[sdiy] Hand-matching capacitors for filter stages

[thx1138]

On 2/9/10 10:57 AM, "Richie Burnett" <rburnett at richieburnett.co.uk> wrote:

> Hi Tom,
> 
>> So the question is - does anyone match capacitors for filters?
> 
> I honestly wouldn't bother taking time to match capacitors in a VCF.
> Variations in the capacitor values only move the real poles of the filter
> slightly.  This alters the knee point (-3dB) point for each pole of the
> filter when there is no feeback (no resonance.)  All a spread of capacitor
> values will do is make the combined knee point slightly softer and more
> gradual.  However, if you think about it, it is very soft and gradual anyway
> with no feedback.
> 
> When you add feedback to poles will group together to form oscillatory pairs
> pretty much as they would if all the capacitors were identical.  The pole
> spreading due to capacitor value variation only alters the amount of
> feedback required to achieve a particular amount of resonance.  A VCF with
> perfectly matched pole positions will self-oscillate with slightly less loop
> gain than one with its pole positions spread out.  But, you generally have a
> resonance pot to adjust this parameter by ear anyway.
> 
> Given that things like transconductance of OTAs will also have a tolerance
> and temperature coefficient, I think that matching of capacitors is not
> worth the effort in a VCF.  If you're not convinced you only have to look at
> Tim Stinchombes analysis of ladder filters.  He shows that the TB-303 ladder
> filter behaves pretty much like any other normal ladder filter regardless of
> the oversized capacitor at the bottom of the ladder.  I guess these
> topologies for VCFs were successful *because* they are tolerant of component
> spread!
> 
> -Richie, 
> 
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Hi ,

I agree with Ritchie.

I can tell you that Ceramic and Electrolytic Caps are a real NO due to cap
value changing over time and temp. Electrolytic are a whole different set of
problems and basically I only use in Power supply designs etc.

Silver Mica might be interesting but I tend to use PolyPropulene
(spelling?).

I have a ton of Teflon caps left over from my old E-Mu days and I am using
them on circuits still.

Regards,

Terry