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VCO Harmonic Interval Switch for MS-20 Mini
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author VCO Harmonic Interval Switch for MS-20 Mini
dubs
Hey Wigglers, Mr. Green

I got an idea for a fairly simple accessory for my MS-20 or any voltage controlled oscillator and would like some feedback. I want to be able to lock my second voltage controlled oscillator to fixed harmonic intervals of my first voltage controlled oscillator. The MS-20 outputs a CV signal from the keyboard that I plan on running to a multi-output switch that I will route to different voltage dividers with a ratio matching the harmonic interval I'm looking for. Finally, that shifted signal will run back to the VCO2 input on the MS-20.

For example, I'll play an A4 on my keyboard, VCO1 will play an A4 and I'll have the keyboard's CV out running to the circuit input. If I want to play a perfect fifth and am only using voltage dividers I'll have to have VCO2 play the lower note of the fifth, so a D3. The A4 has a frequency of 440 Hz and the D3 has a frequency of 146.832 Hz, so following the formula for a voltage divider and setting my first resistor to be 1000 Ohms, my second resistor would have to be about 500 Ohms. This shifted signal would then be routed back to the MS-20 Mini's VCO2 input and play a D3.

Issues I can see with this are that I will need to use an Op-Amp to get higher intervals and that the resolution of the resistors may not be granular enough to get the tuning right. I may need to use pots. If I do use pots, I can have the Op-Amp set up as a voltage follower to boost the signal one octave and then have each of the switch paths have its own pot that I can tune my hand.

Sorry for the novel. I'll post circuit diagrams once I make them.

Thanks!
Starspawn
Well, first of all thats kind of what the tune knobs is for?
But, there is some uses, however going a bit bigger helps then wink

http://www.bartonmusicalcircuits.com/cvspread/
mskala
dubs wrote:
For example, I'll play an A4 on my keyboard, VCO1 will play an A4 and I'll have the keyboard's CV out running to the circuit input. If I want to play a perfect fifth and am only using voltage dividers I'll have to have VCO2 play the lower note of the fifth, so a D3. The A4 has a frequency of 440 Hz and the D3 has a frequency of 146.832 Hz, so following the formula for a voltage divider and setting my first resistor to be 1000 Ohms, my second resistor would have to be about 500 Ohms. This shifted signal would then be routed back to the MS-20 Mini's VCO2 input and play a D3.


This may work for the MS-20 oscillators, which have linear (Hz/volt) voltage control. You'll need to trim the divider ratio carefully. But for exponential (volt/octave) control, which is more common in synthesizers generally, you need to add or subtract a fixed voltage instead to get a fixed musical interval. The Barton CV Spreader is designed for that case.
dubs
Starspawn wrote:
Well, first of all thats kind of what the tune knobs is for?
But, there is some uses, however going a bit bigger helps then wink

http://www.bartonmusicalcircuits.com/cvspread/


I'm trying to be able to snap into a harmonic quickly without fiddling with the tuning knob. Not only will it be more useful for live stuff, but it will save me time when I'm just noodling around at home.

mskala wrote:
You'll need to trim the divider ratio carefully.


I'll make sure to pick up some pots then.
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