[sdiy] Hand selecting capacitors

Oren Leavitt obl64 at ix.netcom.com
Fri Apr 9 17:17:01 CEST 2021

On 4/9/21 1:45 AM, David G Dixon via Synth-diy wrote:
> 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.
> Cheers,
> Doc Sketchy
Eee... what's up Doc?

Of all of the Rs and Cs I've matched, I've never seen that bell curve 
land right on the stated value. Especially with capacitors. It's always 
been some point over or under within tolerance.

Would it be easier to take, say 4 1.03uF caps and nudge any frequency 
determining resistors up/down as needed?

- Oren

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