First Post: Help me make sense of Impedance and Tone Stacks

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First Post: Help me make sense of Impedance and Tone Stacks

Postby Wgtroutman » 08 Jan 2015, 01:02

Hello all,

I have been lurking on this forum for some time and decided to join the discussion (though for the time being I will be doing far more absorbing than contributing). I have learned a ton reading the various threads and am amazed at the amount of knowledge this forum has to offer.

I downloaded the Duncan Tone Stack Calculator and was looking at the Big Muff Pi Tone Stack. My question concerns the "Zsrc." I assume that has something to do with impedance. If I wanted to put that stack in another pedal, how do I determine what number needs to be plugged in there and how much does it matter? Changing that value seems to mess with the control drastically.

Thanks so much in advance. I know little, but some day I hope to contribute to this forum in a way that is helpful to people in the same way many of you have been helpful to me.
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Re: First Post: Help me make sense of Impedance and Tone Sta

Postby grrrunge » 08 Jan 2015, 09:01

I'll try to explain this in a simple way ;)

Zsrc is the output impedance of the previous stage.

You can think of impedances like frequency dependant resistors.

A filters transfer function is calculated by evaluating the complex impedance for every component in the filter, and you end up with one equation for the transfer function, that describes the filters impedance.

As you can see, the filter (which is now represented as one big frequency dependent resistor) sits in parallel with the source. With a little source transformation it becomes apparent, that the source impedance and filter impedance forms a voltage divider.

So there you have it. That's why the source impedance matters.

In general: The smaller your source impedance is, the less it will influence the signal being transferred to the next stage.
Plotting the right vaue in Zsrc's place requires you to calculate the previous stages output impedance.
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Wgtroutman (08 Jan 2015, 16:59)
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Re: First Post: Help me make sense of Impedance and Tone Sta

Postby Wgtroutman » 08 Jan 2015, 16:59

Thanks grrrunge. That clears up what output impedance is and why its important.

Follow up question: If I'm putting this tone stack at the end of a fuzz face (I'm positive I'm not the first to do this), which the below promotional material says it has an output impedance of 2Kohm at 100% volume and 115Kohm at 50% volume, I would have to modify it to make it operate properly, wouldn't I?

http://www.jimdunlop.com/files/manuals/JHF1_manual.pdf
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Re: First Post: Help me make sense of Impedance and Tone Sta

Postby grrrunge » 08 Jan 2015, 20:35

If i were to do something like that, I would bracket the tone control between two buffers, or maybe even amplify the input to make up for any unintentional signal loss through the filter.
Exactly what do you want from it? Perhaps it's an easy fix in the fuzz face, if you're willing to mod it...
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Re: First Post: Help me make sense of Impedance and Tone Sta

Postby Wgtroutman » 10 Jan 2015, 06:50

Would a simple JFET buffer work? Essentially I want the muff tone control to operate with the fuzz face in the same way that it would in a big muff. I'm willing to mod it.

For the time being, I'll wrap the control between two buffers and see how that works. Great suggestion.
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