[sdiy] Suitcase synth updates

[Scott Bernardi]

A 100K pot with a 33K resistor will give you significant loading effects and
warp your response from linear.  Whether that is acceptable or even desirable
depends on what you are controlling with your CV.  For example, I don't think
you would want to do that on a tuning pot for a VCO frequency, because the
response to the control voltage is already exponential. To get a smooth
frequency change as you rotate the pot you want a nice linear voltage change.
For adding level control to a CV input, it's probably OK.
I use log pots for amplitude (like FM level inputs on VCOs, mixers).  You can
make a linear pot act like a log pot by purposely loading it heavily (say load
resistor 1/10th the pot resistance).  I standardize on linear pots in the 10K to
20K range to minimize loading effects.

My website has some useful info about pot selection for CV inputs at
http://home.attbi.com/~sbernardi/elec/og2/partsub_pots.html.

Electronium wrote:

> For what it's worth, if I remember my Electronotes correctly... BH indicates
> a 100k pot & 33k resistor in series for variable CV inputs.  'course
> anything significantly higher than your output impedance should work OK.
>
>  MB
> mike at electronium.com
>
> Patchell wrote:
>
> > Generally, if you want to add level control to one of the CV inputs, I use
> a 100K linear
> > pot.  Wiper goes to CV input of the VCO, one end of the pot to ground, and
> the other end of
> > the pot becomes the input.  With a 100K pot and the 100K input resistor,
> you will not
> > exactly get a linear response from the pot rotation...but, I have never
> noticed it.
> >
> >
> >
> > charlie lamm wrote:
> >
> > > Wow!  The ASM-1 VCO looks great.  Thanks for the suggestion.
> > >
> > > I have a few questions that maybe someone in this group can answer.
> > >
> > > a) I assume to add level control to the various modulation inputs I
> should
> > > put a voltage divider (potentiometer--linear I would guess?) in series
> > > with the 100K resistors.  To give me control of a modulation source, a
> la
> > > the front panel of an ARP Odyssey.
> > >
> > > b) Does anyone know of an already made PCB for a single ASM-1 VCO?
> Core,
> > > modified, or otherwise.  I see a 4-banger here:
> > > http://hem.bredband.net/bersyn/VCO/VCOPCB_l.pdf
> > >
> > > But only need a single.  And of course the ASM-1 VCO's are part of a
> > > bigger synthesizer PCB.
> > >
> > > On Wed, 8 Jan 2003, patchell wrote:
> > >
> > > >     This is one case where it is truely a VCO (we are talking about
> the 3819 aren't
> > > > we?)...(what we commonly call VCO's are generally CCO's, ie current
> controlled
> > > > oscilators).  But to tell you the truth...this VCO (core) is not all
> that much simpler
> > > > that the "Asm1" type oscilator, if you were to strip off exponential
> circuit.  The
> > > > ASM1 oscilator consists of one opamp, one comparator, a FET, a couple
> of caps, and a
> > > > handfull of resistors (for just a sawtooth output).
> > > >
> > > > charlie lamm wrote:
> > > >
> > > > > I really like how simple the VCO is.
> > > > >
> > > > > Can anyone suggest a suitable temp-compensated 1v/octave convertor
> to
> > > > > "slap on" the front of this?  That would make it perfect for a
> project I
> > > > > am working on....
> > > > >
> > > > > On Tue, 7 Jan 2003, Electronium wrote:
> > > > >
> > > > > > Hello all,
> > > > > >
> > > > > > I've made quite a few updates to my suitcase synth project
> webpage:
> > > > > >
> > > > > > http://home.attbi.com/~michaeltbrown1/e_sdiy.html
> > > > > >
> > > > > > Any ideas/thoughts/comments/suggestions/criticisms would be
> greatly appreciated.
> > > > > >
> > > > > > Cheers!
> > > > > >
> > > > > > Mike Brown
> > > > > > mike at electronium.com
> > > > > >
> > > > > >
> > > >
> > > > --
> > > >  -Jim
> > > > ------------------------------------------------
> > > > * Visit:http://www.silcom.com/~patchell/
> > > > *-----------------------------------------------
> > > > *I'm sure glad Merry Christmas comes just once a year
> > > > * -Yogi Yorgensen
> > > > ------------------------------------------------
> > > >
> > > >
> >
> > --
> >  -Jim
> > ------------------------------------------------
> > * Visit:http://www.silcom.com/~patchell/
> > *-----------------------------------------------
> > *I'm sure glad Merry Christmas comes just once a year
> > * -Yogi Yorgensen
> > ------------------------------------------------
> >
> >