Get more from your VCO!
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Please be positive and constructive with your questions and comments.

Re: Get more from your VCO!

by antto on Mon Aug 12, 2013 12:46 pm

i actually changed my mind about extending the octave range

TS272 - that's nothing special just a dual opamp, i'm sure you must have some "nice" opamps there locally
you can try TL072, LM833, anything similar, i think the important part is for it to be able to operate close to the supply rails.. that's all

i had these opamps already as i was playing around with soldering together DIY distortions, and such things, so i bought a bunch of different opamp types
look around at the DIY guitar effects websites/forums, they should have favourite opamps lists, those i would guess would have the properties which are needed for this case here (or just look thru datasheets)

in the stock firmware (thus inherited in all other firmwares, sokkos/n0nx0x/whatever) the normal "c" note (non-transposed low-c) is 23
i would guess that this is identicall in the TB-303, and this is important for the CV-out
now.. if you transpose it down, it becomes 11

in the TB-303 sequencer this is the lowest possible note in the pattern AND at all
now, lets see the highest one - high C with transpose up - 47
however, in pattern Play mode and in track mode, there is pitch shift which can transpose the pattern additionally up to one octave
so the highest possible note on the 303 is then 47+12 = 59

(note: i'm counting from 0 to 63 here)

the x0x stock firmware has one additional octave in the pattern (double-up) which already covers the max range of the 303 sequencer (11 to 59) but it doesn't stop there, it also has an equivalent to pitch-shift, which can transpose the whole pattern up to one octave AND down one octave as well
so then the lowest note is: 11-12 (oops, we went negative here)
and the highest note is: 59+12 (oops, we went beyond 63, the Pitch DAC is 6-bit)

so what i tried to do (long ago in n0nx0x beta) was to extend the octave range but "properly"
i wanted to have pitch-shift up to one octave (like on the 303) but also add an additional octave in the pattern (double-up)
so i "pushed" down all notes till high-C plus double-up + up is 63
then low-c transposed down is not 11 but 3
boom five octaves (61 notes)
so here's where the opamp may f*ck up, at the very low CV values (close to 0V) it can distort and thus those low notes will get out of tune
a "better" opamp fixes this

the only issue with this was that i had to retune the VCO to compensate, AND this also makes the CV-out wrong

however.. a few years later.. as i said, i changed my mind.. this big octave range per pattern (the double-up thing) .. i don't like it
i have it in my soft-synth, but i f*ckin hate it
there is something about patterns having just 3 octaves
it's up to taste of course, but me and a few more people all went to the same conclusion that this extra octave gets in the way too much, plus, the circuit is designed the way it is with the notes ranging to 4 octaves, not 5 (even tho the DAC can do it)
We are here too: irc.freenode.net >>> #x0xb0x
..:: c0nb0x v1.00 ::.. http://forums.adafruit.com/viewtopic.php?f=7&t=29282 (new control app - win/osx)
VCO Tuning: http://forums.adafruit.com/viewtopic.php?f=12&t=24759 (do it the EASY way)

Posts: 1628
Joined: Thu Apr 15, 2010 3:21 pm

Re: Get more from your VCO!

by isak on Mon Aug 12, 2013 1:08 pm

Wow, I see youv been thru a lot there...
Guess if you say it sucks then it sucks :)
I thank you for explaining me this, very informative.

and you right, we always try to band the rules of law and aventually after the big jerny of research come to the conclusion that the original was just fine.
We do it all the time in the name of research :)
Ohh well, next mod (new thread) ;)

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Joined: Wed May 27, 2009 3:54 pm

Please be positive and constructive with your questions and comments.