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<DIV dir=ltr align=left><SPAN class=968231822-10042021>Hi Ben,</SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=968231822-10042021></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=968231822-10042021><FONT color=#0000ff
size=2 face=Arial>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:</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=968231822-10042021><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
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alt="" align=baseline src="cid:968231822@10042021-15BF"></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=968231822-10042021><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=968231822-10042021><FONT color=#0000ff
size=2 face=Arial>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.</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=968231822-10042021><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=968231822-10042021><FONT color=#0000ff
size=2 face=Arial>This all adds about 25% hassle to the build, but it's really
no big deal.</FONT></SPAN></DIV><BR>
<DIV dir=ltr lang=en-us class=OutlookMessageHeader align=left>
<HR tabIndex=-1>
<FONT size=2 face=Tahoma><B>From:</B> Ben Stuyts [mailto:ben@stuyts.nl]
<BR><B>Sent:</B> Saturday, April 10, 2021 7:16 AM<BR><B>To:</B> David G
Dixon<BR><B>Cc:</B> synth-diy@synth-diy.org<BR><B>Subject:</B> Re: [sdiy] Hand
selecting capacitors<BR></FONT><BR></DIV>
<DIV></DIV>
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style="PADDING-BOTTOM: 3px; LINE-HEIGHT: 1.6; BACKGROUND-COLOR: #ffecb3; FONT-STYLE: normal; PADDING-LEFT: 3px; PADDING-RIGHT: 3px; FONT-FAMILY: Arial, sans-serif; COLOR: #000000; FONT-SIZE: 12px; FONT-WEIGHT: normal; PADDING-TOP: 3px">[<STRONG>CAUTION:</STRONG>
Non-UBC Email]</FONT></TD></TR></TBODY></TABLE>Hi Doc,<BR>
<DIV><BR>
<BLOCKQUOTE type="cite">
<DIV>On 9 Apr 2021, at 08:45, David G Dixon via Synth-diy <<A
href="mailto:synth-diy@synth-diy.org">synth-diy@synth-diy.org</A>>
wrote:</DIV><BR class=Apple-interchange-newline>
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<DIV><SPAN style="FONT-FAMILY: Arial; FONT-SIZE: small">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.</SPAN></DIV></DIV></DIV></BLOCKQUOTE>
<DIV><BR></DIV></DIV>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…)
<DIV><BR></DIV>
<DIV>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.<BR>
<DIV><BR></DIV>
<DIV>Ben</DIV>
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