[sdiy] Article "Analyzing the Moog Filter"

Dave dlmanley at sonic.net
Fri Aug 23 18:56:11 CEST 2019


How about the programmable op amp filters based on the lm4250? 

On August 23, 2019 9:09:48 AM PDT, Quincas Moreira <quincas at gmail.com> wrote:
>Don’t forget the Leapfrog topology as implemented by Matthew Skala.
>
>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
>
>Sent from my iPhone
>
>> On Aug 23, 2019, at 11:57, Donald Tillman <don at till.com> wrote:
>> 
>>> On Aug 22, 2019, at 11:30 PM, Brian Willoughby
><brianw at audiobanshee.com> wrote:
>>> 
>>> First of all, where is the specification for the Moog Ladder Filter
>that shows it having accurate voltage control over a defined range? Is
>it really that accurate? Doesn’t it require calibration periodically?
>>> 
>>> How bad is the CV accuracy for other filters? Don’t the others
>calibrate to the same end results?
>> 
>> If a filter is tuned with a transistor's exponential
>base-to-collector function, then it will probably be accurate enough to
>throw into oscillator mode and be used to play a melody.
>> 
>> That's not the case with filters tuned with diodes, FETs or vactrols.
>> 
>> That said, if I'm buying something called a "filter", then it's not
>reasonable to assume that I can play a melody on it.
>> 
>> 
>>> Second, if you were to build a 4-pole SVF, would it be considered
>two 2-pole low-pass sections in series, or would the two dual
>integrators make some folks call it “four” single-pole low-pass
>sections in series?
>>> 
>>> Since I haven’t tried this, I’m not familiar with how global
>feedback might cause problems with 2 SVF in series. I do recall some
>nice discussions around here, though.
>> 
>> There are multiple ways to build a 4-pole SVF. 
>> 
>>  -- Don
>> --
>> Donald Tillman, Palo Alto, California
>> http://www.till.com
>> 
>> 
>>> 
>>> Brian
>>> 
>>> 
>>>> On Aug 22, 2019, at 11:05 PM, Donald Tillman <don at till.com> wrote:
>>>>> On Aug 22, 2019, at 6:07 PM, Tom Wiltshire <tom at electricdruid.net>
>wrote:
>>>>> 
>>>>> 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?
>>>> 
>>>> Consider a description of a filter as a sort of "taxonomy" with
>three layers:
>>>> 
>>>>   Top Layer: the filter spec, number of poles, response
>>>> 
>>>>   Second Layer: the topology that implements that filter function
>>>> 
>>>>   Bottom Layer: implementation details, including the control
>element
>>>> 
>>>> So a Moog Ladder would be:
>>>> 
>>>>  Top Layer: 4 pole, low-pass, with resonance
>>>> 
>>>>  Second Layer: 4 single-pole low-pass sections in series, with
>feedback
>>>> 
>>>>  Bottom Layer: the ladder circuit
>>>> 
>>>> And a State Variable filter would be:
>>>> 
>>>>  Top Layer: 2 pole, multi-mode
>>>> 
>>>>  Second Layer: 2 integrators and an inverter, in a loop
>>>> 
>>>>  Bottom Layer: the circuit, perhaps OTAs 
>>>> 
>>>> And so forth.  
>>>> 
>>>> This analysis also works really well with oscillators and other
>functions.
>>>> 
>>>> Here's a Moog style VCO:
>>>> 
>>>>   Top Layer: VCO with sine, square, triangle, sawtooth waves
>>>> 
>>>>   Middle Layer: block diagram with exponential current source,
>sawtooth core, waveshapers
>>>> 
>>>>   Bottom Layer: the circuit details
>>>> 
>>>> So if I dismiss the implementation details, as defined this way, it
>limits the number of unique filter designs.
>>>> 
>>>> You know I'm a big fan of implementation details.  And you'd want
>to make sure that the implementation details didn't have a significant
>functional effect as you draw these lines.  That's part of the craft.
>>>> 
>>>> But if I'm characterizing filter types, I think it's reasonable to
>pay attention to the implementation topology and ignore the
>implementation details.
>>>> 
>>>> -- Don
>> 
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