graphic equalizer computations

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stepank

Joined Apr 19, 2018
8
hi -
i'd like to re-design this schematic, with different frequency values. i have found some equations for "multiple-feedback bandpass filters" used in graphic equalizers, but i think this one in different. can anyone tell me about the topology or how to "solve" for it?

electroharmonixgraphiceq.jpg

thank you very much.
 
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danadak

Joined Mar 10, 2018
3,913
I at first thought it was a gyrator, but then that would, in the circuit
config, create a notch filter rather than BPF. A series L with C1.


Regards, Dana.
 

crutschow

Joined Mar 14, 2008
24,405
The circuit is indeed a gyrator and it generates a notch or a peak as determined by the setting of the pot with 0dB at the 50% setting.
The LTspice simulation below is for one section with 1kHz values for pot settings of 0, 25, 50, 75 and 100%.

Note that all the opamp input polarities except the one on the left are incorrect on the original schematic.

Also I reduced the value of the pot to 2kΩ to get a more linear adjustment range.
With a 100kΩ pot there was little adjustment between the 10 and 90% range of the pot.

This will help in calculating the values for a frequency other than given in the chart.
Note that R1 and R2 are reversed from the chart and this schematic.

upload_2018-4-23_10-40-57.png
 

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