MOTM

Conductive Plastic and Cermet element potentiometers have greater rotational
cycle life, 25K or greater, sometimes into the millions.  Carbon element
pots have a typical 10K-15K cycle life.  Many manufacturers don't spec that
parameter at all for carbon composition pots, which usually points to lower
durability parts.

Cermet pots have a far better temperature coefficient than plastic or
carbon.  This would only matter in VCO frequency controls.

A sealed pot prevents external contamination.  While it seems easy to seal
any pot, I hardly ever see that except for the higher cost plastic and
cermet elements.  It may be tempting to "seal" a pot with rubber silicon,
but outgassing of silicon is a known fact which may or may not contaminate
the element over time.

Short term the carbon composition pots are very good value for the money.
Long term, it is something of a crap shoot.  I've never seen long range
engineering data that compares the compositions with hard data, or Mean Time
Between Failure studies.

If one wants to deal with anecdotal evidence, then think about how many
scratchy pots you have with 25-30 year old synth equipment, nearly all of
which used unsealed carbon pots.  I replaced one dual ganged PCB mounted pot
recently in a piece of vintage equipment, and because the specific footprint
was no longer available, I spent about an hour reforming leads and soldering
on bus wire to make it fit.  Multiply that by a couple hundred pots in a
large modular, and you may have a very large job ahead of you in a few
decades.

John Loffink
The Microtonal Synthesis Web Site
http://www.microtonal-synthesis.com
The Wavemakers Synthesizer Web Site
http://www.wavemakers-synth.com


> -----Original Message-----
> From: motm@yahoogroups.com [mailto:motm@yahoogroups.com] On Behalf Of
> Richard Brewster
> Sent: Sunday, December 10, 2006 10:19 AM
> To: MOTM List
> Subject: [motm] Pot quality
> 
> I've been wondering about it.  I just splurged $18.00 to buy two of
> these babies:
> 
> http://www.mouser.com/search/refine.aspx?Ntt=652-51AAA-B28-D18
> 
> I needed a couple of audio taper pots for an audio mixing application,
> and it came down to these or some carbon Alpha pots.  I'm pretty sure
> these will work well.  But what if I had built all of my DIY stuff with
> carbon pots?  I would have saved a lot of money.
> 
> I noted some recent posts where people were saying that the Alpha pots
> work fine (they are used by Modcan, Arrick, Blacet, Cyndustries, and so
> on).  So I am wondering what reasons we could give for shelling out more
> $$$ for conductive plastic or cermet sealed units?  Are there
> applications where the extra cost is justified (in say, the MOTM-800)?
> What about pots intended to pass audio without introducing scratchy
> sounds?  What about CV?
> 
> I guess I am asking where I can scrimp on pots by using Alphas, without
> regretting it.
> 
> -Richard Brewster
> http://www.pugix.com