[sdiy] HP from LP ?

Pete Hartman pete.hartman at gmail.com
Tue Aug 1 03:10:03 CEST 2017

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.

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.


On Mon, Jul 31, 2017 at 7:34 PM, David G Dixon <dixon at mail.ubc.ca> wrote:

> 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
> Tillman
> *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
> clipping.
>   -- Don
> --
> Donald Tillman, Palo Alto, California
> http://www.till.com
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