Inverting Active Band Pass Filter - MID EQ

[Lazybone]

Hello dear friends.

I'm on my way modding an overdrive pedal. Tone control was blunt and dull so I went for a 3-way EQ done with an OPAMP stage (active filter) and I kinda messed up,
givin way more bass and treble than the mids that I so adore.
I provide an influence for that and I want someone to help me tune my MIDDLE control. I am trying to understand the math formulas on how to tune it with paper-pen-calculator.
I study electronical engineering but I think I missed something in there.





My calculations where these:
Fc1=1/(2*π*(R14+R11+R12)*C7)
Fc2=1/(2*π*(R14+R10+R9)*C8)

Is this correct? I am trying to control only the mid frequencies, like a mid booster.
I chose this type of filter because it suits the original pedal's circuit, using an OPAMP for the tone filtering stage.
I can provide more if needed. My brain is going to snap!

[Rob Strand]

My calculations where these:
Fc1=1/(2*π*(R14+R11+R12)*C7)
Fc2=1/(2*π*(R14+R10+R9)*C8)

Is this correct? I am trying to control only the mid frequencies, like a mid booster.
I chose this type of filter because it suits the original pedal's circuit, using an OPAMP for the tone filtering stage.
I can provide more if needed. My brain is going to snap!

Formulas are no good for the three band there's toooooo much interaction between the bands.
You need to use Pspice.   Tweak the resistors to determine the boost/cut and tweak the caps to get the frequencies.
The cap ratio of the mid band caps has a strong effect on the amount of mid boost.
It also sets the Q of the mid band.  However the Q is quite low and not very tunable.
Also, if the bass frequencies or treble frequencies overlap the midrange too much they have a strong
effect on the mid frequency.   The circuit is "all connected".

[Lazybone]

This is the new circuitry.
I use passive LPF and HPF filters for Treble and Bass because I don't really need to boost these frequencies, just remove the annoying ones in these bands.
I kept the Middle control from the 3-way EQ. This is still on paper though.

What I want is to filter out bass-treble and boost mid frequencies.
The Opamps are from the original circuit. The left amp is the clipping stage while the other one used to house the Tone control.

[Rob Strand]

I can add some info.   Go to this thread.  About halfway down the fourth page I posted some stuff.
There is a paper by Greiner that gives formulas for common mid-boost/cut eq.
As I recall there's some bugs in the formulas so you have to work through them and fix them.

http://www.diystompboxes.com/smfforum/index.php?topic=24359.60

You can estimate the frequencies for the three-band like this:
- Assuming the bass cap is a short (?), convert the Y network formed by the bass control to a delta network.
Use the Y to delta transformation.    The result of that will be three resistors.   The two that connect
to the -input of the opamp form the same two resistors in the standard mid-control circuit
- Assuming the treble cap is open, complete remove the treble control.
- Use the corrected formulas in the Greiner paper to computer the Mid frequency.
-  Do not use the Greiner paper Q formulas they will have too much error.
- Do not follow the recommendations about the cap size or ratio from the Greiner as the
conclusions are completely wrong for the three-band

I know this procedure works at least for sensible circuit values in the three-band.  If you just chuck
component values into the formulas the approximations are likely to fall in a heap.

[Rob Strand]

This is the new circuitry.
I use passive LPF and HPF filters for Treble and Bass because I don't really need to boost these frequencies, just remove the annoying ones in these bands.
I kept the Middle control from the 3-way EQ. This is still on paper though

Sorry you posted just before my last post.

You will need to add some buffering between each stage.   You might be able to squeeze keeping the LPF and HPF connected together if you make the impedance of the second filter higher than the first to reduce the loading.

For the mid control see the Greiner paper.  For the separate mid control you can follow the recommendations in the paper.  However you don't *have* to stick to them.   Also it is easy to tweak this mid boost/cut circuit quickly in pspice.   I used to write C programs to simulate & tweak those things and it's just unnecessary when you have pspice.

[Lazybone]

This is my new take on the filter.

1) Treble - Bass and an active Mid control. I am wondering if this would work better with the treble-bass filters after the active mid control.
2) It's a parallel Treble-Bass filter and the active Mid control.
3) This is number 2 with a Jfet Buffer for the tone output.

Too late tonight, I am going raster tomorrow morning. Any thoughts on these?

I want to remove low and high frequencies from my sound but I want to have flexibility on mids so the active filter is for scooping or boosting and that I don't know if is going to work properly.
Any ideas on the middle freq boosting/scooping (on one knob) would be a lifesaver.
Thanks and goodnight!

[Rob Strand]

Any thoughts on these?

1) Won't do what you want.   (There a few bugs as well.)
3) Better than (2).   You don't want the mid control to load the filters as it
   won't behave like you expect.

I want to remove low and high frequencies from my sound but I want to have flexibility on mids so the active filter is for scooping or boosting and that I don't know if is going to work properly.

Roughly there's a few ways to control bass and treble. It depends what you want.
Without getting caught up in the finer points:
1) High and Low pass filters.  Like you first idea.  These removed the extremes.  A variable frequency
    cut-off lets you trim how much of the extremes is removed.
2) Standard shelving bass/treble controls.    Bass boosts/cuts below a preset frequency.
    Treble boosts/cuts below a preset frequency.    "preset frequency" is used very loosely here.
    The controls affect the ends and the part in the middle is left largely unchanged.
3) Peaking bass/treble controls.   Bass boosts/cuts at and around a preset frequency.
    Treble boosts/cuts at and around a preset frequency.
4) Band Splitting Bass/treble.   This is like the types you have suggested in your last post.
    Bass controls the level of everything below a specific frequency.   Treble control the level
    of every above a specific frequency.   Depending where you put the frequencies
    you end-up with a mid cut, flat, or mid boost.
    See fig 12 and fig 14
     http://www.guitarscience.net/papers/bigmuff.pdf

Check out
https://www.sweetwater.com/insync/equalization-a-primer/

Any ideas on the middle freq boosting/scooping (on one knob) would be a lifesaver.

The mid circuit you have  (which is missing two resistors to the opamp - input)  will give a mid boost
cut.   The Q is about 0.8 to 1.2 and sounds fairly natural.  If you want a high-Q/narrow scoop then this
circuit wont do it.

[Lazybone]

Having problems...