# Guitar Mid-range Scoop - band stop filter

Discussion in 'The Projects Forum' started by Traverse, Mar 26, 2009.

1. ### Traverse Thread Starter New Member

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Mar 26, 2009
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Hi there...

I'm modifying my electric guitar and would like to install a push/pull switch that allows me to switch between normal volume control and a mid-range scoop switch that uses a band stop filter to cut out some of the middle frequencies while leaving the upper and lower frequencies.

I've read the All About Circuits AC chapter on filters and figure that I can use either use a resistor cap circuit (Twin-T band-stop filter) or an LC Network (parallel resonant band stop filter).

I'm struggling with figuring out what values to use in either situation.
I have a passive circuit with either a 250k or 500k pot. I haven't decided what cap or resistor values as I don't know what is best.

What I am trying to achieve is something you can buy online...
http://www.guitar-mod.com/rg_passive.html
I'd rather do it myself though.

I found a 600 ohm transformer similar to what they use (but not identical - mine has 600 in and out - theirs has 4k in and 600 ohms out).

I'm new to this whole thing but eager to learn.

I don't know the amount of current that will flow through the circuit - just whatever the pickups send.

I'm hoping to find some formulas to help me calculate the inductor, cap and resistor values.

I found the f-notch formula and the Twin-T formulas but can't quite understand how to pull it all together.

Any help from anyone would be much appreciated...

2. ### KL7AJ Senior Member

Joined:
Nov 4, 2008
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The twin-T notch filter is a fine choice, but you might want to put a variable resistor in parallel with it, so you can adjust the Q. Otherwise, you'll just have an infinitely deep, infinitely narrow notch!

Eric

3. ### Traverse Thread Starter New Member

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Mar 26, 2009
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That makes a lot of sense... is a variable resistor just another component part that I can buy or do I have to put one together?

I was hoping to be able to adjust the Q actually.

Also, any idea about how to calculate what specific values I need for the caps and resistor (at least what range for resistor if I can do the variable resistor)?

4. ### Traverse Thread Starter New Member

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Mar 26, 2009
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Part of that was pretty dumb... variable resistor - would be a pot...

Are there smaller resistor sized variable resistors?

5. ### Traverse Thread Starter New Member

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Mar 26, 2009
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Another thought...
I plan to hook up the Twin-T directly to my volume pot on my guitar - 500k push/pull pot... I'm guessing that is what you are talking about.

one lead to the ground and the other to the top half of the switch which is wired across the volume pot - so at pull the scoop is active and at push it is not.

KL7AJ, would that be what you were talking about - would it adjust the Q?

Sorry I'm so new at this, I ask alot of questions.