[sdiy] Hand selecting capacitors

David G Dixon dixon at mail.ubc.ca
Fri Apr 9 08:45:42 CEST 2021

Hey Team,
So, I've now got a ton of orders for my little two-Dome-filter frequency
shifter, which I've figured out how to build in euro format (12HP, if you're
curious).  I hand select all the film capacitors for the Dome filters so
that they all have the same (or nearly so) mantissas -- I try to get them
either all within 0.2%, or 0.4% at worst.  My phase displacement simulation
tells me that this gives very nearly perfect results.
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.
I think I've given myself a fool's errand here.
Doc Sketchy
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