[sdiy] asm-1 pots

[Scott Bernardi]

You probably don't want to use pots as high as 100K due to loading effects. Most
synth inputs have about 100K of input impedence, so using a 100K pot would "warp"
the response. You generally want to use a pot a factor of 10 or more lower (10K)
to avoid loading. See http://home.attbi.com/~sbernardi/elec/og2/partsub_pots.html
for my article on pot substitution.
The other consideration is the taper of the pot. Use log pots when you are
controlling amplitude - such as attenuators on mixers, or level controls (like on
the output of an LFO), or in the feedback loop of an opamp to set gain. Linear
taper for setting control voltage levels - like initial frequency controls on
exponential VCO's.
For an ADSR you would want log pots for the Attack, Decay, and Release controls
(say 1Meg), and a linear pot for the sustain.

On the ASM-1 specifically, here is what I'd do (although I haven't built an
ASM-1. Somebody correct me if you have better ideas):
VCO:
Hook up a 10K linear pot to CV5 input for coarse freq. Run the two ends of the
pot to +/-15v. Measure the voltages on the wiper that give you the frequency
range you want. Calculate series resistors (see article above) to give those end
voltages.
Change R6 to 1M and hook up a 10K pot with the ends from ground to +15v for a
Fine freq control.
VCF:
Hook up a 10K linear pot to CV4 for Frequency control, measure end voltages for
acceptable range, and calculate series resistors to give those voltages.
For the Q, you can do the trick I outline in my article about building a reverse
log taper pot from a dual linear pot.   The "top" of the dual pot would go to the
point labeled Qout, and the wiper to the point labeled Qpot.  A 100K or 50K dual
linear pot will do.
LFO:
Use a 100K log pot for P1 for the frequency control.
ADSR:
Use 1M log pots for Attack, Decay, Release.  The 100K's should be linear.


patchell wrote:

>     What I generally do is use pots that are 10K for supplying control
> voltages, and 100K pots are for attenuators.  Judging from the schematic, you
> may need a few 1M pots in there as well (for the ADSR at least)....
>
> Josh Randall wrote:
>
> > I have most of my asm-1 board completed and am planning on getting Xircon
> > pots.  How do I know what values to get for the different controlls?  Can
> > I get 100K linear pots for the whole thing or will each control require
> > different values?
> >
> > Thanks for the help.
> >
> > Sincerly,
> >
> > Josh Randall
>
> --
>  -Jim
> ------------------------------------------------
> * Visit:http://www.silcom.com/~patchell/
> *-----------------------------------------------
> *I'm sure glad Merry Christmas comes just once a year
> * -Yogi Yorgensen
> ------------------------------------------------

--
Scott Bernardi
sbernardi at attbi.com