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.
(https://www.gordomusic.com/Madbean/MXR-10band.png)
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...
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
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!!)
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
Brilliant!! I was using the okawa-denshi sim as well. Thanks for clarifying.