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guitar signal carried only at the PL tip?


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  • guitar signal carried only at the PL tip?




    yea, i'm quite a novice and building my first DIY pedal. If the guitar outputs an AC signal, why do i hear all over the place that the signal is carried only by the hot lead (tip)? Isn't it present on the shield as well (which functions as the second lead required for carrying an AC signal)? I know i'm getting something wrong here. Thanx.

  • #2

    Yes the shield carries the signal as well. The inner wire is considered to be hot and the outside wire cold or grounded.

    They use a hot and cold wire in your house wiring. They only added the third ground wires fairly recently. Many older homes still have the two wire outlet. 


    What you're probably missing is a conductor can carry AC and DC at the same time. The shield is both AC and DC natural to the circuit and is designed to block the magnetic waves generated by other sources that could get to the hot signal wire and get amplified.  Without a shield the Hot wire acts like an untuned  radio antenna and picks up all the magnetic waves around it. The 60hz wave generated by your house wiring is the strongest and since its within the hearing range between 20hz and 20Khz, it gets amplified by the circuit.

    60hz AC is close to being a "B" note of a musical instrument, and if the cable isn't well shielded the AC hum can mix with an instruments signal and play hell on what you hear. Tuning to concert pitch 440 can cause beating to occur if there's allot of AC hum so you want the cleanest signal possible with the widest bandwidth free of external emf. 

    It also doesn't take much to turn a guitar and amp amplifier into a radio. All you need is a coil, a cap and a diode and you have the makings of a crystal/foxhole radio that will pick up am broadcasts. 

    All you need is a two different metals in contact with each other, a weak solder joint or carbon in contact with steel and you have the diode component and it can convert AC to dc and you can hear AM radio through your amp.

    I have a strat that will do this. The pickups have a specific number of turns that tunes it into radio broadcasts. The pots have a carbon surface with a hardened steel contact and can act as diodes. The capacitance comes from the tone cap, shielded wires and coil capacitance. The strings can act as an antenna and bingo you get radio stations.  There are ways to fix it but I kind of like it so I haven't bothered to eliminate the radio interference. It has a gold anodized pickguard that also adds to the problem.

    I used to experiment making all kinds of crystal radios as a kid from scratch. I'd wind my own coils, make my own caps and diodes. You'd be surprised how easy it is. To make a diode you only need to and use a safety pin with a piece of pencil led on its end in contact with a hardened steel razor blade surface. The hard and soft metals act as a diode to change AC to DC. The earth itself magnetized the coil to provide power and the radio waves coming in on the antenna wire gets converted to an audible signal by the diode.  Then you just need a pair of large plates separated by a piece of wax paper for tuning stations. As you slide the two to one side, there's less capacitance and this tunes your stations in. You connect it to some dynamic headphones and you got a primitive radio that requires no power to hear stations.

    Guess that's what got me interested in electronics. Its the magic of knowing why this unseen thing called electricity does what it does. 

    Its only been recently where scientists have used atomic microscopes to take pictures of electrons moving through different materials and prove what used to be theory. They did have surprises though. Math is blind to some of the things they found. The technology is super expensive but it is this nuclear science that gave us the super fast CPU and memory chips you enjoy in your computers today. Once they were able to see what impurities in silicone that blocked electron flow and produce heat they were able to refine the materials and get less electron friction and faster chips. 

    This is the beauty behind all electronics. The word electronics comes from the word electron. The "ics" at the end is the process of manipulating electrons to so what we want them to. A guitar is a very basic device that channels then electrons generated by its motor. Motion is converted to an AC waveform the same as it is in an AC generator, cutting through magnetic waves in the air and placing those waves on a wire. The rest is all basic science using different materials/elements to manipulate those electrons, and the amplifiers used create a magnified duplicate of the smaller wave.

    The small wave controls a gate or valve and a small change in the gate, creates a large change in a flow of power which is strong enough to move another coil in the speaker to push air and convert magnetic waves to sound waves. Speakers are air pistons moved by electro magnetic. So is a mic diaphragm and coil. Motors can be power generators or motors that move from power being applied. Same goes for dynamic speakers and microphones.

    The only differences are the power capabilities they are designed to carry and the signal strength they are designed to generate. Even piezio elements can both act as pickups and speakers. They simply use a crystalline combination generate a signal or make a disk move. Your old turntables used to use coil cartridges and were probably your first guitar pickups used. I used to tape a crystal cartridge to my acoustic guitar to record it.

    This is all cool stuff you can study and get a good mental picture going to remove some of the mystique surrounding it, but it remains a wondrous process no matter how well you know it as do most natural things we encounter in life. When you run out of questions of why things work the way they do, your mind is closed to change and instead of being a pioneer, you become a slave to the technology.


    • blindopher
      blindopher commented
      Editing a comment

      wow, what a cool answer. So may i please ask you once again:


      1. Does AC require 2 conductors, or can it flow in one as well.

      2. When my pickups output AC into a pedal running on DC, what is the current output of the pedal - AC or DC. I'm assuming AC otherwise we wouldn't need to rectify at the amp? Is the pedal circuit a DC circuit? I'm only assuming that because it's usually powered by DC.

      3. If the shield in the instrument cable carries the signal as well, why isn't it reversible with the hot lead?


      thanx again!