[sdiy] Moog Rogue question...

[Pete Hartman]

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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