Potentiometers: what do you suggest??

Christopher_List at Sonymusic.Com Christopher_List at Sonymusic.Com
Wed Jun 11 09:57:20 CEST 1997

>> Furthermore, I've always heard that RS pots have a
>> dead zone at the CCW end of travel; the resistance doesn't change at all
>> for quite a few degrees. Mouser pots don't seem to have this problem.
> I think that's a feature.  I tested a bunch of pots I've got sitting
> around and the only one that didn't have a dead zone at both ends was
> still reading resistances of 20 to 30 ohms between the wiper and the
> ends at full rotation, i.e., it wouldn't turn all the way off.  In
> most cases I would rather have a dead zone than not be able to turn a
> pot all the way off.  The Clarostat 53C3 pots had by far the largest
> dead zones.

The pots you are referring to are "Xicon @ Alpha" pots - it says it on the
top of the catalog page. I have only these Mouser pots in my ASM-1. The
pots say "Mouser" on the back, but they are made by Xicon. Now I don't know
if they're the same as the RS parts, but come on, they're $1.25 each!!! You
can't expect a lot from that kind of pot.

I was, in fact, just working on "the dead zone" problem last night. Seems
the 1M audio taper pots I have for the attack and decay are at 0 ohms for
the first few fractions of a turn, then suddenly jump to about 3.5K! - OK,
I know that 3.5K isn't alot relative to 1M, but it's noticable when the
attack goes from about .1ms to 10 or 15ms!

I don't much care for the pop created by a superfast (.1ms) attack, and 10
- 15ms is too slow, so I was trying to figure out what to do. The way I see
it, my options are;

1. Put an 800ohm resistor in series with the pot so that the min attack is
around 4ms. The down side is that this is intentionally degrading the
performance of your synth - something the synth gods frown upon. I could
put a bypass switch in, but this is a waste of real estate for something
that'll rarely get used...

2. Lower the value of the cap in the ADSR so that when the pot jumps to
3.5K, the attack time only jumps up to 5ms or so. This results in shorter
max attack - again degrading synth performance.

3. Get a perfect pot. Must be 1M, audio taper, 1/4" shaft - these are hard
to come by. I could go with a linear pot (like the RV4), but I'm almost
always in the lower 50% of the pot travel when it comes to attack - so this
would be bad. Actually, this is a good reason to go with option 2!

4. Put a LPF on the CV for the VCA to filter out pops. Again degrading the
performance of the synth - I'm also not sure how to do this given the fact
that the summing amp for the CV in the VCA doesn't have a feedback resistor
and is current output....

- Any other ideas?

- CList

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