[sdiy] Weird Serge Resonant Filter topology in upper bands

[René Schmitz]

On 15.12.2022 10:43, Lanterman, Aaron D via Synth-diy wrote:
>> On Dec 15, 2022, at 2:56 AM, Guy McCusker <guy.mccusker at gmail.com> wrote:
>>
>> Isn’t Rod Elliott’s Fig 7 exactly what Don drew?
> Yup! But it looks like if you try it with the values given in the Serge Resonant Filter schematic it’s not quite peaky enough. Rene’s trick of flipping the + and - inputs gives the correct Q.

I managed to get the same response now with the inputs arranged as in 
Don's and Elliots drawings. With the values from the Serge 2.8kHz band.

With the AD549 as before it did not work that way round, but after 
changing it to an AD711 it did. Neither model worked for both 
arrangements...

This circuit is jinxing ltspice it seems.

>
> Now, I tried the Elliott/Don schematic with the values in the Elliott schematic and it did give a super peaky filter. I guess the devil is in the details of what values you pick.
>
> Although, it kind of boggles my mind that the filter operates in a linear fashion either way. I’d think that one of the ways should make it freak out and either rail out or turn into an oscillator. I still feel like I’m missing something.

I think it could get stuck near the rails with too much positive 
feedback. Which also would explain the weird AC plots.


Best,

  René


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