Is this parametric EQ schem suitable for guitar?

[Morocotopo]

Hi people, I found this parametric EQ circuit, and I want to know if it is suitable for making it into a stompbox. I see no pulldown resistors, they should be added, right? And I don´t know if the circuit has input/output buffers (I don´t understand this circuit very well, or most of them... ), AND I don´t know if the input / output impedance is right (don´t know how to calculate it...). Anything else to take into consideration? Any help is greatly appreciated. By the way, I´ve built the geofex parametric EQ (if anyone wants a PCB layout, I can post it), it works allright, but what I like about this one is that it covers the whole freq spectrum (might be harder to dial the freq exactly, though, too much sweep in a single knob...) If confirmed as usable, I will make a PCB layout.
Oh, and would it be possible to make the 47K gain pots 50K? (The blue outlines are just for my reference, don´t pay attention to them)
Thanks in advance.

Morocotopo

[R.G.]

It's a nice setup. Looks well behaved on the sim. Simulated inductor setup. It uses a lot of opamps and pots, though. But the performance might be worth it.

You need to read the article on "wrappers" at GEO for all of the things that go around a basic circuit. It needs an input buffer to present a high impedance to guitar signals, power supply and switching, indicators, all that kind of stuff. The pulldown resistors go on the outsides of any buffer and output stage.

It will have a limited range of boost and cut if you power it from 9Vdc and ground. The raw circuit shows gains of about +/- 20db. That's enough to force distortion on peaks with only a 9V supply . It wants at least a charge pump inverter to get -9V for a bipolar +/-9V supply.

[Morocotopo]

So, I think, I could use the adjusticator circuit for before/after buffering, the usual switching/LED/power switching/pulldowns/etc, and a 7660S power inverter, and it would work? With the power inverter, I would connect -9V to all ground points, right? This circuit doesn´t seem to use 1/2 Vcc, so I wouldn´t need a Vr circuit, right?
Sorry for the many questions...thanks for your answers R.G.

[Transmogrifox]

I would connect -9V to all ground points, right? This circuit doesn´t seem to use 1/2 Vcc, so I wouldn´t need a Vr circuit, right?

If you used a charge pump to get -9V, you would want to ground the battery (-) terminal.  Then you would have +9V, -9V, and a point 1/2 way in between that you could reference to ground.  This is what 1/2 Vcc circuits are simulating.  The 1/2 Vcc circuit essentially makes a +/- 4.5 bipolar supply out of a 9V battery.  The caveat is that these circuits still use the negative supply terminal as ground.  The 1/2 Vcc reference is capacitively coupled to ground by large bypass capacitors.  This is why it is called a "virtual ground".  It essentially looks like ground to AC voltages because the large capacitor looks like a short circuit to ground for rapid changes in voltage (such as an audio signal), but does not look like ground for DC variations.  This is a good thing, because we can set the signal's '0' reference right in the middle of the positive and negative rails so it doesn't have to go any further positive to clip than it does negative.  This way we maximize headroom from our power supply.

With that being said, you could just as well connect ground to -9V and establish a 1/2 Vcc point to recreate the equivalent of having an 18V battery...but why?

Instead it makes more sense to use your low impedance path to the voltage source (battery '-' terminal) as your ground reference.

[Morocotopo]

OK transmogrifox, thanks for the explanation, I won´t say that I understand it fully, but I´m trying....
So, reading your responses, I came up with this. All numbers connect with each other (3 with 3, etc). What I´m concerned about is if I got all the grounding and plus/minus 9V right. Would this work? At all?  Is it Ok to use a single supply and a bipolar supply circuit together with the same power source? Errors? Everything in red is my additions/connections to the original circuits.

Thanks for your responses

Morocotopo

[Transmogrifox]

Aha.  Now I see what you're asking.  Thanks for posting the schematics.

Yes, you could connect all of the adjusticator "grounds" to supply negative and it would probably work.

There is a better way to do it, though:

1)  Forget about the 10k/10K divider.  Just connect that 100k resistor to ground (NOT -9V).
2)  Leave the 2.2M resistors connected to ground as they are.
3)  Connect the negative supply pin on the op amp to -9V.

Everything on the EQ as you marked it looks good.  It appears to me you have a good plan, so go for it.  Just a simple matter of connecting everything right.

[Morocotopo]

Thanks Transmogrifox, I think this is gonna work thanks to you people helping me!!
I have one more question and I´ll stop bothering you 
Is this right? I modified the adjusticator part according to your instructions, so I think now it works on bipolar power, right? What about caps marked 1, 2 and 3 in blue? are they neccesary or they could be left out?

Thanks again. If/when succesfully made, will post a PCB.

[Morocotopo]

Bumppp!!
Transmogrifox, are you there...? 

[Seljer]

You can drop #1 and #3 out.

The #2 should stay there (and I think you'd want to add other one from the -9V line to ground)

[Morocotopo]

Thanks Seljer!

[Transmogrifox]

Bumppp!!
Transmogrifox, are you there...? 

No 

Looks like Seljer has you good to go.

[toneman]

cap#1 & cap#3  have *both* sides going to gnd   

As Moro says "not needed" , anyway.

cap#2 depends on whatCha use for a pwrsupply.
9V batt?? & -9inverter??
Probably fine the way it is..... 

Hey, U gonna fit *all* this in a GeeTar   

stay gyrated!!
T

[Morocotopo]

Oh, so you´re not here Transmogrifox. OK.



So, cap #2 goes in, and another one of same value from -9V to ground, BUT WITH THE PLUS SIDE POINTING TO GROUND (0V), RIGHT, ´CAUSE THAT (GROUND) IS NOW MORE POSITIVE THAN -9V, RIGHT?
Slowly, slowly, I´m learning (did I say slowly?)

Toneman, yes, 9V batt (or power supply) and -9V inverter (ICL7660S). See above. I´m not planning to put all this inside a guitar, just in a stompbox, I guess the thread title is misleading... my mistake. Edited. But, now that I think about it... hmmmm... can someone bring me the hammer and chisel, please?

[toneman]

"So, cap #2 goes in, and another one of same value from -9V to ground,
BUT WITH THE PLUS SIDE POINTING TO GROUND (0V), RIGHT,
´CAUSE THAT (GROUND) IS NOW MORE POSITIVE THAN -9V, RIGHT?"

By Jove, I think he's *got* it!!

Build it.
*THey* will come.......
(whoever "they" are)
but, *U* will play....
don't play with your food, son.
play with the guitar!!
U know who U are(?)
whoooooo *are* U; who who? who who?....