[sdiy] Moog Rogue question...

[Pete Hartman]

What advantage does that configuration have over a 10k trimmer wired
as +12 / -12 divider, with the wiper in series to 620K to the summing
node?



On Tue, May 6, 2014 at 1:42 AM, Roman Sowa <modular at go2.pl> wrote:
> 2.7M resistor in series with 300k trimmer from -12 to summing node (2 of
> U2A).
>
> Roman
>
> 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.
>>
>> Thanks!
>>
>> Pete
>>
>> 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).
>>>
>>>
>>> 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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>>>
>>>
>>>
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