[sdiy] HP from LP ?

David G Dixon dixon at mail.ubc.ca
Tue Aug 1 02:29:38 CEST 2017

As others have said, it depends on the filter.

For a four-pole cascaded-stage filter, you can get four different highpass
modes by summing the filter stage outputs and the feedback + input in
various gain ratios.  If the gain ratios are given for the {Feedback+Input,
Stage 1 Output, Stage 2 Output, Stage 3 Output, Stage 4 Output}, then:

 6dB HP: {1,1,0,0,0}

12dB HP: {1,2,1,0,0}

18dB HP: {1,3,3,1,0}

24dB HP: {1,4,6,4,1}

For example, the 24dB HP can be realized by taking the 5 signals into an
inverting opamp with a 30.1k feedback resistor with the following 1%
standard input resistor values:

{30.1k, 7.50k, 4.99k, 7.50k, 30.1k}

The challenge is to find a set of resistor values which give the proper
ratios.  I've used 30.1k for the Polaris (which generates about 22 different
modes using this scheme) and have found 1% values which get close for all
the required ratios (1, 2, 3, 4, 6, 8, 12, 16, 24 and 32).

Of course, an SVF gives a highpass output directly.

> -----Original Message-----
> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On 
> Behalf Of Michael Zacherl
> Sent: Monday, July 31, 2017 11:10 AM
> To: synth-diy mailing list
> Subject: [sdiy] HP from LP ?
> 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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