[sdiy] Serge Resonant Equalizer mystery solved

[Lanterman, Aaron D]

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
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