[sdiy] Article "Analyzing the Moog Filter"

Tom Wiltshire tom at electricdruid.net
Fri Aug 23 03:07:44 CEST 2019


I don’t like to disagree with you, Don, but I’m not sure what you’re thinking. Moog or SVF are definitely *not* the only two filter options.
OTA+cap-to-ground+buffer? VCA+Integrators?

I think you’re being a bit unfair to the rest of the synth filters. Ok, so most of them are just 4-pole lowpass, but that doesn’t mean they’re all Moog ladders. Most of them *aren’t*. And the State Variable makes a reasonable appearance by virtue of its versatility, but is more common in modular synths than in any classic keyboard.As we know, Oberheim was the only manufacturer who pushed 2-pole filters, and even then the later instruments offered that as an option, not the only choice. The famous Rolands (Jupiter8, Junos, SH101, etc) Korg (Polysix, MonoPoly, Trident) Sequentials (Prophe...Everything…) are all four-pole OTA filters. There are one or two outliers like the Jupiter 6, but they are outliers for exactly that reason. For the whole era of “classic polysynths” only Moog could produce Moog filters since the patent was still in force. And the SVF was a possibility, but an unusual choice. Hence there aren’t many that use one of those two options alone.

Tom

==================
       Electric Druid
Synth & Stompbox DIY
==================

> On 22 Aug 2019, at 21:51, Donald Tillman <don at till.com> wrote:
> 
> I think he's got a point.  How many filter topologies are there that are analog, audio range, and accurately voltage controlled over a substantial range?   I count two.
> 
> The Diode Ladder is essentially derivative of the Moog, even though it works differently.  
> 
> There are VCFs based on the Sallen-Key, or any other active filter topology, hacked up for voltage control, sure.  Though they have less precise tuning.
> 
> In practice, don't the Moog Ladder and the State Variable account for 95% or more of the filters used in synths?  What am I missing?
> 
> I have several filter topologies of my own, of course, so I'm not saying there are inherent limitations or anything.
> 
>   -- Don
> --
> Donald Tillman, Palo Alto, California
> http://www.till.com
> 
> 
>> On Aug 22, 2019, at 12:52 PM, Kylee Kennedy <kmkennedy at gmail.com> wrote:
>> 
>> Honestly it was hard to get past "Learn about one of the only audio filter topologies out there, the Moog filter!"
>> 
>> Kylee
>> 
>> 
>> On Thu, Aug 22, 2019 at 12:33 PM Ian Fritz <ijfritz at comcast.net> wrote:
>> Not many synth filters? That could make many people unhappy. Oh well 
>> it's just a kid.
>> 
>> Ian
>> 
>> On 8/22/2019 12:51 PM, Ingo Debus wrote:
>> > Anyone seen this?
>> > https://www.allaboutcircuits.com/technical-articles/analyzing-the-moog-filter/
>> > https://www.allaboutcircuits.com/technical-articles/small-signal-open-loop-transfer-function-moog-filter/
>> > 
>> > Ingo
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