Cgs synth

I just built an LM337 based -2V DC circuit in order to add offset to my .com's oscillators
(because they don't go low enough unless you switch them into "LO" mode which is really
hard to tune properly). It works perfectly when I use two of the CV ins on the oscillators to
mix the MIDI>CV voltage with the -2V offset voltage.

For convienience and to save patching, I want to physically "piggyback" the board on my
MIDI>CV converter module, install a switch to turn the -2v source on/off, and mix the -2v
with the MIDI>CV converter's pitch output voltage to achieve the offset (i.e. so it all appears
on the single MIDI>CV output jack.

When I tried using the DC mixer board to mix the two voltages, it threw off the keyboard
scaling by about a half step over a three octave span. Is there anything I can do to maintain
the correct scaling, or is the DC mixer circuit not capable of doing so?

(btw, my DC mixer doesn't have any pots in the circuit at all, just 100k resistors)

thanks in advance!

Mitchell Sigman

You need to add a pot in the feedback loop. Reduce the 100k to 82k and put
in a 50k trim pot. This will allow you to adjust the gain for exactly unity.
Chances are your 100k resistors don't quite match.

Ken

Dude, where's my post? I posted an answer an hour ago. Friggin' Yahoo...
--
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Thanks, will do.

Mitchell

--- In cgs_synth@yahoogroups.com, sasami@... wrote:
>
> You need to add a pot in the feedback loop. Reduce the 100k to 82k and put
> in a 50k trim pot. This will allow you to adjust the gain for exactly unity.
> Chances are your 100k resistors don't quite match.
>
> Ken
>
> >I just built an LM337 based -2V DC circuit in order to add offset to my
> .com's oscillators
> >(because they don't go low enough unless you switch them into "LO" mode
> which is really
> >hard to tune properly). It works perfectly when I use two of the CV ins on
> the oscillators to
> >mix the MIDI>CV voltage with the -2V offset voltage.
> >
> >For convienience and to save patching, I want to physically "piggyback" the
> board on my
> >MIDI>CV converter module, install a switch to turn the -2v source on/off,
> and mix the -2v
> >with the MIDI>CV converter's pitch output voltage to achieve the offset
> (i.e. so it all appears
> >on the single MIDI>CV output jack.
> >
> >When I tried using the DC mixer board to mix the two voltages, it threw off
> the keyboard
> >scaling by about a half step over a three octave span. Is there anything I
> can do to maintain
> >the correct scaling, or is the DC mixer circuit not capable of doing so?
> >
> >(btw, my DC mixer doesn't have any pots in the circuit at all, just 100k
> resistors)
> >
> >thanks in advance!
> >
> >Mitchell Sigman
> >
> >
> >
> >The CGS Modular Synth home page: http://www.cgs.synth.net/
> >
> >Yahoo! Groups Links
> >
> >
> >
> >
> _______________________________________________________________________
> Ken Stone sasami@...
> Modular Synth PCBs for sale <http://www.cgs.synth.net/>
> Australian Miniature Horses & Ponies <http://www.blaze.net.au/~sasami/>
>