[sdiy] Moog Rogue question...

[Pete Hartman]

So to be sure I get it:  90.9/2700 x -12V = -.4V == not quite 5
semitones down, and 90.9/3000 x -12V = -.36V == a little more than 4
semitones down.  When you figure in tolerances, this puts us in the
right range to pull the F down to C.  90.9K is the feedback resistor
on the summing node.

I don't actually have any 2.7M or a 300K trimmer so I will add some
resistors together and work out what does roughly the same thing with
the values I have.

Thank you :)


Pete

On Tue, May 6, 2014 at 8:12 AM, Roman Sowa <modular at go2.pl> wrote:
> Such trimmer would have tuning range of about 4 octaves, impossible to tune
> precisely. And since you need to turn it only to negative side (-12) anyway,
> so better have it from GND to -12, hence 2 octaves range.
> It's only about reducing full rotation tuning range so it's easier to tune
> precisely down to lower C.
>
> Roman
>
> W dniu 2014-05-06 14:28, Pete Hartman pisze:
>
>> 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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