[sdiy] Serge Resonant Equalizer mystery solved

[Michael E Caloroso]

Nice find.  And a very good example of why one should never take schematics
for granted.

MC

On Mon, Dec 19, 2022 at 10:28 AM Benjamin Tremblay via Synth-diy <
synth-diy at synth-diy.org> wrote:

> I am working on a Multivox FR6M (TR-66) which has a mix of active and
> passive tone generators. It’s clear the only reason the inductors were used
> was to save cost when semiconductors were just a tiny bit more expensive
> than coils. This unit mostly uses active components to achieve the T filter
> circuits, but op amps were not used. Clearly, by the time op-amps were
> cool, there was no reason to consider using inductors. (I’m guessing coils
> used to be made in high volumes.)
>
> On Dec 19, 2022, at 4:09 AM, Eric Schlappi via Synth-diy <
> synth-diy at synth-diy.org> wrote:
>
> For those also about to google, the TI app note referenced (I think) is
> sboa093a
> https://www.ti.com/lit/pdf/sboa093?keyMatch=SBOA093A
>
> and it has some delightful illustrations in it
> <image.png>
>
> thanks for all the research Aaron, this is great stuff!
>
>
> On Mon, Dec 19, 2022 at 9:27 AM Lanterman, Aaron D via Synth-diy <
> synth-diy at synth-diy.org> wrote:
>
>> So, yes, the top three filters on the CGS schematic are incorrect. And it
>> looks like that error long predates Ken Stone’s schematic; Serge found a
>> pencil sketch of the circuit that had the correct connection from the
>> positive input of the 1st op amp to the input of the 2nd, but he had erased
>> it and erroneously connected the output of the 1st op amp to the input of
>> the second.
>>
>> I finally found it on the filter on this TI app note (have to take Figure
>> 5-5 and substitute it into Figure 5-10 to see it; it’s also Circuit 12 on
>> p. 78), where it goes by the name "INIC Band Pass Active Filter,” where
>> INIC stands for "ideal current-inversion negative-immittance converter.”
>>
>> So it’s like the filter in a Wein bridge oscillator, but with this INIC
>> thingamabob stuck in the middle.
>>
>> Someone else discovered it in Fig. 8.16 on p. 308 of “Burr-Brown
>> Operational Amplifiers Design and Applications” by Graeme & Tobey. (The TI
>> app note was originally a Burr-Brown app note, so that makes sense). Serge
>> recalled having a copy of that book, and he’s pretty sure that’s where he
>> must have seen it.
>>
>> Serge went to this topology for the higher bands since he had trouble
>> with limitations of the gain bandwidth product of the op amps he was using
>> from giving him the performance he wanted with the multiple feedback
>> topology he used in the lower bands.
>>
>> 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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