volume control treble loss

Discussion in 'General Electronics Chat' started by secretagentman, Aug 14, 2012.

  1. secretagentman

    Thread Starter New Member

    Aug 14, 2012
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    secretagentman here.

    Greetings. I have made many modifications to the circuitry of electric guitars, over 32 years. The first of these was very simple. It's called a "bright" cap, or a treble by-pass. I use it on all of my personal guitars.
    It is a .001uf cap placed across the in and out legs of the volume pot.
    This might be a "dumb" question, but I am new at making a formal attempt to learn some theory besides Ohm's Law, and new to this forum.
    I haven't learned the "ropes" yet. Question : "Why is treble "lost" or attenuated by turning a guitar's volume pot down" ? Any advice or opinions are very welcome. Thanks in advance for your time.
     
    Last edited: Aug 14, 2012
  2. #12

    Expert

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    The capacitance of the cord is significant. A volume control can be in the range of 100,000 ohms to 500,000 ohms. When you put that in series with 18 pf per foot, the highs start leaking out.

    That's why I invented the CTA...cord tip amplifier. It's a jfet source follower that fits in the 1/4 inch male plug.
     
  3. ErnieM

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    Apr 24, 2011
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    Huh, so you're going to make me guess what value the pot is? In that case I'm just gonna wave my hands to explain this till some kid with a spice program prints out some graphs.

    A pot works for volume control since it makes a natural voltage divider. If you pick a cap of the right value it works in parallel with the upper resistance to make it effectively lower at higher frequencies, which lets thru more signal, hence it "brightens" the music.

    As to why it seems to attenuate at low volume... that's not so simple to guess at. It may be some interplay with the driving source impedance (zoom, over your head today, sorry) or may just be the resistor getting so large it lowers the frequency where the boost starts, so it doesn't seem so bright in a relative sense.

    A simple RC filter has a "break frequency" (the frequency where it begins to be effective) of:

    F = 1 / ( 2 * pi * R * C) Low-pass filter From Wikipedia, the free encyclopedia

    And welcome to the forums!
     
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  4. secretagentman

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    Aug 14, 2012
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    I don't have a quote, but I am in complete agreement with yours. This may not be proper procedure here, but I welcome correction if it is not.
     
  5. #12

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    No need to guess. Here's the math.
    When an audio taper pot is set to "5" (out of 10) the resistance to ground is 10% of the total resistance.
    Assuming a 250k pot and
    the average capacitance of the guitar cords I have measured is 26 pf per foot
    and the guitar cord is 12 feet long
    F = 1/(2Pi C Xc)
    F = 1/ (2 Pi 312e-12 225k)
    the frequency at which the loss of voltage is 50% is equal to 2.267 kilohertz

    the frequency at which 90% of the voltage is lost is
    1/ (2 Pi 312e-12 25k)
    or: 20,404 Hz

    and these are conservative numbers. No stretching to prove (fake) my point.
     
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  6. ErnieM

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    Apr 24, 2011
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    Looks like a guess to me, just using fancier words. :D

    As you drew, the cap is breaking with the Thévenin equivalent resistance, which is all 3 resistors (2 from the pot, one from the load) in parallel. So for 10% and 90% the resistance is 16.9K ohms and the break frequency is the same and it's above 30KHz.

    The worst drop off is at a 50% setting (I have no idea what number setting this is as it's logarithmic scale) where the two ends of the pot are equal and 125K. As that setting the resistance is 32.6K ohms break is 15.6KHz.

    However, this is not the question secretagentman is asking.
     
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  7. #12

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    Thanks for cleaning up the math for me.

    Most guitars contain a 250k volume control. That's why I "guessed" that 250k would be a useful number. If you wish, you can guess that the other commonly used value is 500k. The only way to not guess would be for secretagentman to name the value of the volume control in his guitar. However, that doesn't seem important now that you have established that the attenuation of audio frequencies does not happen when one turns down the volume control on their guitar. secretagentman is apparently chasing an error in perception, not a real loss of high frequencies.
     
    Last edited: Aug 15, 2012
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  8. Audioguru

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    Dec 20, 2007
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    A guitar pickup is inductive. It resonates with the capacitance of the output cable at about 4.5kHz so high frequencies are attenuated.
    Players in a band are deaf to high frequencies anyway.
    People who play with guns are also deaf.

    "Wau' 'id oo 'ay?" ("What did you say" without any high frequencies).
     
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  9. ErnieM

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    A good friend of mine is a life long drummer. A hearing test a decade ago discovered he has absolutely no sensation at the frequencies of his cymbals.
     
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  10. Audioguru

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    I am almost 67 years old and I can hear high frequencies up to about 14kHz.
    Because I protected my hearing from loud sounds.
    A hearing test goes up to only 8kHz. So you can hear basic speach, not hifi music.

    Most oldies are lucky to hear a 3kHz smoke detector blasting.

    But I cannot understand what some teenagers say. They speek a different language.
     
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  11. #12

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    But seriously, loss of high frequencies when turning the volume control down (on a guitar) is well known among people that actually play guitars. For instance, that is why Eddie Van Halen refuses to use a cord that is more than 9 feet long.

    I just checked with my guitar repairman and he s... "guesses" that 99% of all guitars have either a 250k volume control or a 500k volume control. It is easy enough to guess twice and do the math for both values.

    Still, I understand that 100,000 guitar players must be wrong, so I will not argue any further.
     
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  12. Ron H

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  13. Audioguru

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    The Fetcher Munson curves show that a person with normal hearing is most sensitive to the 4kHz sounds of a baby crying or the horrible shriek of an electric geetar and its distortion.

    A person who is deaf to high frequencies might not hear a baby crying but might like the muffled sound of the electric geetar.
     
  14. Ron H

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    I don't understand that graph that I linked to. It implies that the ear's frequency response is greatest at 20Hz, and would be even higher at frequencies lower than that.
    The graph was in this article. It wasn't even mentioned in the text.
     
  15. Audioguru

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    The opposite. The graph shows how much sound level is required so that all frequencies sound at the same level.

    We are most sensitive to 4kHz (a baby crying or the shriek of an electric guitar) and we are not sensitive to low level low frequencies. We are also not very sensitive to low level high frequencies.

    Many stereos have a "loudness compensation" circuit that boosts the bass and sometimes boosts the treble when the volume control is turned down.
     
  16. Audioguru

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    The opposite. The graph shows how much sound level is required so that all frequencies sound at the same level.

    We are most sensitive to 4kHz (a baby crying, a smoke detector beep or the shriek of an electric guitar) and we are not sensitive to low level low frequencies. We are also not very sensitive to low level high frequencies.

    Many stereos have a "loudness compensation" circuit that boosts the bass and sometimes boosts the treble when the volume control is turned down.
     
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  17. Ron H

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    OK, now it makes sense.
     
  18. secretagentman

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    Aug 14, 2012
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    Greetings, and thanks for your reply. I am pleased to know that simeone has an understanding about the well known issue of treble loss when one turns down the volume on a guitar. most guitars are either 250kOhms rt 500kOhms most are 500kohms in my experience. I actually use a 1Meg pot
    which allows a stronger outut sugnal from the pickups. Doin't ask me why, it's another of those mods that I read about in the early 80's, tried, and found to be quite effective. It;s also another mod that I've never heard explained. in my youth I wasn't nearly curious enough about these things, and now am now trying to understand better. When I was in my early 20's, I didn't care about anything except results, much to my sorrow now.
    I really appreciate your time in replying. The answer to the question is probably a simple one, and sooner or later somebody out there will know the answer. All things come with patience. have a great day. secretagentman.
     
  19. #12

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    I tried showing how this works but was proven wrong with mathematics. You can try a 3 foot to 9 foot guitar cord and see if that makes any difference. If it works for you, the math is irrelevant.
     
    Last edited: Aug 17, 2012
  20. ErnieM

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    While completely admitting I am new to this party and have never even picked up an electric guitar... The whole issue of treble loss due to the cord length would disappear by just having a lower input impedance on the amplifier.
     
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