I love it
already.
--Adam
--
Adam Schabtach
www.studionebula.com
think is a simple modification, and tweak it in the manner described under
"Notes from John Loffink". I've realized that I've gotten to a point at
which I might end up chasing my tail in circles, substituting resistors and
never ending up quite where I'd like to be.
The instructions and schematic show a pot for controlling the frequency, and
a CV input jack. I would like to add a second CV input with a pot for
attenuating its signal. The idea is to use the unattenuated input jack for
keyboard tracking at 1V/oct, and the attenuated input for adding contour
with an envelope generator. I'm not fussy about the tracking being exactly
1V/oct, but something close to that would be nice.
John's notes suggest that he took a similar approach. I have a couple of
questions, which are largely directed at John himself, but perhaps someone
else knows the answers.
The first question concerns the statement "I also added 220K of series
resistance to the FREQ knob." Does this mean in series with the resistive
element of the pot, that is, between the pot and +15V? Or does this mean in
series with the pot's wiper, that is, between the wiper and the CV pad on
the PC board?
The second question is, how should I add another pot to create the
attenuated input? Should I just reproduce the FREQ pot and circuit shown on
the schematic, but connect the "upper" end of the pot's element to the (new)
input jack, rather than to +15V? Does the base of the transistor act as a
summing node? Would I be better off using a simple op-amp summer to add the
three control voltages and apply their sum to the base of the transistor
instead?
Thanks in advance for any help. This filter sounds really great--I love it
already.
--Adam
--
Adam Schabtach
www.studionebula.com