[sdiy] HP from LP ?

[Walker Shurlds]

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.

Walker

On Jul 31, 2017, 16:31 -0500, Tom Wiltshire <tom at electricdruid.net>, wrote:
> I’m getting the feeling here that just a simple subtraction of the lowpass response from the input isn’t enough (ahem). If you want better than -6dB/oct, you need to do some more sophisticated summing of the input and the various outputs, and you’d better make sure you’ve got accurate resistor values and signal levels too, since that provides a limit to how m such attenuation you can expect in the stop band off you high pass filter. How good is your signal cancellation? -40db? -60dB? Unlikely to be better than that.
>
> That’s not to say that it’s not worth it.. As Paula said, the Xpander service manual is the original source, and Oliver Gillet’s documents on the Four Pole Mission board for the Shruthi expand on that:
>
> https://mutable-instruments.net/archive/documents/pole_mixing.pdf
>
> Basically, generating a multi-pole highpass from a multipole lowpass response isn't as simple as just taking one off the other. These are both great filters and worth doing.
>
> HTH,
> Tom
>
>
> ==================
> Electric Druid
> Synth & Stompbox DIY
> ==================
>
> > On 31 Jul 2017, at 20:50, Julian Schmidt <elfenjunge at gmx.net> wrote:
> >
> > Have a look at this paper on page 64, Multimode Ladder Filter
> > https://www.native-instruments.com/fileadmin/ni_media/downloads/pdf/VAFilterDesign_1.1.1.pdf
> >
> > You can mix the different poles with a simple opamp mixer.
> > input -4 * Pole1 + 6 * Pole 2 -4 * Pole 3 + Pole 4 will yield a funky looking HP filter
> >
> > julian
> >
> >
> > Am 31.07.2017 um 20:10 schrieb Michael Zacherl:
> > > Hi,
> > > I discussed that briefly in one of the last SDIY meetings,
> > > did some experiments but never came to a sound conclusion:
> > >
> > > Is there a ‘cheap’ way of turning a low pass filter into a high pass?
> > >
> > > How’s that possibly approached?
> > > Just something like subtracting the LP’s output from its input signal?
> > > If so, what about resonance?
> > >
> > > My point about doing that cheaply is the (upcoming) release of two synths with Moog ladder filter featuring a switchable high pass mode.
> > > To my knowledge, there was no resonating HP filter from Moog so far in any synth.
> > > Until now.
> > > Since both synths (Moog Mother-32 and Behringer Model D) are on the budget side in terms of money I’d rather expect a simple solution compared to a, say, DSI Pro-2 with two fully featured filters in it.
> > >
> > > Any hints?
> > > Thanks, Michael.
> > >
> > >
> > > --
> > > http://mz.klingt.org
> > >
> > >
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