[sdiy] Serge Resonant Equalizer mystery solved
btremblay at me.com
Mon Dec 19 16:23:36 CET 2022
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 <https://www.ti.com/lit/pdf/sboa093?keyMatch=SBOA093A>
> and it has some delightful illustrations in it
> 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 <mailto: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 <https://edupocalypsenow.wordpress.com/>
> My blog on Electronics and Programming: https://lantertronics.blogspot.com <https://lantertronics.blogspot.com/>
> My YouTube channel on Electronics and Programming: https://www.youtube.com/c/lantertronics <https://www.youtube.com/c/lantertronics>
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