Cap chat

[Grant Richter]

Here's my $0.02

tempco = temperature coefficient

The polystyrenes have around a -120 ppm tempco. This is great if you are
making a
resistor-capacitor time base for a monostable because it nearly cancels the
+100 ppm
tempco of a metal film resistor.

In a VCO, where you go to great effort to remove tempcos from the
exponential converter,
you don't need a compensating tempco in the cap. I have had the best luck
with ceramic COG
caps that have a 0 ppm +/-30 ppm tempco (KEMET makes some that are
reasonably cheap).

In a sample and hold, you are less concerned with tempco and more with
dielectric absorption
and leakage. Polystyrene, polycarbonate and Teflon are the choices here.
Teflon is expensive
and polystyrenes are increasingly hard to get. A polycarbonate will work
well at a reasonable cost.
The low dielectric absorption offers good accuracy also.

A portamento circuit is less demanding and a mylar can be used to save
money, although a better cap
will not hurt.

----------
> From: Michael Lloyd <michaellloyd at lucent.com>
> To: tomg <vco at mindspring.com>; synth-diy at mailhost.bpa.nl
> Subject: re: EFM Load; polystyrene caps availability
> Date: Friday, May 21, 1999 9:15 AM
> 
> Tom:
> 
> You message triggered something I seem to remember reading from a few
years
> ago, that that the cap manufacturers were stopping the building of
> polystyrene caps.  Production cost and difficulty making surface mount
parts
> that can take soldering were what did 'em in I think. Are far as high
> performance poly* types go, I suppose the preferred stuff is
polypropelene
> and polycarbonate. What are folks using for caps for VCOs, VCF's and
> Keyboard Voltage Processing (S/H, Portamento, etc.) anyways?
> 
> Michael
> mlloyd1 at enteract.com
> 
> ----- Original Message -----
> From: tomg <vco at mindspring.com>
> To: <synth-diy at mailhost.bpa.nl>
> Sent: Friday, May 21, 1999 4:44 AM
> Subject: EFM Load
> 
> 
> 
> >
> > I've also noticed that it's getting harder to get parts in my neck of
the
> woods. I may just be
> > paranoid but the bins for poly 1000pf and 2200pf at my local surplus
place
> are empty....they were
> > half full a week or two ago. I made a call to another local place for
> lm393s...the guy said "You
> > know that's funny. I haven't sold any for months and yours is the third
or
> forth call today."
> >
>