Modifying 4-band semi-parametric to fully parametric with shelving?

[diymixer]

hey all-

I found a schematic for my mixer, but the only problem with it is that it's four bands of peaking, semi-parametric equalization. (schematic shown as an attachment below)

What I would like to do is add bandwidth control (I would prefer bandwidth, but Q control also works for the purpose of the mixer) and also make bands 1 and 4 switchable between shelving or peaking. In peaking mode, the bandwidth knob would be active so I would still have fully-parametric equalization when in peaking mode.

How would one go about modifying the EQ?

thanks,
Mike

P.S. How do I change the frequency range? I know it has something to do with the caps, but what's the formula between farads and frequency range?

 

[Mooly]

You need to read up on "gyrators" as this is how your equaliser operates. It "synthesizes" an inductor by using opamps instead.

http://cas.web.cern.ch/cas/Denmark-2010/Caspers/gyrator overview from web CAS2010.pdf

Gyrator - Wikipedia, the free encyclopedia

Also this might be of interest, the last part,
Spectrum Analyzer and Equalizer Designs

 

[diymixer]

the equalizer in that last link sounds perfect except for two things:
-there's still no way to make the first and last bands shelving
-how does one change the upper and lower extremes of the frequency range?

also, would I connect the signal ground from the EQ to the mixer's main signal ground bus?

 

[Mooly]

The values of R and C in each gyrator determine the frequency of operation. Typical values such as,
10-Band Graphic Equalizer Using Gyrator Circuit Diagram | circuitwiringdiagram.com

"also, would I connect the signal ground from the EQ to the mixer's main signal ground bus?"

Signal grounds are normally connected together paying attention to correct practice... no loops etc and returning critical signal grounds to a common point or star.