[sdiy] Hand selecting capacitors

Florian Teply usenet at teply.info
Fri Apr 9 11:01:50 CEST 2021


Am Thu, 8 Apr 2021 23:45:42 -0700
schrieb David G Dixon via Synth-diy <synth-diy at synth-diy.org>:

> Hey Team,
>  
Hey Dave,

> 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.
>  
You're not the first one to observe the rather skewed distribution of
tolerance in passives: Even though manufacturing yields aproximately a
gaussian distribution of values, what ends up in distribution pretty
commonly has the central part of the distribution missing. The
reasoning behind is pretty simple: those parts, that qualify for a
tighter tolerance are sold as such, as they can charge more for tighter
tolernce groups. And you also get economies of scale by not running
separate lines or manufacturing batches for different tolerance groups.
On the other hand, if you're buying larger quantities (quantities way
beyond what one would need for DIY, that is), you sometimes end up with
a batch of tighter tolerance parts thrown in depending on what they had
in stock.

So, when one needs very tight matching between parts, there are (at
least) two solutions: a) buy excess quantity and select parts (excess
here meaning usually upwards of three times the number you want to end
up with), or b) divide critical parts into parallel and/or series
combinations of at least two devices. This could be either an
approximate 50/50 split or 90-95 % plus one for fine-tuning. For a
50/50 split you'd need either some luck with your purchase (as you
mention nearly all your parts are on the upper end of the tolerance
range) or again sufficient excess, while for the fine-tune approach
(95/5 split) you'd need a decent stock of smaller value parts to meet
the final value. Matching to specific target values becomes easier when
you have more devices to work with (either in form of stock or in form
of several devices connected together to form the target value), but of
course you quickly reach a point of diminishing returns as sorting and
selection is both money- and time-intensive such that in most cases
it's moe economical to buy tighter tolerance parts in the first place.

Doesn't help much in the situation you're facing right now, but worth
considering in the future nonetheless...

Best,
Florian


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