<div dir="ltr">Digital switches in ladder filters are problematic though, because the additional resistance, even when it's low, can significantly impact the response. Working with the Moog 904 trying to find a digital rather than physical solution (because the physical many-gang switches are a size problem for Euro), I found that just switching capacitor values required me to use VN104 transistors as the switching element for the lower three stages and a mechanical switch gang for the highest. The voltages at the highest were offset enough I couldn't get good pinch off in the FETs, so that's why the mechanical switch for that level. <br><br>I also tested with DG408's and similar digital switches with low on resistance, but they all seemed to have worse impact on the response than the VN104's.<div><br></div><div>Pete<br><div><br></div><div><br></div><div><br></div></div></div><div class="gmail_extra"><br><div class="gmail_quote">On Mon, Jul 31, 2017 at 7:34 PM, David G Dixon <span dir="ltr"><<a href="mailto:dixon@mail.ubc.ca" target="_blank">dixon@mail.ubc.ca</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><u></u>
<div style="WORD-WRAP:break-word">
<div dir="ltr" align="left"><span class="m_6245575500380861963472393300-01082017"><font color="#0000ff" size="2" face="Arial">Most lowpass filters can be converted to highpass filters by
swapping the position of the R and C elements. If this were done with
digital switches, it shouldn't be too much of an
expense.</font></span></div><br>
<blockquote style="BORDER-LEFT:#0000ff 2px solid;PADDING-LEFT:5px;MARGIN-LEFT:5px;MARGIN-RIGHT:0px">
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<hr>
<font size="2" face="Tahoma"><b>From:</b> Synth-diy
[mailto:<a href="mailto:synth-diy-bounces@synth-diy.org" target="_blank">synth-diy-bounces@<wbr>synth-diy.org</a>] <b>On Behalf Of </b>Donald
Tillman<br><b>Sent:</b> Monday, July 31, 2017 3:26 PM<br><b>To:</b> Walker
Shurlds<br><b>Cc:</b> <a href="mailto:synth-diy@synth-diy.org" target="_blank">synth-diy@synth-diy.org</a><br><b>Subject:</b> Re: [sdiy] HP
from LP ?<br></font><br></div><div><div class="h5">
<div></div><br>
<div>
<blockquote type="cite">
<div>On Jul 31, 2017, at 2:42 PM, Walker Shurlds <<a href="mailto:walkershurlds@gmail.com" target="_blank">walkershurlds@gmail.com</a>>
wrote:</div><br class="m_6245575500380861963Apple-interchange-newline">
<div>
<div>
<div name="messageBodySection">In theory, I would think you could follow up
a two pole LP with two differentiators, and you'd have the right transfer
function for a two pole HP. Same for four, with four differentiators.
Differentiators tend to be impractical though–I wonder if this has been
tried.<br></div></div></div></blockquote>
<div><br></div>
<div>Quick answer: </div>
<div><br></div>
<div>No, no, no, no, no.</div>
<div><br></div>
<div>More helpful answer:</div>
<div><br></div>
<div>First off, you're going to need to tune those four differentiators to
track the ladder filter.</div>
<div><br></div>
<div>So that would effectively be a filter on it's own, similar to the Moog
high pass filter.</div>
<div><br></div>
<div>If your ladder filter is tuned to a frequency of, say 100Hz, then at 1kHz
the signal will be down 80dB, and at 10kHz the signal will be down 160dB.
So your differentiators would need a gain of 100 million at the high end
to bring the signal back up. </div>
<div><br></div>
<div>That's a crazy amount of gain. The tiniest noise would be amplified
to clipping.</div>
<div><br></div>
<div> -- Don</div></div>
<div>
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<div style="WORD-WRAP:break-word"><span style="LINE-HEIGHT:normal;TEXT-TRANSFORM:none;FONT-VARIANT:normal;TEXT-INDENT:0px;BORDER-SPACING:0px;BORDER-COLLAPSE:separate;WHITE-SPACE:normal;LETTER-SPACING:normal;COLOR:rgb(0,0,0);WORD-SPACING:0px" class="m_6245575500380861963Apple-style-span">
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<div><font face="Monaco"><span style="FONT-SIZE:11px">--</span></font></div>
<div><font face="Monaco"><span style="FONT-SIZE:11px">Donald
Tillman, </span></font><span style="FONT-FAMILY:Monaco;FONT-SIZE:11px">Palo Alto,
California</span></div>
<div><span style="FONT-FAMILY:Monaco;FONT-SIZE:11px"><a href="http://www.till.com" target="_blank">http://www.till.com</a></span></div>
<div><font face="Monaco"><span style="FONT-SIZE:11px"><br></span></font></div></div></span></div></span></div></div></div></div></div><br class="m_6245575500380861963Apple-interchange-newline"></div><br></div></div></blockquote></div>
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