[sdiy] HP from LP ?

David G Dixon dixon at mail.ubc.ca
Tue Aug 1 02:34:33 CEST 2017

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.


From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Donald
Sent: Monday, July 31, 2017 3:26 PM
To: Walker Shurlds
Cc: synth-diy at synth-diy.org
Subject: Re: [sdiy] HP from LP ?

On Jul 31, 2017, at 2:42 PM, Walker Shurlds <walkershurlds at gmail.com> wrote:

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.

Quick answer: 

No, no, no, no, no.

More helpful answer:

First off, you're going to need to tune those four differentiators to track
the ladder filter.

So that would effectively be a filter on it's own, similar to the Moog high
pass filter.

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.  

That's a crazy amount of gain.  The tiniest noise would be amplified to

  -- Don

Donald Tillman, Palo Alto, California

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