[sdiy] Hand-matching capacitors for filter stages

[Tom Wiltshire]

On 9 Feb 2010, at 19:30, David G. Dixon wrote:

>> I often use polystyrene
>> capacitors for filters, since these are widely regarded as the best,
>> and breadboard experiments I've done do tend to support this.
>
> How are they better, exactly?  Does the filter sound better?  Is the
> filter's transfer function closer to the theoretical?
>
> I've never used (or even seen) a polystyrene cap, because they are too
> inconvenient to obtain, and I loathe inconvenience.  I've used  
> silvered
> mica, metallized polypropylene, and C0G ceramics, and they all seem  
> to work
> pretty well.  Polystyrene caps are famous for having low leakage,  
> but many
> others are nearly as low.  What specific quality of polystyrene is so
> desirable for filters, and how exactly does this enhance the filter's
> performance?

David,
I'll let others more technically informed than I chime in on which  
parameters of polystyrene make them desirable for audio. My knowledge  
here extends to trying various types in filter circuits, on my  
breadboard and in my synths. This is where you get into subjective  
"by ear" testing, and each to their own and all that.
Example: I replaced ancient ceramic caps in the CEM3020 filter in my  
Pro-One with polystyrene and it made an immediate and significant  
difference. The sound which previously I would have described as  
"biting" or even "brittle" became much rounder and fuller. In short,  
polystyrene caps sound good. Are they worth the extra money? You decide.

Richie,
Thanks very much, you make some very good points.

Terry,
I completely agree about avoiding matching components. Personally, I  
hate trimmers too. However, I'm not in a commercial setting, so  
actually a little extra time or money doesn't matter if it makes  
something better. That's the luxury that we SDIYers have that  
companies do not, and I believe it should enable us to build better  
stuff - which is not the same as saying that we manage!

In summary, then, a bit of variation in your filter caps probably  
doesn't matter as much as you think it does (just turn the res up  
more!) and as a result hand-matching capacitors is a waste of time  
and effort.

Thanks all,
Tom