[sdiy] Article "Analyzing the Moog Filter"

Donald Tillman don at till.com
Fri Aug 23 21:09:10 CEST 2019

On Aug 23, 2019, at 9:09 AM, Quincas Moreira <quincas at gmail.com> wrote:
> Don’t forget the Leapfrog topology as implemented by Matthew Skala.

I don't know it.  'Gotta schematic?

> Also the Steiner Parker, the Wasp filter. The EMS diode ladde and Arp 2600 filters maybe closely related to the Moog but there are differences. Still cool to see a young mind tackle this analysis

The Steiner Parker is a rare exception.  It's a classic Sallen-Key filter hacked up with biased diodes as controlled resistors.

The Wasp filter is a State Variable with 4069 inverters replacing the inverting opamp in the integrators.  Certainly the overdrive characteristics of the 4069 inverter are different than an opamp, but it's in a local feedback loop, and in a global feedback loop, and I think the OTA overdrive will predominate anyway.

The EMS diode ladder is the same as the Roland diode ladder, and they're both Moog Ladder knockoffs, knocked off sufficiently to get around the patent.

The original ARP2600 filter is a clone of the Moog Ladder.  

The later ARP2600 filter has the same filter topology, but implemented inelegantly to get around the patent. 

So the last three are, in fact, Moog Ladders with legal issues.  :-)

  -- Don
Donald Tillman, Palo Alto, California

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