[sdiy] Hand selecting capacitors

Ben Stuyts ben at stuyts.nl
Sat Apr 10 16:16:07 CEST 2021


Hi Doc,

> On 9 Apr 2021, at 08:45, David G Dixon via Synth-diy <synth-diy at synth-diy.org> wrote:
> 
> Anyway, I ran low on capacitors to the point where I couldn't put together decent collections for the 18-stage filter (I need 4 102s, 4 103s, 4 104s, and 6 105s for it).  I bought 250 each of the smaller values and 200 of the 105s.  Tonight I measured all of the 105s, thinking that they would ideally form a Gaussian distribution around 1.000uF.  Well, they were nearly all clustered between about 1.030uF and 1.050uF, and in fact, about 20% of them were off spec (higher than 1.050uF, and these are 5% caps).  Not a single one of them was less than 1.020uF.  They are Kemet caps.  So now I've got to hope that the other values also tend on the high side of their tolerance ranges, or else I'm going to have to buy even more caps.

It is a pity they are all over the nominal value. Otherwise you could perhaps add a small parallel cap. (Adding a series cap in this case would not be practical…)

On the other hand, if you add a small cap to make them all 1.1 uF (e.g. 1.03 uF + 68 nF) the tolerance on the smaller cap would be much less of an issue. And then change the corresponding resistor values to match.

Ben

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