[sdiy] Hand selecting capacitors

David G Dixon dixon at mail.ubc.ca
Sun Apr 11 00:33:57 CEST 2021

Hi Ben,
That would involve changing my layouts, which simply cannot be changed,
because I've covered virtually every 0.1" square area on the board with
something, and I've got all the circuitry fitting onto PCBs which are
eurorack compatible.  Getting an 18-stage Dome filter onto a board 4.3" x
2.1" was no mean feat, I can assure you!  Here's the layout (note: the grey
boxes on the left are pass-through pins from the PCB above to the PCB below)
-- I've used 30.1k for all of the unity gain resistors, and all the other
resistors are custom based on my phase displacement model:

I've actually come up with a solution.  After measuring all of the 102, 103
and 104 caps, I found that they all fall more or less randomly within the
960 to 1030 range of mantissas, as I expected.  It's only the 105s which are
high.  Hence, what I'm going to do is make sets of equal mantissas for the
102/103/104 caps (I now have lots of possibilities for that), then use
whatever 105 caps I have on hand, and adjust the R values accordingly just
for those caps.  Also, since I'm using pairs of timing resistors in series
for each filter stage, I can actually just install the same values for the
large resistors (a little smaller than my custom values shown above), and
then just customize the smaller resistors.  That's just 6 resistors that
must be customized for this board.  I can use my simulation spreadsheet to
find the optimum resistor values.  I'm collecting every 1% resistor in the
E96 series from 1k to 1M anyway, so that won't be a problem.  In this way,
I'll be able to get rid of all my 105 caps, including the ones where I only
have one or two sparse values here and there in the table.
This all adds about 25% hassle to the build, but it's really no big deal.


From: Ben Stuyts [mailto:ben at stuyts.nl] 
Sent: Saturday, April 10, 2021 7:16 AM
To: David G Dixon
Cc: synth-diy at synth-diy.org
Subject: Re: [sdiy] Hand selecting capacitors

[CAUTION: Non-UBC Email]	
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.


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