[sdiy] Obtaining Bandpass and High-Pass Functions From A Four-Pole Low-Pass

Tom Wiltshire tom at electricdruid.net
Mon Oct 15 15:34:49 CEST 2018

Olivier’s article covers the same responses as the Xpander filter, since the Shruthi “4 pole mission" filter was done the same way, with a 8-to-1 switch selecting resistor sets and an extra SPST switch turning the first stage of the filter on or off. With this circuit you can’t have both the input signal and the 4th pole output at once - hence no 4 pole highpass response is possible.

I can’t answer the question about whether Electronotes is the first mention of the technique. It would hardly be a surprise if it was…


       Electric Druid
Synth & Stompbox DIY

> On 15 Oct 2018, at 11:44, Guy McCusker <guy.mccusker at gmail.com> wrote:
> I looked back at Olivier's article as soon as I read Mark's email and
> was a bit surprised to notice that there is no 4-pole HP in there
> either. The difference seems to be that Olivier's circuit does not
> include the input signal as one of the taps to be mixed, so the
> numerator of his transfer function has degree 3 rather than 4, hence a
> 4 pole HP is not available.
> A question for people who know the history: is the Electronotes
> article the first the world saw of this idea? It pre-dates the
> Oberheim stuff by a few years, but perhaps the idea was around before.
> On Mon, Oct 15, 2018 at 11:22 AM Neil Johnson <neil.johnson71 at gmail.com> wrote:
>> Hi Mark,
>>> A while back there was talk about getting a hipass response using cancellation of a lopass filter. I was surprised to find out that you cannot simply cancel a 4 pole lopass to get a 4 pole hipass. In fact, I still can’t really understand why. But, I was reading Electronotes about something unrelated and I discovered an article about just this subject.
>>> In Electronotes #85 there is a technique for getting a 4 pole hipass from a 4 pole lopass by mixing all 4 pole outputs. I still don’t really understand the math, but it is proven in this article. As far as I can see, the Oberheim Xpander filter doesn’t ever get a 4 pole hipass. So, this may be the only implementation of one using pole mixing.
>> Read Olivier's paper here for anotherr perspective on the same approach:
>> https://mutable-instruments.net/archive/documents/pole_mixing.pdf
>> Neil
>> --
>> http://www.njohnson.co.uk
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