[sdiy] HP from LP ?

[Donald Tillman]

> 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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