[sdiy] VC Sallen-Key with a 2164
Andrew Simper
andy at cytomic.com
Wed Apr 27 04:53:36 CEST 2016
PS: the first stage in Olivier's circuit optionally switches out the
first capacitor, which is what the Oberheim XPander did. This is the
take weighted sums of the 4 output stages to generate different
responses, so just leave that cap in the circuit like the other stages
for what I propose.
On 27 April 2016 at 10:48, Andrew Simper <andy at cytomic.com> wrote:
> Hi Pru,
>
> There is no way to build an MS20 type structure with virtual grounds,
> but there is another structure I came up with independently and have
> called SMP in my software. The equations are:
>
> 0 = g (v0 - v1 + k (v1 - v2)) - s C (v1 - 0)
> 0 = g (v1 - v2) - s C (v2 - 0)
>
> but because each stage has an inverting op-amp virtual ground buffer
> this turns into:
>
> 0 = g (v0 - v1 + k (v1 + v2)) - s C (v1 - 0)
> 0 = g (-v1 - v2) - s C (v2 - 0)
>
> where g is the cutoff gain, k is the resonance gain (from 0 to 2), and
> C is the value of the capacitor, which in turn alters how much g needs
> to be.
>
> Olivier has done a 4 pole cascade using 2164, so the setup is similar
> but you just need the first two one pole low pass stages, but the
> resonance path becomes the sum of the first and second stage outputs
> (which is actually the difference because of the inverting buffers),
> gained up by the resonance amount, limited in some way, and fed back
> to the first stage input:
>
> http://mutable-instruments.net/static/schematics/Shruthi-Analog-PoleMixing-v02.pdf
>
> All the best,
>
> Andrew Simper
>
>
> On 27 April 2016 at 08:53, prudence prendergast
> <prudence.prendergast at gmail.com> wrote:
>> long time, first time.
>>
>> i was hoping for some advice. is there a way to build a VC sallen-key
>> low pass filter with a 2164? since 2164s require a virtual ground at
>> the output, the matter seems complex.
>>
>> i think dave dixon might have written an article on feeding true
>> buffers with the 2164, if he's here to weight in.
>>
>> -pru
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