[sdiy] [OT] Analog synths with 2 pole filters

[Scott Nordlund]

My understanding is that a 2 pole filter technically shouldn't self-oscillate. Compare the root locus of 2 pole and 4 pole systems. 4 pole filters oscillate when feedback is increased to the point that two of the poles enter the right half plane (which indicates an unstable system). This never happens with 2 pole filters. Of course this assumes linearity. There's inevitably going to be clipping if the resonance is increased enough, but I think it's really only the designs that have a fairly severe nonlinearity that are going to oscillate. So this is limited to the "cheap and crude filter designs that sound all distorted and cool".

Second, some of the filters shouldn't be included in the list. The "cascade" ones are really 4 pole, tapped at the second stage. It's a 2 pole response with no resonance, but it's still a 4 pole filter. An exception to this is the filter in the Korg Poly 61, which is a sort of 2 pole cascade (which I think doesn't oscillate).

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> Date: Thu, 21 Feb 2013 14:58:05 +0800
> From: andy at cytomic.com
> To: synth-diy at dropmix.xs4all.nl
> Subject: [sdiy] [OT] Analog synths with 2 pole filters
>
> This is a repost with [OT] in the subject, sorry for not adding that
> in the first place.
>
> Hi Guys,
>
> I'm having a discussion with a friend who said that "most analog
> synths with 2-pole filters won't self-oscillate". I know quite a few
> analog synths with 2-pole filters that do self osc, so I thought I'd
> write up a list to shed more light on the situation.
>
> Can you guys please help me fill in the considerable missing gaps by
> naming some more and/or correcting what I already have? I know that
> you can make ladder / cascade / svf / sallen key 2-pole analog filters
> that self oscillate, this is really just seeing how most production
> synths have been designed.
>
> Analog synths with 2 pole filters that self oscillate
> --
> Korg MS-10 / MS-20 / Monotron / Monotribe (sallen key)
> Steiner Parker Synthacon (sallen key)
> Oxford Synth Company OSCar (svf)
> Electronic Dream Plant Wasp (svf)
> Oberheim Xpander (cascade with summed taps)
> Formanta Polyvoks (svf)
> Arturia Minibrute (sallen key)
> Elektron Analog 4 (svf)
> Dark Energy v2 (svf)
> Future Retro XS (svf)
> ARP Odyssey whiteface mk1 (svf)
> Oberheim OB-Xa (svf)
> Oberheim OB-8 (cascade-ish - need to adjust res trimmers)
>
> Analog synths with 2 pole filters that don't self oscillate
> --
> Oberheim SEM (svf)
> Roland JP6 (svf)
> Alesis A6 (svf)
> Prophet 8 / Prophet 08 (cascade)
> Yamaha CS 80 / CS 40 (svf)
>
> Thanks!
>
> Andy
> --
> cytomic - sound music software
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