[sdiy] Shruthi 4PM (was re something else...)

[Olivier Gillet]

>  but does its 4 pole LP setting really have the same qualities as a normal 4pole OTA filter?

The difference is that the 2164 is linear so it doesn't bring the
slight coloration of OTA filters when they are used closed to their
saturation point. I still kept an OTA in the resonance feedback loop
since it works very well as the non-linearity element that prevents
the resonance from blowing up. Speaking of OTAs as a gain control
element for resonance: the fact that it has differential inputs, to
subtract the original signal from the feedback signal and get loudness
compensation is another thing I like with this configuration.

The same design, without the fancy pole-mixing, is used in my Eurorack
module Ripples, and it looks like people are quite happy with it. I
also use it on the Ambika (the "4P" voicecard) - this gives a
well-behaved, low-part count 4-pole filter.

Olivier

On Wed, Nov 27, 2013 at 11:33 PM, Michael Bachman <bachmanm50 at gmail.com> wrote:
> I am thinking of building the 4p Misson board. (or some variant)
>
> What I wonder is, with all those mixing resistors in the circuit
> bleeding between pole outputs, what affect it may have.
>
> True , if it provides a wide range of nice sounding filter functions,
> then it has value to build, but does its 4 pole LP setting really have
> the same qualities as a normal 4pole OTA filter?
>
> Mike
>
> On Wed, Nov 27, 2013 at 2:06 PM, Dave Manley <dlmanley at sonic.net> wrote:
>>
>>
>> Tom Wiltshire <tom at electricdruid.net> wrote:
>>
>>>2) THAT2162 is non-inverting. There are a number of occasions where the
>>>inverting nature of the 2164 is very helpful. Olivier Gillet's "Four
>>>Pole Mission" Xpander-style filter is a good example:
>>>
>>>http://mutable-instruments.net/shruthi1/build/4pm
>>>
>>
>> This is a very nice implementation.
>>
>> One thing I've droned on in the past about is the sensitivity of the different modes to the summing resistor tolerances.  With exact values the calculated responses are obtained.  With real world tolerances perhaps not so much.  Has anyone measured the actual filter responses to see what they look like?
>>
>> It would be interesting to see the results from multiple boards.
>>
>> Dave
>>
>> P.S. yes, before anyone states the obvious, all that really matters is what it sounds like.
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