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<DIV dir=ltr align=left><SPAN class=472393300-01082017><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>
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<FONT size=2 face=Tahoma><B>From:</B> Synth-diy
[mailto:synth-diy-bounces@synth-diy.org] <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> synth-diy@synth-diy.org<BR><B>Subject:</B> Re: [sdiy] HP
from LP ?<BR></FONT><BR></DIV>
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<DIV>On Jul 31, 2017, at 2:42 PM, Walker Shurlds <<A
href="mailto:walkershurlds@gmail.com">walkershurlds@gmail.com</A>>
wrote:</DIV><BR class=Apple-interchange-newline>
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<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>
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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">http://www.till.com</A></SPAN></DIV>
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