Buffer less filter stages ,was stupid buffers, Oberheim Xpander.

[Bjorn Julin]

Hi Jurgen dude and list duedes/dudettes!

>JH wrote, why using stupid buffers!

>All right, for the last of 4 stages you probably want a buffer stage,
>because you're driving the feedback loop, and a VCA and whatever.
<But there would still be no reason for a different cap value and for
<resistor dividers, no ? One could just insert a FET (and source resistor), 
and then drive the OTAs for VCA and VC feedback with 20mV as well.

>>BUT ! (And that's a rare occasion when you'll find me using capital 
>> >letters) BUT if you do this for the last stage as well, you get some 
>><feedthru from the input *backwards* thru the feedback OTA to the <output. 
>>The result is that your filter will not sound as smooth and <"dark" as it 
>>should The rolloff has the 24dB / oct at first, but <after a while it 
>>becomes flat and you get your high frequencies thru <with a fixed 
>>attenuation of a few dozen dBs. That's why I think the <chip designers 
>>used a different scheme for
<<the last stage. There is even a chip intended for 2pole and 4pole <use 
which has alternately unbuffered and buffered OTA stages. (Might <be in the 
JP-6, but I'm not sure.)

>I may be completely wrong with my interpretation of course, but then >at 
>least the explanation fits nicely in retrospect. I'd love to see >this 
>confirmed or denied from SSM and CEM designers - let's see if >there will 
>be some reaction.

You are not wrong in your conclusion!
Just take a look at patent no US04514704 of Mr Curtis.

Personaly i think its a wery nice patent, SSM tok their way of
implementing a "sort of" variation on the moog ladder, Curtis was cerinaly 
influenced by the moog ladder but this is more a traditionaly OTA filter. He 
has removed the buffers between each GM cell, and as Juergen speculates Mr 
Curtis uses just a final buffer at stage 4,
who can be removed if a high impadence load is driven!

This filter is a second generation with some wery interesting compensation 
cirquits because of some artefacts due to buff'les GM stages. Perhapps some 
of you academic educated dudes understand this better then me but he talks 
about compensating errors in each filter cell by using biases on each stage.

But why does he use only a tracking bias compensation cirquit for
the first GM cell(input), and static compensation for the remaining?
I asume the error adds up due to +N stages and transported from cell to 
cell?

Another thing is that there is a third generation filters,the ones
who compensates for pass band loss as Q increases, im most eager
to see how Curtis and SSM have done in their pass band compensation cirquit? 
did they use input amplitude increase/reduction? Variable pole feedback? 
Anyone who have been looking into that?

I was thinking of do a discreete version of this patent, is it OK to use a 
resistor as bias cirquit (point 164) for each stage or would it be more 
aproiate to have a constant currents source instead due to variation in the 
input impedance on the nex following GM cell?

Besides how much do the impedance variate?

BJ
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