[sdiy] Weird Serge Resonant Filter topology in upper bands

[René Schmitz]

On 13.12.2022 07:54, Donald Tillman wrote:
> Hey Aaron,
>
> I don't recognize it either.
>
> And the filter circuit, as presented, doesn't make any sense.  If you 
> work out the transfer function, it's not a bandpass filter; it's some 
> highpass bandpass combo with a low Q.  And like you said, the opamp 
> buffer doesn't make any sense either.
>
> Hmmm, y'know...
>
> If the schematic was in error...
>
> And the filter's output signal was not the opamp output as shown, but 
> instead tapping off of the RC network that leads to the opamp's + 
> input, that would make more sense.
>
> Something like this:
>
>
>
> That's a proper bandpass filter.  And it would require a buffer.


I had a similar hunch, but couldn't quite figure it it out.

When I simulate it as above, I find that the gain is way down in the mud 
at -50-ish dB, and the peak is very flat and broad.

But if you swap + and -, and tap at the -input, then it works like I'd 
expect.


Best,

  René


PS. "Look ma', no algebra"


>
>   -- Don
> --
> Donald Tillman, Palo Alto, California
> https://www.till.com
>
>> On Dec 11, 2022, at 5:11 PM, Lanterman, Aaron D via Synth-diy 
>> <synth-diy at synth-diy.org> wrote:
>>
>> I’ve been looking at the Serge Resonant Filter:
>>
>> https://electro-music.com/wiki/pmwiki.php?n=Schematics.SergeResonantEqualizer
>>
>> The lower 7th bands are standard bandpass Multiple Feedback filters. 
>> But the 3 top band (see rightmost two on the bottom row, rightmost 
>> one on the top row) are something else weird I haven’t seen. They 
>> vaguely remember a Wein-bridge-style filter in the Boss SP-1 Spectrum 
>> pedal, but instead of having a capacitor in series in the positive 
>> feedback loop, Serge has it in series with the input.
>>
>> Does anyone recognize this filter? I have an urge to give things names.
>>
>> I tried to work out the transfer filter under the assumption that the 
>> negative feedback is stronger than the positive feedback so “golden 
>> op amp” rules hold. But when I plot the frequency response of my 
>> transfer function, I get peaks that are way too broad — they’re more 
>> curves than peaks. So I’m doing something wrong somewhere.
>>
>> My transfer function has a numerator of
>>
>> c*s*(r+rp*(1+r*c*s))*rn
>>
>> And a denominator of
>>
>> rp*r^2*c^2*s^2 + (rp*2+rn)*r*c*s + rp
>>
>> Where rn is the resistor in the negative feedback look, rp is the 
>> resistor in the positive feedback loop, and the input resistor and 
>> resistor to ground are the name and denoted by r.
>>
>> Serge doesn’t remember where he got the filter from. He said he went 
>> to this topology for the higher bands because limitations in the open 
>> loop gain bandwidth product prevented the MFB topology from working 
>> well at the higher frequencies.
>>
>> One interesting thing is he felt the need to add an op amp buffer at 
>> the output of the filter, which suggests to me that non ideal op amp 
>> effects are in play.
>>
>> Aaron Lanterman, Prof. of ECE, Georgia Tech
>> My blog on Education and Innovation: 
>> https://edupocalypsenow.wordpress.com
>> My blog on Electronics and Programming: 
>> https://lantertronics.blogspot.com
>> My YouTube channel on Electronics and Programming: 
>> https://www.youtube.com/c/lantertronics
>>
>
>
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--
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