[sdiy] Moog Rogue question...

[eidorian at aladan.net]

Oops, forgot to answer your second question: no, I don't think you need 
it.  Presumably the summing op-amp is an inverter so you will need a 
positive voltage to offset the F down to the C below it (that's what I'd 
do, anyway).

Cheers,
A.


On 2014-05-05 20:11, Pete Hartman wrote:
> I'm working on a Moog Rogue...  One thing that bothers the owner is
> that the Rogue's voltage range is 'rooted' at F -- the lowest key on
> the keybed is F and that's 0V -- but most CV sources are 'rooted' at 
> C
> (for example both of the MIDI-CV modules I have generate 0/1/2/3/4/5V
> at C0, C1, C2, C3, etc on the keyboard).  This can be worked around
> with the pitch wheel and tune knob in concert, but then you can't
> really use the pitch wheel for anything else if you want things to
> stay in tune.
>
> My thought is that it ought to be feasible to add another voltage
> source that is switched between 0V and whatever is necessary to 
> offset
> things so that the incoming voltage maps to an actual "C" for the
> Rogue on the even 1V boundaries.  I expect that something with a
> trimmer that matches the pitch wheel's input to the CV summing op amp
> ought to be good.  Switch the "input" from 0V for no effect to the
> trimmer for the correct offset.  (obviously I'll be testing this
> theory with clips before taking an iron to it).
>
> However, there's a detail in that section whose purpose I'm unsure 
> about.
>
> http://elmegil.dynathome.net/~elmegil/rogue-pitch.png
>
> The pitch wheel is a simple +12V / -12V divider that goes through
> parallel diodes pointing in opposite directions, then through a
> resistor into the summing node.  I don't understand what the diodes
> are intended to accomplish, and I'm unsure whether the specific type
> is important or not.  It's hard to read in the screenshot, but they
> are FDH333--available inexpensively, but with some delay and shipping
> expense.
>
> So the questions are:
>
> 1) what does this diode configuration accomplish?
>
> 2) Given whatever that purpose is, is the type of diode important?
> This isn't a common signal diode, the datasheet describes it as "high
> contraction, low leakage".  So low leakage is obvious enough, but I'm
> not clear on what "high contraction" is.
>
> Thanks for any insight :)
>
> Pete
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