potentiometers

[Tony Clark]

   Pots are going to be your friend and your enemy.  Friend because once 
you get them installed, you're going to have countless fun tweaking 
them.  But for now, they'll be your enemy, because good pots are 
EXPENSIVE!!!!
   I've been doing some research of my own to replace the pots on my 
micromoog (as I'm still in the process of racking it).  What I want for 
it (and my ASM-1) are some bloody durable pots that aren't going to go 
flakey on me 5 years down the road.
   I won't discuss linear vs. exp because that is usually a circuit 
issue.  You gotta use what you gotta use.  I will talk a bit about what 
various kinds of pots there are.
   
   1) Porcelain vitreous-enameled pots.  Also known as rheostats.

   Unless you are either insane, dirt poor, or someone bent on building 
a synth circa '50's, I wouldn't suggest these at all.  I can find tons of 
these things lying around everywhere, and for good reason.  They have 
graphite brushes which will wear out.  Contacts that can get worn and 
irregular.  One rheostat can take up the space of 4 to 10 normal panel 
mounted pots.  The plus side is, think of how much muscle you'll be 
building in those arms tweaking those things all the time!

   2) Wirewound pots

   Probably what you'll end up choosing in the end.  These kinds of pots 
usually have very good rotational life, from 300,000 to 2 million shaft 
rotations (depending on manufacturer and type).  Resistance tolerances of 
3 to 10%, and power rating of up to 5 Watts (again depending on 
manufacturer and type).  The bad side of these, and practically all good 
panel mount pots are that they cost anywhere from $8 to $30 each.  The 
ones that I've looked at the most costs about $12-14 each.

   3) Conductive plastic pots

   The difference between these and the wirewound ones are just the type 
of resistive element inside.  These offer a much longer rotational life, 
anywhere from 5 million to 20 million shaft rotations.  The downside is 
the cost.  Most I've seen start at $14 and go up from there.  Average 
cost is probably around $18.  Also, I haven't seen precision resistive 
tolerances on these kinds of pots.  Usually 10-20%.

   4) Cermet pots

   These type are generally what your trimpots are made of.  And as such, 
usually aren't found in the larger panel mount size.  They typically are 
not outstanding in their rotational life, but have pretty good resistance 
tolerances and maybe are a tad (talking cents here) cheaper than the 
other kinds.

   There are hybrid type pots out there, but they really don't offer you 
much in the way of being different than the types listed above.

   What I look for in a pot is as follows:

   rotational life greater than 1 million shaft rotations.
   resistance tolerance of %5 or better
   metal bushings (no plastic crap)
   the ability to get special types (center tap, linear/log, ganged)

   BI Technologies carries a pretty good selection of pots that meet the 
above criteria.  Most of the other companies are just way too expensive.

   Hope this helps.

   Tony
   

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I can't drive (my Moog) 55!
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Tony Clark -- clark at andrews.edu 
http://www.andrews.edu/~clark
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> So, here I am with a couple of big boxes of knobs (for 1/4" shafts), but no
> potentiometers upon which to place them.
> 
> Can anyone suggest a decent type of pot to use for the ASM-1? There are so
> many variables here-- I need something panel-mountable, obviously, with a
> 1/4" shaft. What should I look for or watch out for? I need something that
> will stand up to repeated tweaking, since I don't expect this to be a
> set-and-ignore kinda synth... Just general pointers will be gladly
> accepted.
> 
> Also, what would work best as linear or exponential pots? There are only a
> few notes-- what have people found useful?