On Fri, 22 Feb 2008, korgpolyex800 wrote:
> The Moog Slayer mod seems to be picking up pin 7 of the op amp (IC4)
> that is being driven by DCO1 output. This I believe to be the square
> wave output of DCO1.
=============
the FM-800 mod...
> Did you ever try to use pin 1 (DCO1 triangle wave)?
==============
IIRC, i tried that but the results were better (more harsh and extreme)
taking a signal from pin 7. with this type of effect the goal is to use a
modulating signal with the most harmonics; square wave vs triangle, square
wave wins ;)
> You are feeding that DCO1 wave into VCF LIN (pin 13). I suspect this is
> a linear control line for the VCF cutoff.
===============
i believe that's correct.
> How did you arrive at the 5K POT and 1K fixed resistor values? Was it
> trial and error or what method did you use to arrive at those values?
===============
theory: the 5K pot is acting as a voltage divider so in theory anything
5-100k should work fine. the 1k resistor provides isolation from the knob
"grounding out" pin 13 of IC1, and shouldn't be too critical.
practice: the combination of 5k linear pot and 1k resister is the "sweet
spot" where the range adjusts very smoothly from no effect to full effect
(full effect being about what you'd get if you shorted IC4 pin 7 to IC1
pin 13). also it has little/no influence on the filter settings when the
knob is turned all the way down.
IIRC, i went through a lot more experimenting than i'd expected before i
got that combination, and everything else had severe nonlinearities when
adjusting the pot through its range, didn't cover the full range and/or
adversely affected the range of the filter when the knob was fully CCW. it
also surprised me that a linear pot gave the best response.
> The reason I ask is because the digital pot I am considering using is a
> 10K and so I may need to rework the bias resistors. Any light you can
> shed on this would be much appreciated.
==============
that's a tough one... would bias resisters affect the "curve" of the pot?
maybe two 10k linear digital pots in parallel??
you may be able to experiment and find a replacement for the 1k resistor
that works with a 10k pot... since the resistive/reactive impedance of IC1
pin 13 is unknown, i would speculate that experimentation would be far
more productive than calculating.
another option would be this... have an op-amp with the gain controlled
either digitally or via your digital pot; the input of the op-amp comes
from IC4 pin 7; the output of the op-amp pass through a 1k resistor on
it's way to IC1 pin 13. you should be able to find a sweet-spot where this
gives the same effect as the original mod... and... with an amplifier in
the circuit you could even go beyond the range of the original mod ;)
ascii art (may not survive yahoo):
gain
control
|
___ | _______
IC4| |\| |IC1
pin7|---| >--\/\/\/--|pin13
| |/ 1k |
--
...atom
________________________
http://atom.smasher.org/ 762A 3B98 A3C3 96C9 C6B7 582A B88D 52E4 D9F5 7808
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