[sdiy] VCO Tuning goals

[David G. Dixon]

It is my understanding that most high frequency trim circuits involve
feeding a very small bit of expo current back to the CV summer, and that
this is typically done through a diode in order to feed back only positive
currents.  Wouldn't this imply that one should be able to adjust the scale
trim at lower frequencies (i.e., below zero volts of CV) regardless of the
HF trim setting?  Then, the HF trim could be adjusted independently at high
frequencies (i.e., well above 0 volts of CV).


> In brief, I just finished building a trio of MOTM 300 ultra VCO's into a
> Frac Panel
> 
> http://www.xmission.com/~dingebre/page8.html
> 
> As I have been tuning them, I have a question on the concept of tuning a
> VCO.
> 
> I understand all too well the scale will always have errors, and my
> understanding is that an error of +/- 0.2% in the frequency is likely not
> detectable, BUT... as one plays, it seems most successive notes are within
> the same octave and often within a few notes. The question then is:
> 
> Is it more important to have better accuracy octave to adjacent octave or
> between 4, 5, or more octaves? That is, when I tune the oscillators,
> should
> I be more worried about a 0.2% difference between C0 and C1 or C2 and C3
> than between C0 and C5? I hope this makes sense.
> 
> I've been making a chart, and have been playing with Paul Haneberg's high
> frequency trim modification
> 
> http://www.wiseguysynth.com/larry/mods/VCO_track.htm
> 
> or
> 
> http://dragonflyalley.com/images/MOTM300/VCO_track.pdf
> 
> (but I am using a multi-turn trimmer in series with a fixed 1M resistor
> instead of swapping resistors).
> 
> I've noticed there is some interesting interaction between the scale and
> high frequency trim and I just want to make sure I'm approaching the
> tuning
> in the best musical sense. I'm curious musically/theoretically if it is
> better to worry about the error between several octaves and let the note
> to
> note difference be what it is, or work on minimizing the "octave to
> adjacent
> octave"/"note to note" error. Reducing the high frequency error between,
> say
> C0 and C5, increases the inter-octave error between C1 and C2, or C3 and
> C4
> somewhat.
> 
> The accuracy of the 300 makes some of this argument moot, but I am curious
> about the whole zen of VCO tuning.
> 
> David
> 
> David M. Ingebretsen M.S., M.E.
> Collision Forensics & Engineering, Inc.
> 2469 East Fort Union Blvd. STE 114
> Salt Lake City, UT 84121
> www.CFandE.com
> 
> 801 733-5458 Office
> 801 842-5451 Cell
> 
> dingebre at CFandE.com
> dingebre at 3dphysics.net
> 
> 
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy