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MXR 6 band EQ filter calculations

Started by gordo, November 13, 2020, 02:25:30 PM

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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...
Gordy Power
How loud is too loud?  What?

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!!)
Gordy Power
How loud is too loud?  What?

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.
Gordy Power
How loud is too loud?  What?