MXR 6 band EQ filter calculations

[gordo]

I'm trying to change the frequency bands on an MXR 10 band EQ.  The first 9 bands make sense in terms of the math.



If each of the bands is considered a Sallen-Key high pass then tweaking, for example, the C23/C24/R26/R27 values will shift the 8k band frequency and Q-factor.  That part makes sense and I can calculate what to change to shift.  What has me totally confused is how the 10th band is calculated.  Is it just a high pass filter?  If so how do I calculate it's specs?  I'm missing how it works without it's own op amp...

Thanks if you can point me in the right direction.  I'm starting to understand how little I understand...

[mauman]

Yup, just a shelving low-pass filter, which doesn't need the op amp gyrator (fake inductor) like the passbands do.  470R and 22nF set the knee @ 15.392 kHz.  If you delete all the other 9 band filter hardware and hook RV10 lugs 1 & 3 directly to pins 2 & 3 of U5A op amp, it might look more familiar.   Mike

[gordo]

Damn Mike, I still can't get the math to work.  How are you coming up with that?  (And thanks very much for the help!!)

[mauman]

Hey Gordy, I usually cheat and use a calculator.  If you put in the values of C25 (.022 uF) and R28 (470R) here, it will give you the corner frequency:  http://www.muzique.com/schem/filter.htm . 

The underlying math is shown here:  http://sim.okawa-denshi.jp/en/CRlowkeisan.htm , for low-pass filters the corner frequency Fc in Hz is 1/[2piRC] (one over two pi R C) with R in Ohms and C in Farads.  In this case, Fc = 1 divided by (2 * 3.141 * 470 Ohms * 0.000000022 Farad) = 15,395 Hz.  Mike

[gordo]

Brilliant!!  I was using the okawa-denshi sim as well.  Thanks for clarifying.