Cermet vs plastic pots..

[jorgen.bergfors at idg.se]

Excellent post!
This is the sort of information that I want to read.
It is very easy to assume that the most expensive components are automatically 
the best. But as you see, that's not allways the case.
I'm always suspicious to pepole who recommend things on the basis that they are 
more expensive and therefore must be better.

/Jorgen





MIME:majmoog at synthfool.com on 98-04-06 10.24.51
To: synth-diy at mailhost.bpa.nl @ SMTP 
cc:  (bcc: Jorgen Bergfors/IDGSE)
Subject: RE: Cermet vs plastic pots..

>The issue is temperature drift, and stability over time (say 5 years). If
>you have to get a 'cheapie' pot, BE SURE it's conductive plastic.

>Stay away from the 70 cent Mouser carbon pots. 3-5 years from now, you
>will have noisy pots that drift.

I don't mean to be the devil's advocate here, but I have to speak up here.
In case some of you don't know me, I've restored MANY modulars and synths.
So my background is not that of design, but of repairs, maintenance and
restoration.

A few observations. I'm not trying to flame anyone here. I welcome comments-

I've seen the Mouser ones last much longer than 5 years, even in smoky
environments, but the worst and noisiest pots I've encountered in synth
restores- Emu modulars- Conductive plastic.
They *should* be great. They look great on paper, they are wonderful when
new, but they just don't seem to hold up.

>There is nothing wrong with conductive plastic. Cermet pots (used in MOTM
>and EMu modulars) have>10 times less temperature drift than conductive
>plastic. They also tend to 'feel' better. They also have about 10 times
>more resistance stability over time (years). It's just the best pot you
>can get.

I've replaced so many Cermet pots that I've lost count.
Possibly it's that they are often "homed" on one particular area of the
resistive material.
Cermet trimmers are very susceptible to this and so are front panel tuning
pots.
They just don't seem to last as long in the real world as they "should" on
paper.

>NEVER use anything on VCO tuning pots but cermet. All the temperature
>compensation in the VCO >will be worthless if the tuning pots are drifting
>all over.

Again, longevity is a question in my book.

>Now, in the older Moog stuff (modulars and Minimoog) what was used was
>called an Allen-Bradley Type J pot
>(which you can still get). These are either carbon or wirewound. What
>makes them different (they are about $7
>now) is that they are fully sealed against moisture and dust. They have
>stainless steel shafts. This is NOT the same construction as the 'open
>frame' Mouser stuff.

True. But for some strange reason, I've replaced more of these sealed pots
than any other type of pot.
Literally hundreds. In fact, in Minimoog restores I've rarely had to
replace the open frame pots, but very often have to replace the J's.
Sometimes they will "come around" by exercising them, but why would a
sealed pot require that.
>
>Why does a pot get noisy? #1: dust #2 moisture.

And overuse.

I just wanted to interject that what looks good on paper isn't necessarily
going to be so in 10 or 20 years. I'm repairing this stuff every single day
and so far... good or bad....the cheapy japanese open frame pots are
staying quieter than the Ohmite or AB sealed J or RV4 pots by FAR.
They feel better too, IMHO.

So, what should you use?
That's up to you.

K