[sdiy] Moog Rogue question...
modular at go2.pl
Tue May 6 08:42:50 CEST 2014
2.7M resistor in series with 300k trimmer from -12 to summing node (2 of
W dniu 2014-05-06 06:01, Pete Hartman pisze:
> Yah, it's an inverting op amp.
> I'll try simulating this setup so I get a better feel for it, but that
> makes sense.
> On Mon, May 5, 2014 at 10:34 PM, <eidorian at aladan.net> wrote:
>> 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).
>> 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.
>>> 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
>>> 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 :)
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