[sdiy] Hand selecting capacitors

Brian Willoughby brianw at audiobanshee.com
Fri Apr 9 17:50:01 CEST 2021


The trick is to get the data sheet for the capacitors that you've decided on, see the range of tolerances that are available for that particular family line, and always buy the best tolerance on the data sheet. You might see one line of capacitors offered in 5%, 10%, and %20, so buy the 5%. Another line might range from 1% to 10%, so you'll have to buy the 1% to avoid missing values.

Basically, each line of capacitor is made with the same process. They then sort through what comes out, and charge differently according to how close the value is. If you care about matching, you're going to have to buy the best tolerance available from that assembly line. The good news is that you don't always have to pay for 0.1% tolerance (if that's even available for your values) because a given line may not offer 0.1% - just check the data sheet to know how the manufacturer has sorted them.

There's no sense in taking the worst group after the manufacturer has sorted them, and then try sorting them again yourself ... at least not unless you're really flush with time and short on money. However, don't underestimate the amount of money lost by parts that cannot be used because they're too far out. The amount you save by paying less for wider tolerance might easily be eaten up by the lost money on unusable parts.

I don't think you mentioned a part number. Kemet has many lines of capacitors, even in the same value and type. It would be interesting to know the exact part number and then check out the data sheet.

Brian


On Apr 8, 2021, at 23:45, David G Dixon via Synth-diy <synth-diy at synth-diy.org> wrote: 
> 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.



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