I tried using both Rod Elliott's MFB-filter program and the spreadsheet from Project 136 to calculate the filters. Both of these tools give negative resistor values for R2 when Q=.7 and the gains & center frequencies needed for the Palette controls are input. Did you end up using non-equal C MFB filters instead? There are solutions with non-equal C filters.
I will check up what values I used exactly at some point, but what you also should know, is that I put the gain in some separate opamps, and let the filters be filters.
Thinking back I even think I calculated each filter to be -6dB down and added that to the gain, because the filters had both high-pass and low-pass attacking the same frequency ( -3 +-3dB = -6dB)
Here are my filter values attached.
Notice the Q of 0.707 of all the filters, which gives a gain of 0.5 in the filter stage. This must be made up for in the "input/output stage" circuitry of Rod Elliots project 75.
Well...it is on the shelf.
It was working fine but I build another preamp (...can't stop building new projects )
My hope was to remake it with discrete jfet circuits (no opamps) as the sound is not the best I have ever heard if used without setting the EQ pots.
But I guess upgrading the opamps could also make it happen as well, so if you want to make PCB for the opamp version and share with the group, then please go ahead.
If you make it for 8 pin dual opamps it would be very practical, because then people can use their favorite opamp.
Here is a version done with state variable filters. The problem with other designs presented is that the 'Q' did not match that of the Cello. This one is a subtle variation on the Barry Porter studio parametric EQ, a well known classic circuit. I have built this circuit before and was really happy with the results. Unfortunately a stereo version contains lots of opamps. You would want to use quiet ones such as the LM4562. I have a stereo version using all thru hole parts in the works. The PCB is out for fab now. The PCB for both channels fits into a 1U chassis.