concerning theory : purely resistive circuits and frequency.

[tonequester]

tonequester here.


In trying to understand electronics theory better, a question has come to mind. This may be a very elemental question(s) with a simple answer, but please bear with me. I understand that resistance increases with an increase in frequency. Why then, is it that a common modification made to volume controls on an electric guitar
is to place a .001 cap across the input and output legs of the volume pot so that higher frequencies can by-pass the pot. This being done to compensate for loss of treble when turning the volume down. This has been said to occur more noticeably(treble loss) with log, or audio pots than with linear pots(which have their own reasons for not
being good choices for volume pots). Any explanations of opinions are greatly appreciated. Thanks. tonequester.

[Enzo]

I understand that resistance increases with an increase in frequency.

Where do you understand that from? Resistance is not reactive and has no frequency component. There must be more to the concept you propose.

A cap across a volume control is what we often refer to as a brightness cap.

[Tom Phillips]

... I understand that resistance increases with an increase in frequency...

That is not necessarily true. Some will say “not true at all” depending on how you interpret the semantics of the statement.
The resistance of an ideal resistor does not change with frequency.
The “impedance” of a capacitor goes down with rising frequency and the impedance of an inductor goes up with rising frequency. The measurement units for impedance and resistance are both expressed in Ohms which can be confusing until you understand the concept of “reactance.”
All this should be well explained in the components chapter of the ARRL handbook that you purchased a while back. Give that section a read and then post more questions if needed.

...Why then, is it that a common modification made to volume controls on an electric guitar is to place a .001 cap across the input and output legs of the volume pot so that higher frequencies can by-pass the pot...

Because the capacitor has a lower impedance at higher frequencies. Thus the higher frequencies find an easier path thru the volume pot / bright cap circuit. That is, until the volume pot is turned all the way up in which case there is theoretically no resistance left to bypass around and all frequencies are already passing through with minimum attenuation. By the way the bright cap value is more like 100pF which is 0.0001 µF.
Cheers,
Tom

[Austin]

Where do you understand that from? Resistance is not reactive and has no frequency component. There must be more to the concept you propose.

A cap across a volume control is what we often refer to as a brightness cap.

I think maybe he means because the resistance of the pot can interact with the capacitance of the long cord you plug into the amp with making a low pass filter?

[Tom Phillips]

...should be well explained in the components chapter of the ARRL handbook that you purchased a while back...

Tonequester,
A little better direction: In my 1965 version of the Radio Amateur's Handbook the chapter I referred to is "Electrical Laws and Circuits", subsection "Reactance."
Cheers,
Tom

[Enzo]

Austin, that is why I asked, because by itself the statement he made is not correct. If it is in the context of combining with cable capacitance, all well and good, as long as we specify that as a condition.

[J M Fahey]

To begin with, the post title refers to

purely resistive circuits

so there's no way to guess otherwise.

[Tom Phillips]

If we literally want totally pure resistance we are in for a difficult quest. Even a short bond wire has inductance and parasitic capacitance. This is of special concern to a microwave circuit designer.

[tonequester]

tonequester here.


Enzo. i know you don't like long replies, but after reading all of the comments I feel that I must "defend" myself. I don't write down all of my sources or I'd get nothing else done. So I Googled up some for you. First. www.furakawa.co.jp<home>r & d. Article : The Integrated New Products Family "KANZACC" of Kyowa Wire Co.,ltd. "KANZACC anga" Sulphuration Resistant Silver Plating Film. Figure 9., Frequency-Resistance graph(plotting 2 conductors) The graph shows resistance increasing with frequency. A quite noticeable increase at 10khz, and hugh increase at 100khz. I figured a wire company might be a fair source. I realize that the 10khz is above the range of a 24 fret guitar, including it's 2nd and 3rd order harmonics, ....but still. Second, www.analog.com/library/analoguedialogue/ard. Under Figure 12.31 : "The skin effect has the consequence of increasing the resistance of a conductor at high frequencies. Note also that this effect is separate from the increase in impedance due to the effects of self-induction of conductors as frequency is increased." Third : ENGINE ROOM wikipedia.org/wiki/electrical_conduct. "Another complication of AC circuits is that resistance and conductance can be frequency dependent." "One reason, mentioned above is skin effect(and the related proximity effect)." "Another reason is that resistance itself may depend on frequency(see Drude model, deep-level traps, resonant frequency, Kramer-Kronig relations atc.)".
I could list more, but in digging deeper I've determined that there is no concensus as to if this 'skin effect" makes much difference at audio frequencies or not. Some say it does, and some say it doesn't. However, it is undeniable fact that at high ENOUGH frequency, resistance increases. Even my 1957 ARRL handbook maintains that it does at radio frequencies. I wish I had
dug deeper before posting, as until doing so I had never found anything except the simple statement : Resistance increases at higher frequencies. Being totally AUDIO oriented, I just assumed higher audio frequencies. Thats what happens when you try to learn theory on your own, and on the fly at that. Mr Phillips, I believe, was on to something in his reply as he mentioned microwaves. At least I now know that R increases with fq. It probably doesn't make that much difference with an audio frequency signal, but I have not read ANYTHING
that proves that one way or another. As this was mostly a misunderstanding on my part, I would like to re-phrase the original question. I fully understand the "bright" cap and have always used one on every guitar I've owned. My ears here an improvement in treble with their use. Why...does a volume control at low volume setting "lose" treble, or brightness ?
Thanks for the replies folks, especially to Enzo for making me dig deeper. I won't forget the lesson learned. Now....about that treble loss thing??????? tonequester.

[J M Fahey]

VERY short answer: forget all about skin effect.
Even the name.
Absolutely irrelevant in MI amplification.
Is that clear?

[tonequester]

tonequester here.


Already stated as much in my last posts(2). Is THAT clear ! The point is.....the statement was CORRECT. Thanks JM.

[km6xz]

No, the statement is in error. You specified resistance and frequency having a relationship and that premise is wrong. Resistance is a calculated parameter, not a thing. Be definition there is no time or period factor in its calculation so can't be frequency dependent.

If you search for terms and get an answer you think supports your belief, you can find anything about any subject and be totally wrong. Consider that your search and the results you got are not on point of your belief requiring you to jump to a conclusion that was in error. Making a search that is not so prone to errors requires knowing enough of the subject to be able to separate the 95% inappropriate information or just dead wrong information from relevant information.


Why not ask a direct question that you want the answer to instead making assumptions that mislead you in how to ask the question?
For example, you would be a lot less confused and have less inappropriate information if you wanted to know the purpose and function of the cap in a guitar tone network, just ask what the cap does.

When you brought in "resistance" it has a specific meaning and has nothing to do with frequency. But what you did not realize that any real circuit with a resistive element, also as inductive and capacitive reactance which are related to frequency. The values of reactance in resistors, wires and pots is low to the point that they can be ignored for low frequency applications. Skin effect also has little impact on very high frequencies in practical circuits because designers use hollow wires or tubing that has very little difference between low frequency current flow and high frequency current flow. Resistance of a conductor is directly related to cross-sectional area. Since high frequency energy tends to concentrate current flow towards the surface, there is a difference in direct current flow capacity which would flow evenly over the whole cross section and high frequency energy which would see an effective smaller cross section because the center would have less concentration of electron flow. The solution is simply to use wires or conductors which have a high surface area to diameter ratio such as a hollow tube or a flat foil where current flow concentration would be essentially the same for DC and very high frequency energy. None of this has ANYTHING to do with low frequency energy that you will be dealing with. And frequency does not relate to resistance.

[tonequester]

tonequestr here.


Greetings km6xz. Thanks for the reply, and the thourough confusion ! (humor intended) Steve Conner replied as well. He's an electronics engineer, and he says that I am correct in my statement that resistance increases at high frequency(s). Nobody knows better than I what confusion arrises when you confer with a bunch of experts' on any subject.
I was told that if I would only check out my newly required ARRL Handbook,circa 1957 under components, that I would find this so. I quote the handbook on page 19 under skin effect :
"The resistance of a conductor is not the same for alternating current as it is for direct current." "When the corrent is alternating there are internal effects that tend to force the current to flow mostly in the outer parts of the conductor." "This decreases the cross-sectional area of the conductor, WITH THE RESULT THAT THE RESISTANCE INCREASES." (emphasis mine)
So much for the handbook, as I'm sure there were no "experts' in 1957. I now fully understand that this "effect" has no bearing on my original post, and probably has no effect on audio frequencies. I did point this out in my replies, to all of the replies on my post.. I gave examples of source material that backed up mY STATEMENT, not that I was any longer laboring under the impression that skin effect was of concern to me. I am, and have always been aware that there is no such thing as pure resistance.
There is no perfection in resistors/conductors, electronics, or anything else on God's green earth. I am also aware that when it fits their needs, men of science will make use of an impossible
entity like "pure resistance" to prove a point or to get around an obstacle. Read Steve Conner's reply and then you can correct him. I being an admitted idiot, will reply to every comment made on the outstanding posts that I have on this forum, while never making another. For every reliable piece of information I received which I could make use of from the forum, I received
double in WRONG information, bad attitude, uncouth manners, and plain old grouchiness. My time in dealing with this is too much of a waste, and tends to raise my anxiety level to an inappropriate level.I was a Master Printer for twenty years, considered an "expert" in my field. I taught and trained many who became Master Printers in their own right. I NEVER instructed
anyone in an ill-mannered or insulting way....not ONCE ! There are too many "experts' on this forum, including the guy that runs it. I've always been a seeker of knowlege. Therefore, I am not afraid to ask questions that may not be easily answered. I believe that skin effect is a whole topic that is yet to be completely understood. There is too much disagreement on the internet concerning it. If you had read my replies, you would know that I already re-phrased my original question. The question is now put forth, and I paraphrase here : Why, when a volume cotroll on a guitar is turned down, is there treble loss. My not merely rolling over on Enzo's reply, was because I could just see him thinking that I had pulled "that original statement"
out of my" posterior." I had no other intention than to prove that I had found that statement all over the web, from a variety of sources like wire/conductor manufacturers, articles about 400hz power transmission, speaker wire vendors, Wikipedia, and the list goes on. I could really care less about who is right on the subject and who is wrong. Skin effect be damned ! I have
"bigger fish to fry". One thing I won't allow if possible, is to let someone get away with denying my honesty, or questioning my motives. I came here to learn a few things, and the most important thing that I've learned is that this forum is more about inflated ego's than sharing real world knowlege on electronics. I actually would have thought that you would have checked
my reply to Enzo before complicating the matter unnecesarily by not having all of the facts of the situation, BEFORE giving yet another expert opinion. That's another thing I've discovered about this forum. opinion out numbers fact exponentially. I"ll post no longer because opinions are free anywhere you go, and the quality of the opinions you get here are so much "straw" in
my estimation. Thanks for your interest, and all of the other advice you have given me. I truly appreciate it.The next time you wish to lay down some law of physics to someone, please, get all of the correspondence, as well as facts together first. Especially when it concerns a controversial subject like Skin Effect. Nobody deserves to be talked down to, even by an expert.
tonequester.

[tonequester]

Rtonequester here.

Hey Tom. I don't care about the impedance of a capacitor although I understand it well. Everybody is up in arms about my statement concerning higher resistance
at higher frequency.Tthere's no such thing as an ideal resistor. The concept is used in science when someone, usuallky an "expert" wants to qualify or quantify something that he can't do otherwise. There is nothing perfect, ideal, por absolutely pure on god's green earth. My statement stands and if you read my reply to Enzo you'll perhaps understand why. Right here on this forum, and concerniong this post we have two experts in disagreement, 180 degrees, about the subject. I,ve already stated that I now belived that Skin Effect" does not cause noticeable effects in audio apps. I will pass along another thing that I've already stated. The 1957 ARRL Handbook has this to say on page 19, under skin effect : The resistance of a conductor is not the same for alternating current as it is for direct current. When the current is alternating there are internal effects that tend to force the current to flow mostly in the outer parts of the conductor. this decreases the effective cross-sectional area of the conductor, with the result that THE RESISTANCE INCREASES my emphasis. The difference between D.C. and A.C. is that A.C. has a component known as "frequency". Steve Conner, an electronics engineer, says my statement is correct. Check my reply to Enzo for much more on this subject( a very controversial subject). I have already admitted my error in applying this to audio frequency, although I have found sources which believe that it does. I have already re-phrased my question to, and I paraphrase here : "Why does a guitar's volume pot when turned down cause treble loss." I'll leave all "experts" and those who think that they are to "hash" this one out.
No disrespect intended. thanks for the reply. tonequester.

[J M Fahey]

"Why does a guitar's volume pot when turned down cause treble loss." I'll leave all "experts" and those who think that they are to "hash" this one out.
No disrespect intended. thanks for the reply. tonequester.

This Forum is *so* good that your question was answered in full 6 hours *before* you asked it.
How's that?
Just read post #3 on the thread

http://music-electronics-forum.com/t30388/

It even includes answers with and without the skin effect.
And includes a Radio Amateur specific example, in the spirit of the 1957 ARRL book.