[HAM] AO-28 Sounds

[David Anderson]

On Jan 19, 2008, at 4:48 PM, J Wacek wrote:

>  David,
>
> With that said, what's a "better" coupling cap; ceramic or  
> polypropylene?
>
> Jim

Jim,

It depends on the application. The ceramic capacitor picture is much  
more complicated because of the many different types available.

Here's what I know: there are ceramic capacitors made with a pure  
ceramic dielectric, often found listed as C0G or NP0 (that's a zero  
there, not the letter 'o'--for zero temperature coefficient). I think  
"Class 1" is also a term for them. These C0G/NP0 ceramics are usually  
excellent capacitors, but they are most often found only in values up  
to a few hundred picofarads due to size limitations. Contrary to some  
opinions, ceramics are not inferior to mica capacitors; in fact,  
ceramic specs are usually better, and mica is much more expensive.

The "bad" ceramic capacitors are those that are doped with compounds  
to get more capacitance in a smaller package (Z5U, Y5U, etc...).  
These compounds cause all sorts of strange effects from variation of  
capacitance based on applied signal to microphonic/piezoelectric  
effects. They are the reason ceramic capacitors have gotten a bad  
reputation in audio.

So, the answer is: if you need a picofarad value capacitor like the  
24 - 220pF values you'll find in an AO-28, use a C0G ceramic. You  
won't find polypropylene capacitors in those values, and it's even a  
bit small for polystyrene. Polystyrene is a great dielectric, too,  
but they are very easy to damage by soldering due to polystyrene's  
low melting point.

For values like .047uF and .33uF, use polypropylene, either  
metallized film or film & foil. Try to buy the grades of  
polypropylene capacitors with copper leads rather than steel since  
steel can pick up EMI from nearby transformers.

David