Are the "Q" variables in Unit of Charge and Wah alike?

Stompboxes circuits published in magazines, books or on DIY electronics websites.
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JohnBlakeArnold
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Post by JohnBlakeArnold »

I've been boning up on my actual electronics learning. I came across this nifty definition for Charge, and the International Unit of Charge, the coulomb.

"1.2 Charge The unit of charge is the coulomb (C) where one coulomb is one ampere
second. (1 coulomb D 6.24 ð 1018 electrons). The coulomb is defined as
the quantity of electricity which flows past a given point in an electric
circuit when a current of one ampere is maintained for one second. Thus,
charge, in coulombs Q = It
where I is the current in amperes and t is the time in seconds."

Electrical Circuit Theory and Technology, Second Edition Revised

My question is: when I have seen "Q" in various inductance designs such as Wah pedals, can I now understand that this mysterious "Q" as being representative of Charge which changes within the circuit as it the pedal moves? 8)

Thanks.

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Post by lolbou »

Nope. It's a quality factor...
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Post by DrNomis »

In a filter, the "Q" of the circuit is basically the bandwith, a Wah pedal has a circuit which is basically what's called a "Bandpass-Filter", although the filter can be swept up and down in frequency, the higher the "Q", the narrower the bandwith and vice versa..... :thumbsup
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Post by JohnBlakeArnold »

"although the filter can be swept up and down in frequency, the higher the "Q", the narrower the bandwith and vice versa.."

Isn't a "change in the quantity of electricity" ~Q~ [via a variable resistor] the exact mechanism used to "sweep" through the filter?

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Post by lolbou »

Nope again. The wah is a LC filter. Though the variable capacitor is somehow "simulated" by the pot circuit (variable resistor).

Check the technology of wahs by RG Keen...

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Post by JohnBlakeArnold »

Since I come to these forums asking a question in order to learn, can someone give me a few examples of where in a basic guitar circuit Q in coulombs would be found and measured?

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Post by DrNomis »

The "Q" in Coulombs that you're talking about is relating to the amount of charge,in Coulombs, that a capacitor can hold, and is given by the following mathematical equation (if my maths, and memory are correct):


Q= C X V

Where:

Q= Amount of Charge in Coulombs.

C= Capacitance.

V= Applied Voltage.


(Can someone check that to see if that's correct?...thanks in advance.... :thumbsup )
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Post by DrNomis »

This webpage may be of interest to you:


http://m.everythingscience.co.za/grade- ... .cnxmlplus
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Post by JohnBlakeArnold »

I'm not saying anything about capacitance at all. I think there is a difference in scope in considering the defined value of Q at any point in time or physical location on the circuit, and how the values on the circuit change as the value of Q changes.

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Post by deltafred »

JohnBlakeArnold wrote:Since I come to these forums asking a question in order to learn, can someone give me a few examples of where in a basic guitar circuit Q in coulombs would be found and measured?
I don't recall the last time I used coulomb calculations, probably when I was at college. Quality factor however is used extensively in filter design.
I'm not saying anything about capacitance at all. I think there is a difference in scope in considering the defined value of Q at any point in time or physical location on the circuit, and how the values on the circuit change as the value of Q changes.
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Post by JohnBlakeArnold »

From http://en.wikipedia.org/wiki/Q_factor

"In physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency. Higher Q indicates a lower rate of energy loss relative to the stored energy of the resonator; the oscillations die out more slowly. A pendulum suspended from a high-quality bearing, oscillating in air, has a high Q, while a pendulum immersed in oil has a low one. Resonators with high quality factors have low damping so that they ring longer."

Thanks to DeltaFred for the actual statement of the word "Q Factor", this definition shows that Q is not dependent on electromagnetism, via the pendulum example.

I also found these resources:

http://analogfilter.blogspot.com/2008/0 ... actor.html

http://www.sengpielaudio.com/calculator-bandwidth.htm

Also this "Q is only defined for second and third order stages. For second order stages, Q is defined as Wo divided by the sum of the real part of the pole locations. For third order stages, the two complex poles are used to calculate and display Q. If all poles are real, the two smaller poles are used. Q provides a measure of sharpness to the stage frequency response. High Q implies a very narrow sharp frequency response. Low Q provides a wide shallow frequency response. The minimum value for complex poles is 0.5. Q is dimensionless.

Stages are sequenced such that low pass and low Q stages are physically position prior to high Q and hi pass stages. Low pass takes precedence over low Q. Stages with equal Q are sequence by increasing Wo. This minimizes the possibility of op amp slew rate limiting, and internal signal ringing. Stages may be physically sequenced by the user by clicking on any op amp of any stage and selecting the stage number of the desired stage to switch places with."

And this:
"Quality Factor Q and Filters:
*Changing R changes the resonance width
*Controlled by the energy loss per cycle
*called the damping factor
*Define the quality Factor Q = 2Pi (max energy stored / energy lost per cycle)
*the Q measures **** 'how good a circuit is' **** <---- what does this mean!?!
*the higher the Q, the sharper the peak "
from http://www2.ensc.sfu.ca/~glennc/e220/e220l21b.pdf

Thanks for the efforts, I believe the post has brought better info to this subject.
John Blake Arnold

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