[sdiy] Hand-matching capacitors for filter stages

[Tom Wiltshire]

On 9 Feb 2010, at 21:25, David G. Dixon wrote:

>> 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.
>
> OK, now we're gettin' somewhere!  Of course, "ancient ceramic caps"
> definitely falls well outside of the margins of what would be  
> considered
> "acceptable" for VCF timing.  Did you happen to measure the  
> capacitance on
> any of those old caps, and if so, was it anywhere near the  
> advertised value?

No, I didn't measure them. I just threw them away without a thought  
for their value to science! They were cheap brown ceramic discs that  
probably weren't that close to the advertised value when they went  
in, and certainly wouldn't have got any closer in the 20+ years they  
were in there.
I regret not having made a few test recording before and after,  
actually. That would have been cool.

> Some of them could have become seriously degraded, to the extent  
> that one or
> more of the filter stages might not have been contributing  
> significantly.
> Perhaps the "brittleness" you heard was actually a loss of cutoff  
> slope.

Maybe so.
I'm out of my depth in detailed discussions of capacitor parameters,  
but it seems to me like some people here are trying to reduce the  
'sound' of a given capacitor at a particular time and temperature to  
a single parameter - its actual value. Is that really the whole  
story? Are we saying that any two capacitors of exactly equivalent  
value (measured value, not marked value) will have exactly the same  
sound? That seems unlikely, given the wide variety of dielectrics. If  
not, what else makes a difference?
As an analogous example (and it may not be a good analogy for  
technical reasons I don't get yet) different diodes have quite  
different sounds, as anyone who has built fuzzboxes knows. The  
relevant parameter (volt drop in the diode case) is far from being  
the whole story. Different types produce all sorts of weird wriggles  
in the waveform when you turn it up a bit and watch on the 'scope.

> You have definitely piqued my curiosity, to the extent that I might  
> actually
> have to lay my hands on some polystyrene caps and try a little  
> experiment.

Good luck! Start with the oldest, most rubbish-sounding equipment  
first so that you get some actual benefit!

T.