[sdiy] Hand-matching capacitors for filter stages

[Neil Johnson]

Hi,

John Mahoney wrote:
> Perhaps some caps have frequency-related non-linearities? Like,  
> maybe type X is has fairly uniform capacitance across the audio  
> bandwidth whereas the capacitance of type Y gets lower at higher  
> frequencies. Seems like a long shot, since 20 kHz isn't a very high  
> frequency, but small differences can add up.

Basic ceramic caps (at the cheaper end, e.g. Y5V) are highly voltage- 
dependent - the capacitance decreases with applied voltage.  The  
effect in a filter at various amplitudes is left as an exercise for  
the reader, but do check out the AVX datasheets for Y5V ceramics and  
see how much the capacitance changes over a 0-10V range.

As for various capacitor dielectrics, I think what you're most likely  
going to find is differences between the two main dielectric families  
- ceramics and plastic film.  The differences between polystyrene and  
polypropylene will be minimal compared to ceramic and poly[ester| 
carbonate|styrene|propylene].

If anyone is bothered to check through the archives you'll find this  
subject thrashed to death many many times.

Neil
--
http://www.njohnson.co.uk