Feedback on circuit design

[Marc Nostromo [M-.-n]]

Hi Guys,

As I wrote yesterday, I successfully controlled my monotron from a
teensy with some very basic circuit (and actually some very basic
software too). I'm probably going to do a small write up on it, so it
can be a starting point for other people like me with very little
experience.

But before, I'd like to run my schematics against you guys so I can
double check I didn't do any major mistake.

The monotron schematics are over here:

http://3.bp.blogspot.com/_rScBRKlTdoE/TNQraMhovzI/AAAAAAABsz8/PJkljExPMXE/s1600/monotron_sch.jpg

And my circuit is over here (thanks punkpauldisco):

https://dl.dropbox.com/u/323652/monoteensy.png

Like I said, it seems to work alright but I wonder the following:

1) Do I need any buffering stage between my resulting signal
(cut-off/pitch) and the monotron's vc points ?
2) There's this strange 'bias' solder point in the monotron's
schematics that people seem to use and is apparently some kind of
virtual ground. I'm not using it but I wonder if I should. Right now
the cutoff signal influences the cut-off in both directions (with
respect to the monotron's knob setting). Maybe if I used it it would
be only in the positive part ?

here's a schematic using the bias point:

http://beatnic.jp/manuals/monotron-midi/img/mono++midi-schem.PNG

3) I'm planning on using the first channel of the mcp4822 to generate
a DCO that will be fed in the monotron's line input to beef it up with
a bunch of DCO / noise / whatever goes in my mind. Do I need to buffer
it after the mcp ? (I guess it's pretty much the same question as 1)

4) A side question. How do you choose resistor values for voltage
dividers ? I've put a 47k because I thought it would minimise the
current and therefore consumption but it's a blind choice and I don't
like that too much :D

>From there on, I'll have a nice platform to do a pretty interesting
Digilog synth.. even if it will miss a VCA... but I don't think I mind
too much. And I could add it later on too.

Thanks for any feedback/criticism/suggestions !
/M

--
http://marc-nostromo.com