[sdiy] Moog Rogue question...

[Roman Sowa]

The line that goes to summing node via 100k resistor is marked in the 
schematics "1V/oct" so to reach 5 semitones shift we need to source 
4.16uA from the summing point. Sunk by -12V that gives resistance value 
of about 2,88M. So 2M7 with 300k trimmer should catch needed value more 
or less in the middle.

OTOH in the same schematics there are 2 lines marked as 1V/oct, and 
there is 0.909 gain between them, so an error in the schemo, or another 
unsolved mystery (until you measure it).

Roman

W dniu 2014-05-06 15:36, Pete Hartman pisze:
> 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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>>>>>
>>>>>
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>>
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